From crutcher at astro.uiuc.edu Wed Mar 1 12:34:25 2000 From: crutcher at astro.uiuc.edu (Richard Crutcher) Date: Wed, 01 Mar 2000 11:34:25 -0600 Subject: [asac] Polarization paper for ALMA Science Advisory Committee References: <200002091839.NAA00674@polaris.cv.nrao.edu> Message-ID: <38BD54A1.698CEAA9@astro.uiuc.edu> POLARIZATION OBSERVATIONS WITH THE ATACAMA LARGE MILLIMETER ARRAY Richard M. Crutcher, University of Illinois W. J. Welch, University of California at Berkeley Larry D'Addario, National Radio Astronomy Observatory 1. INTRODUCTION Because of its enormous sensitivity and imaging capabilities, the ALMA will be the premier instrument at millimeter and submillimeter wavelengths. Polarization observations will likely be carried out far more frequently with the ALMA than with present telescopes because the sensitivity of the ALMA will make such observations (which always have to deal with signals only a few percent of the total intensity) possible for a much larger set of radio sources. However, polarization observations place significantly more stringent requirements on instruments than do total intensity measurements. Carefully consideration of the instrumental requirements for successful polarization observations should therefore be given high priority in the design of the ALMA. 2. POLARIZATION SCIENCE Major scientific areas that will benefit from excellent polarization capabilities of the ALMA include the following: Star formation. Theoretical and observational work has shown that magnetic fields can play a significant and perhaps essential role in the formation of interstellar clouds, in their evolution, and in the star formation process. Needed are observations of the morphology and strength of magnetic fields in molecular clouds. Techniques available include: (1) measurement of linearly polarized emission from dust grains aligned by magnetic fields; (2) measurement of linearly polarized spectral line emission (both in thermal lines due to the Goldreich-Kylafis effect and in maser lines such as SiO); and (3) measurement of circularly polarized spectral-line emission produced by the Zeeman effect. The first two techniques yield information about the morphology of magnetic fields in the plane of the sky, while the third gives the magnitude of the line of sight component of the field. Supernova remnants. Synchrotron emission from SNRs is linearly polarized, and the polarization is used to measure the direction and estimate the strength of magnetic fields. Normal galaxies. Synchrotron emission from the interstellar medium in normal galaxies may be used to map magnetic fields in external galaxies and study the morphology and estimate the strengths of extragalactic magnetic fields. Such studies may lead to an understanding of the amplification of magnetic fields in galactic dynamos. Radio galaxies. Radio lobes produce polarized synchrotron emission that may be used to map the morphology and estimate the strength of magnetic fields. Circular polarization observations will probably be primarily Zeeman line work carried out for that special purpose at a small number of frequencies. Certainly the 3-mm CN lines, and perhaps the CCS line at 33 GHz, the 1-mm CN lines, and several SO lines would be of interest. Other lines may of course also prove to be useful as the tremendous sensitivity of the ALMA is exploited. Except for the Zeeman effect, all of the above science drivers for polarization observations with the ALMA involve linear polarization. Requirements on the instrumental polarization are much more severe for continuum linear polarization mapping than for Zeeman observations. Moreover, for many if not most of the observations that will be made with the ALMA, the polarization of thermal dust continuum or synchrotron emission will be of scientific value EVEN WHEN THE POLARIZATION DATA ARE NOT THE PRIMARY PURPOSE OF THE OBSERVATIONS. Thus, optimization of instrumental characteristics of ALMA for routine linear polarization observations would be of the greatest scientific value. 3. REQUIREMENTS Requirements fall into three areas: (1) sensitivity - zero or minimal loss of sensitivity when doing polarization observations; (2) Fourier sampling - ability to obtain and include zero and short spacing polarization data in order to carry out full synthesis mapping; and (3) accuracy - the ability to calibrate instrumental polarization easily and accurately (0.1% or better) over the entire primary beam. We briefly describe these requirements in this section, and in section 4 discuss specifics of instrument design and calibration needed to meet these requirements. 3.1 Sensitivity The very great effort going into giving the ALMA very high sensitivity for mapping of total intensity also will yield high sensitivity for polarization work so long as that sensitivity is not compromised by the instrumental design. The fact that the ALMA will have dual receivers with feeds sensitive to orthogonal polarizations is the first necessary step. But if that system is to achieve its potential for polarization work, the design must have a focus on the effect on polarization of all aspects of the system. Polarization is usually less than 5%, and over large spatial areas the percentage polarization is 1% or less. Hence, the dynamic range that can be achieved is automatically significantly lower than for intensity observations. In order not to further reduce sensitivity, one would like to be able to map polarization to the limits set by thermal noise rather than instrumental polarization. 3.2 Fourier sampling A large fraction of polarization mapping with the ALMA will be of extended objects. Hence, procedures for obtaining short and zero spacing polarization data that will not degrade the quality of the interferometric data are essential. Single-dish polarization observations have traditionally been done by rotating a polarizer and detecting the total intensity of the time-modulated signal. Because this involves subtracting two big numbers (intensities in two different polarization states) to determine a small number (a Stokes Q, U, or V), it is very difficult to achieve calibrated instrumental sensitivities of 0.1%. New methods of single-dish polarization mapping must be developed for the ALMA. 3.3 Accuracy The goal should be to map Stokes V, Q, and U limited by thermal noise and not by instrumental effects. As a practical matter, the goal should be instrumental polarization effects of < 0.1%, after calibration. Moreover, this spec must be met over the entire primary beams of the telescopes in order to map over the single primary beam and to mosaic map. A significant difference between standard intensity (Stokes I) mapping and polarization mapping is instrumental polarization. For intensity mapping, the primary beam is a relatively simple and stable function, so the instrumental response (dirty beam) can be predicted from the UV coverage. Knowledge of that instrumental response can therefore be used to deconvolve it out of the final maps. The instrumental response in Stokes parameters Q, U, and V depends in addition to the UV coverage on the polarized instrumental response over the primary beams of the various antennas, and in general this may vary strongly and in a complicated manner with position in the primary beam, time, pointing position, etc. In order to deconvolve the polarized dirty beams out of the final polarization maps, the polarized dirty beams must be known at the noise level of the maps. If the instrumental polarization due to the antennas is stable in time, one can measure it once and take it out. Time variable instrumental polarization (due to elevation effects for example) requires great loss of sensitivity due to time spent on calibration and/or limitations on polarization fidelity. Failure to know the polarized response of the instrument over position and time is the major limitation on the accuracy of polarization mapping. 4. MEETING THE SCIENCE REQUIREMENTS 4.1 Instrumental polarization issues As noted above, science drivers imply that most polarization work will be in linear polarization. The main science driver for circular polarization work is Zeeman work, for which the requirements are less severe (see below). Thus, if it is necessary to optimize the ALMA for observations of linear or circular polarization, the science implies optimization for linear polarization observations. If this is not possible for all bands, consideration should be given to optimization for linear polarization observations at a prime polarization band; perhaps the 345 GHz band is best. The science goal is that the total instrumental polarization be less than 0.1% without major loss of observing time for calibration. This tolerance cannot be met without calibration, but achieving the closest possible approach to zero instrumental polarization must be a design criterion in order to meet the science goal. Meeting this goal requires consideration of the following areas: - Absolute polarization of each of two (nominally orthogonal polarization) ports. - Orthogonality of the polarizations of the two ports of one antenna. - Uniformity of polarization among antennas of the array. - Orthogonality of opposite ports between antenna pairs of the array. - Variation of each of the above with direction of arrival over the main beam. - Temporal stability of each of the above, short- and long-term. - Effects of elevation dependence; designs that call for the antennas to be stiff or that allow them to sag with refocusing both require attention to the polarization effects. Although one often speaks of linearly or circularly polarized feeds, it should be noted that "feeds" are never purely linearly nor purely circularly polarized, though they are often a close approximation to one of these. The mathematics makes it clear that so long as the telescopes have orthogonal polarization receivers, one can derive the full polarization information (i.e., all four Stokes parameters). One can choose any pair of orthogonal polarization states as "basis" states, so that any arbitrary state is describable as a linear combination of them. To be accurate, it is the polarization state of the whole antenna that matters. For most radio telescopes, this includes the main reflector; subreflector; other mirrors (flat or curved); other optical elements (including wire grids and lenses); and finally something to convert the free-space, multi-mode beam into a guided, single-mode wave. The last element is often a polarization-insensitive horn followed by a "polarizer" with two single-mode ports, each coupling to a different polarization of a plane wave incident on the whole antenna. Each of these cascaded elements affects these final two polarizations. Those elements that have sufficient symmetry can be treated as polarization-insensitive. In the simplest case only the polarizer is significant, but in practice the situation is often more complicated. The sensitivity can be reduced if the polarizer introduces noise, or if a significant fraction of the observing time must be devoted to calibrating the instrumental polarization in order to achieve the required sensitivity. The BIMA system, which has only a single receiver per telescope, employs a transmission polarizer consisting of a grooved dielectric plate in front of the receiver to select the desired polarization basis state; this plate adds significantly to the noise of the system. Second, if the polarization state of each antenna is complicated (for example, if it differs significantly from the desired basis state or varies both in time or over the field of view), a large fraction of the observing time must be spent in calibration, which will significantly reduce the sensitivity. Hence, a design that has the lowest instrumental polarization and the lowest possible, most time stable instrumental polarization will maximize sensitivity. The optical design is crucial for polarization mapping over extended areas. The best optical system is a "straight through" design, with no off-axis elements or oblique reflections. Both will produce instrumental polarization that varies over the primary beam of the telescopes. If an off-axis system is necessary, careful calibration of its instrumental polarization effects will be necessary. Since this will be time consuming, it will be important that the optical system be kept invariant so that a calibration may be used over a long period of time. It would make sense to choose a primary band for linear polarization work (probably 345 GHz would be best) and optimize the optics of that band for polarization. Again, ideally, this would be on axis. If that is impossible, at least a dual-mirror system should be chosen with reflections designed for the polarization basis state of each channel. Having reflections as close as possible to normal (to the mirror) for the primary polarization band should be a design consideration. Another issue is whether there is a significant advantage to a choice as close as possible to a linear or a circular basis state, and second, what deviation from a particular basis state may be tolerated without making the calibration less accurate and/or more difficult and time consuming. Although in principle even large instrumental polarization effects may be calibrated, in practice the best approach is to have the polarization state of each antenna to be intrinsically as close as possible to the desired ideal state. In practice, accurate polarimetry must account for the actual polarization state of the antenna; extraordinary efforts to produce a basis state that approaches circular or linear to high accuracy is not important. Cotton (1998; MMA Memo 208) discussed calibration of interferometer polarization data and the merits of linear or circularly polarized feeds. There are a number of strong disadvantages of linear feeds, including especially the facts that p-q (orthogonal polarizations) phase fluctuations can significantly increase the noise in linearly polarized data, that no polarization "snapshots" are possible since extended observations are required to measure calibrator Q and U, and that any p-q phase difference corrupts polarization data. Circularly polarized feeds overcome these disadvantages for polarization work, and have the additional advantages that calibrator polarization only weakly affects gain calibration, that there is good separation of source and instrumental polarization with parallactic angle, and that instrumental polarization can be determined from a calibrator of unknown polarization. If, as argued above, linear polarization science observations will be the most important, having the polarization basis states as close as possible to circular would be best. Since Zeeman observations are spectral-line observations, the observed polarization is a relative measurement. That is, the circular polarization as a function of frequency must be measured. The most important instrumental polarization effect is beam squint - the pointing of the two circularly polarized beams in slightly different directions. More generally, beam squint may be considered to be the total (including sidelobes) difference in instrumental positional response between the two senses of circular polarization. In the presence of velocity gradients in molecular clouds, beam squint will produce false Zeeman signatures. However, so long as the primary beam squint is not too bad, and especially if it is known and stable, its effects can be calibrated and corrected. Small (< 5%) impurity in instrumental circular polarization and difference in gain between the two polarization channels can be calibrated out using standard Zeeman analysis techniques. Moreover, simultaneous observations of thermal continuum and/or of non-Zeeman spectral lines within the observation window may be used to calibrate the instrumental circular polarization. 4.2 Calibration issues Since the instrumental polarization tolerances will not be zero, what is the best overall strategy for calibration to determine the actual polarization of each antenna? Moreover, besides knowing polarizations of the antennas, it is also necessary to know the complex gains of the receivers. To a large extent, this is the same as is required for observations of sources that are assumed unpolarized or where only total intensity is to be measured. An exception is that polarimetry requires knowledge of the ratio of the complex gains of the two channels, whereas total intensity measurement does not. Conventional astronomical calibration determines the amplitudes of these gains separately (and hence their ratio) provided that the calibrator's polarization is known (preferably unpolarized); it can determine the phase difference only if the calibrator is appropriately polarized (preferable strongly so). What, then, is the best overall strategy for receiver gain calibration? These points must be considered in the contexts of both interferometer mode observations and single-dish mode observations. The single-dish mode is the more difficult. For the ALMA, it may be that the engineering reality is that all receivers will be connected to antenna ports that are approximately linearly polarized, and thus a poor approximation to being circularly polarized. MMA#208 states that the principal reason for this is that it allows larger bandwidth; this is roughly true at centimeter wavelengths, but it is not correct for the ALMA. At the shorter wavelengths, various antenna elements besides the polarizer are either impossible to construct or are excessively lossy if they operate on waves that are nearly circularly polarized. An element that selects a single linear polarization with very low loss and very large bandwidth is easily built (a wire grid), whereas nothing similar exists for circular polarization. It is possible to insert a "quarter wave plate" to convert circular to linear polarization with good accuracy over a narrow band, but with some noise penalty due to ohmic losses. Thus, engineering reality may preclude the possibility of having the ALMA optimized for linear polarization by having near-circular polarization feeds, except as a potential add-on, with limitations. It should be clear that this is an engineering limitation and not a decision that optimizes for polarization science. Many of the difficulties cited by Cotton in MMA#208 would be overcome by having a calibration source of known polarization with a very strong linearly-polarized component (assuming that we are more interested in mapping the linear polarization component than the circular one of unknown sources). Although such things do not exist in the natural sky, it should be straightforward to have one built into each ALMA antenna. One attractive possibility for the calibration of the dual polarization receivers is to provide an intense millimeter wavelength CW signal that can be coupled into the receivers at their inputs. Such a signal could be coupled into the receivers through a small aperture in the middle of the secondary mirror. It could be highly linearly polarized but at a position angle of 45 degrees, so that it couples equally and coherently to both the horizontal and vertical polarization receivers. In this way, it could provide a very accurate relative calibration of the two receivers. A total power spectral correlation measurement would provide both amplitude and phase calibration between the two receivers. Presumably this CW millimeter wavelength signal could be tuned to different frequencies as needed. A further possibility would be that the same coherent millimeter CW signal could be injected into every front end. For example, the signal might be provided as the beat note between two optical laser signals. In this case, the coherence of the signals would allow the phase (and amplitude) relative calibration of all the receivers, including their two polarizations. This internal polarization calibration source would of course calibrate the system from the feeds on; instrumental polarization of the primary and secondary reflecting surfaces would have to be calibrated astronomically. In order not to spent excess time on such calibrations, the design should focus strongly on making the instrumental polarization that must be calibrated astronomically as stable in time, elevation angle, and position over the beam as possible. Obtaining single antenna and short spacing polarization data will be a challenge for the ALMA. A plan to obtain such intensity data by "on-the-fly" mapping with the ALMA antennas should work for polarization also so long as full polarization information is obtained and the system is sufficiently stable. A stability of at least 1 part in 10,000 seems to be necessary, sufficient, and achievable, but this spec needs to be investigated specifically for polarization calibration. A system to cross-correlate the signals from the orthogonally polarized receivers on each antenna in order to produce single-dish polarization data while "on-the-fly" mapping is being carried out should work, but needs to be investigated. A system which requires physical rotation of polarizers should be avoided; it would be difficult to achieve the required accuracy and would be time consuming. 5. RECOMMENDATIONS The sections above describe the science drivers and the required polarization performance of the ALMA. Specific recommendations have been discussed in section 4. However, millimeter-wave polarimetry is not yet a mature field. We therefore strongly recommend that the systems for polarization observations with the ALMA be implemented and tested at the earliest possible time. Use of existing millimeter-wave interferometers is likely to be useful, but implementation of polarization capabilities from the beginning on the first ALMA test interferometer is essential if the ALMA is to fulfill its promise for polarization. -- Richard M. Crutcher Chair, Department of Astronomy University of Illinois 1002 W. Green St. Urbana, IL 61801 Voice: 217/333-9581 Fax: 217/244-7638 From wilson at physics.mcmaster.ca Thu Mar 2 15:32:14 2000 From: wilson at physics.mcmaster.ca (Christine Wilson) Date: Thu, 2 Mar 2000 15:32:14 -0500 (EST) Subject: [asac] report on infrared water vapour monitor for ASAC meeting Message-ID: Dear Colleague, (I'm sending this message again, since I'm not confident it was distributed properly the first time, due to the size of the attached files.) I have placed a report on the infrared water vapour monitor for our meeting in March in my public web site at http://physun.physics.mcmaster.ca/Xfer_Wilson/ The report on the infrared water vapour monitor consists of 9 files: the report itself (6 pages, postscript), 7 postscript figures, and one jpg figure. The files are available individually or as a gzip'd tar file containing all the files. The names of the files are IWVM_report.ps (6 pages) Fig1.ps Fig2.jpg Fig3.ps Fig4.ps Fig5.ps Fig6.ps Fig7.ps Fig8.ps Please note that you may have difficulty viewing Fig4.ps with ghostview; hopefully it will print out OK (it did at McMaster), although you may perhaps get two pages, the main figure and a one page error. The remaining figures should print without errors. Chris Wilson From jsr at mrao.cam.ac.uk Fri Mar 3 12:03:46 2000 From: jsr at mrao.cam.ac.uk (John Richer) Date: Fri, 3 Mar 2000 17:03:46 GMT Subject: [asac] Water Vapour Radiometry paper Message-ID: <200003031703.RAA02485@katje.ra.phy.cam.ac.uk> Dear SAC members, A water vapour discussion document for next week's meeting in Leiden, principally written by Richard Hills, is now available, at: http://www.mrao.cam.ac.uk/~alma/wvr/ALMA_Radiometers.ps or http://www.mrao.cam.ac.uk/~alma/wvr/ALMA_Radiometers.doc Take your pick of formats, and let me know if you have any problems printing it. Regards John -- John Richer Astrophysics Group, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE Tel: +44-1223-337246 Fax: +44-1223-354599 http://www.mrao.cam.ac.uk/~jsr/ From wilson at physics.mcmaster.ca Mon Mar 6 09:56:14 2000 From: wilson at physics.mcmaster.ca (Christine Wilson) Date: Mon, 6 Mar 2000 09:56:14 -0500 (EST) Subject: [asac] A4 version of IWVM report In-Reply-To: Message-ID: Dear Colleague, Neal Evans brought to my attention that some of you in Europe may be having difficulties printing out the report on the infrared water vapour monitor because of difference in paper sizes. A version of the report that has been formatted for A4 paper (which Neal processed and has successfully printed) is available now at http://physun.physics.mcmaster.ca/Xfer_Wilson/IWVM_report_A4.ps Chris Wilson From awootten at NRAO.EDU Mon Mar 6 10:37:15 2000 From: awootten at NRAO.EDU (Al Wootten) Date: Mon, 6 Mar 2000 10:37:15 -0500 (EST) Subject: [asac] Leiden ALMA Meeting Message-ID: <200003061537.KAA18650@polaris.cv.nrao.edu> Dear Colleague, Plans for the ASAC meeting are close to final and this mailing includes some information in addition to that sent earlier. I am sending this to the asac mailer and to those who are coming but not included in the mailer. If you get this as part of the ASAC, but are not coming, it is because of my using the mailer. The following will be appended at the end of this message. 1. Agenda (see http://www.cv.nrao.edu/~awootten/mmaimcal/asacleidenagenda.html which inludes hyperlinked documents to read) 2. List of Participants 3. Partial list of relevant reports and documents for ASAC members to read. 4. Directions to Observatory Be prepared for rain (or possibly snow) and temperatures between 0 and 10 C. Those arriving Thursday can either have dinner at the hotel or go into central Leiden, where there are many restaurants. Please tell the hotel when you check in if you are staying there for dinner. Because other groups are meeting at the castle, some of you will be at the Mayflower Hotel Thursday; if so, you will have been notified by Janet Soulsby. The Mayflower is close to many restaurants. Maps of Leiden are usually available at the hotel. For those staying at the castle who want to contact people at the Mayflower, the phone number is 514-2641. If you are here (and awake) during the day Thursday, feel free to come to the Oort/Huygens Buildings where the Observatory is located. Directions are appended. Ewine is in Rm 505 of the Huygens lab and Neal is in Rm 536 of the Oort lab. Ewine's phone number is 527-5814. Neal's phone number is 527-5865. Neal will be in his office most of the day, except that he gives a colloquium on protostellar collapse at 4 PM. Those arriving in the morning may not be able to check in right away, and it probably won't be possible to leave things in rooms past check-out time because other groups are arriving. At the Mayflower hotel you can get into your room about 1 p.m. and at the conference centre possibly a little before lunchtime. Checking out is before 10:30, Mayflower, and 10:00 at the conference centre. (By the way you are kindly asked to please check in at the Mayflower before 6.p.m. on the day of arrival) The meeting room has an overhead projector and flip chart board. If you need any other A-V aids, let us know right away. There is a xerox machine at the meeting center, but it is expensive. If you have paper to distribute, please bring enough copies; if this is not possible and you appear on the agenda Friday afternoon or Saturday, you can give things to us Thursday afternoon or Friday morning to have duplicated at the Observatory. For those staying over Saturday night, you are invited to a buffet supper chez Evans/Geballe. Please reply by email to nje at strw.leidenuniv.nl if you are likely to attend and indicate if you are vegetarian, have food allergies, etc. Directions will be given at the meeting. We look forward to seeing you in beautiful Leiden, where the flowers are starting to bloom! Neal and Ewine Agenda for ALMA Scientific Advisory Committee Leiden March 10-11, 2000 Assume 30 min per event, except as noted Presentations must leave 5 min for questions Sessions in the Wapenzaal, second floor of Kasteel Start 1030 March 10, coffee available 10:30-10:40 Welcome and establish plan for writing report (10 min) (Ewine van Dishoeck, Neal Evans) 10:40-11:10 Interim Report on Definition of Phase 2, Schedule, Estimated Cost (Dick Kurz) 11:10-11:40 Report from ALMA Liaison Group (ALG) on meetings in Grenoble (Dec99) and Tokyo (Feb00). (Stephane Guilloteau or Dick Kurz and Ryohei Kawabe) 11:40-12:30 Discussion* 12:30-1:20 Lunch, coffee 1:20-2:00 Receivers (band boundaries, sensitivity goals, priorities, update D&D work, plans for mass production, ...) (40 min) (Wolfgang Wild) 2:00-2:30 Q-band Receivers (Why ALMA should have them) (John Carlstrom) 2:30-3:00 Report on Polarization (Dick Crutcher) 3:00-3:30 Discussion* 3:30-4:00 Break, coffee 4:00-4:30 Report on System Reviews (Feb00) (Darrel Emerson or Jaap Baars) 4:30-5:00 Discussion 6:30 Bus to The Hague 7:00-9:30: Dinner at Indonesian Restaurant Garoeda, hosted by the Netherlands Organization for Scientific Research (NWO) Return to Oud Poelgeest before 10 pm Start 0830 March 11, coffee available 8:30-9:00 Report on Configurations, Long Baselines (Min Yun or Karl Menten) 9:00-9:30 Report on Antennas (Dick Kurz) 9:30-10:00 Break, coffee 10:00-10:30 Report on Total Power, Nutating Secondaries (Jack Welch) 10:30-11:15 Report on Water Vapor Radiometry (John Richer, Chris Wilson)(45 min) 11:15-12:00 Discussion* 12:00-1:00 Lunch, coffee, walks in the woods 1:00-2:00 Writing assignments and subgroup meetings* 2:00-4:00 Meeting of the whole, reports, election of new Vice-Chairperson* 4:00 Adjourn at 4 PM Agenda items with * following imply that we will split into two groups: one includes the ASAC members, the observers from Chile and Japan, and only those members of the project staff that the committee wants to confer with; the other group will include the remainder of the project staff. Participants Baars, Jaap Benz, Arnold Blake, Geoffrey A. Booth, Roy -- Mayflower Thursday Bronfman, Leonardo Brown, Robert Carlstrom, John Cox, Pierre -- Mayflower Thursday Crutcher, Richard Dishoeck van, Ewine Emerson, Darrel Evans, Neal Fukui, Yasuo Guilloteau, Stephane -- Mayflower Thursday Gurwell, Mark Kawabe, Ryohei Kurz, Richard -- Holiday Inn Rafael Bachiller Menten, Karl M. Nakai, Naomasa Richer, John -- Mayflower Thursday Scoville, Nick Shaver, Peter Walmsley, Malcolm Welch, Jack Wild, Wolfgang -- Mayflower Thursday Wilson, Christine Wootten, Al Yamamoto, Satoshi -- Witte Huis Hotel Yun, Min S. Directions from the Kasteel Oud Poelgeest, Oegstgeest conference centre to the Leiden Observatory, J.H. Oort Building, Niels Bohrweg 2, Leiden. If you prefer to walk rather than take a taxi it will take you between 15-20 minutes. The lab is near the bottom left corner of the map sent to you earlier. >From the conference centre walk down to the main road and turn left (Laan van Oud Poelgeest). Straight on at the roundabout. (The name of the street changes to Warmonderweg. Continue straight on and at the T-junction straight across into a narrow one way street Nachtegaallaan. Continue on and walk along the path at the end of this street, passing on your right first a wooden building with children's play equipment outside and then further down horse riding stables. You are now at the roundabout on the Wassenaarseweg. Cross the road and turn right walking along the Wassenaarseweg. You will see a tall chimney stack coming up on your left. Take the second left - Niels Bohrweg and the entrance to the J.H. Oort Building is across a bridge through the car park on your left hand side. There you will find the Reception desk. The Observatory occupies floors 4 and 5 as well as the 5th floor of the Huygens Laboratory which is joined to the Oort building. FOR DIRECTIONS AND MAPS FROM THE TRAIN STATION TO LEIDEN OBSERVATORY, SEE HTTP://WWW.STRW.LEIDENUNIV.NL SUGGESTIONS FOR RESTAURANTS IN LEIDEN (French/Continental): (ask taxi driver to take you there or walk from Mayflower hotel) - Jill's restaurant: popular with live music on some evenings Morsstraat 6 - Restaurant La Cloche: best French restaurant in town, but also most expensive Kloksteeg 5 - Restaurant Het Prentenkabinet: nice setting in old mansion next to Kloksteeg 25 Pieterskerk and plaque of Pilgrims fathers - Fabers Restaurant Kloksteeg 13 - Restaurant Koetshuis-de Burcht: nice setting at foot of old fort in Burgsteeg 13 center of town There are many other types of cuisine (Italian/Pizza, Greek, Chinese, Spanish/Argentina, etc.) in restaurants scattered throughout Leiden. From awootten at NRAO.EDU Tue Mar 7 11:01:07 2000 From: awootten at NRAO.EDU (Al Wootten) Date: Tue, 7 Mar 2000 11:01:07 -0500 (EST) Subject: [asac] Nutator Questions for Leiden Meeting Message-ID: <200003071601.LAA04554@polaris.cv.nrao.edu> Jeff Kingsley forwards some possible choices on subreflector options for discussion in Leiden: All, Listed below are the questions I hope you can get answered at the ASAC meeting later this week. It is important that we get clear science guidance on these issues so that they can be properly budgeted and work can start on getting the nutator developed. I have added a few possible option for the nutator with rough estimated costs that I hope will be useful in making the cost performance tradeoffs of the science requirements. 1.) Do the nutator requirements listed below agree with the science requirements? Switching Rate: 1-10 Hz variable Transition time: 10 msec Throw: +/- 3 acrmin on sky Reaction less: Yes Pointing requirement: Meet all Antenna RFP pointing specifications Multiaxis: ? Budgeted Cost: ? 2.) Are nutators required for the prototype antennas? 3.) How many nutator will be required for the full array? 4.) Several nutator options are listed below with the estimated production cost. What option best meets the science requirements considering the cost trade off? does the group have any alternate option, idea or proposal for the nutator? A.) For a single axis reactionless nutator similar to the SMA but scaled for ALMA is expected to to cost about a $100k for production version. B.) A simple multiaxis nutataor might use a reactionless nutator similar to the SMA but scaled for ALMA along with a rotator about the optical axis. The rotator would have a range of about +/-90 with the slow motion in rotation . This type of multiaxis system would be the most economical and simplest to implement. (The 12-meter at Kitt Peak originally had a similar type system.) The estimated cost of production models of this system is about $160k. C.) Same as above but with full rotation about optical axis at a fast rate of about 10 rpm. This system would require slip rings and cost about $210k. D.) A gimbaled reactionless system similar to the JCMT could be implement but the system would be very massive and might require modification to the antenna feed legs and structure resulting in additional antenna cost. The estimated production cost is about $260k not including possible antenna modifications E.) A completely new reactionless hexapod nutator system could be developed but this would be very expensive to develop. The system would be light weight and dynamically be very agile. The production cost is expected to be similar to items A or B but developmental cost would be about $400k and take about 20 months. The nutator options list above are rough estimates but the best that we have right now. Other option exist but have not yet been explored. I wanted to confirm with Simon on the above estimates but have been unable to get a hold of him. Once I talk with him I might revise the costing. Good luck at the meeting and I look forward to hearing the results. Best wishes, Jeff From awootten at NRAO.EDU Tue Mar 7 11:37:05 2000 From: awootten at NRAO.EDU (Al Wootten) Date: Tue, 7 Mar 2000 11:37:05 -0500 (EST) Subject: [asac] Updated agenda Message-ID: <200003071637.LAA05309@polaris.cv.nrao.edu> Folks It appears that our mailing list discriminates against messages over 40 KB in size, so the following messages from Stephane bounced back to me. I will distribute them piecemeal in subsequent messages. They are also linked to the non-public agenda WWW site at http://www.cv.nrao.edu/~awootten/mmaimcal/asacleidenagenda.html Stephane wrote: Dear ASAC members, The ALMA-Japan Liaison group has been set up to discuss possible = contributions of Japan to an "enhanced" ALMA project. Two meeting have = occured so far, and a 3rd one is scheduled on March 9, just prior to the ASAC meeting. The outcome of the ALG activities is one of the items for the ASAC = meeting. Final documents will not be available by that time. In order to = provide a basis for discussions, I attach the documents in draft form = with this E-Mail. We will try to make printed copies of the (revised) = documents available in Leiden, if possible. Best regards, Stephane Dr. Stephane GUILLOTEAU ALMA European Project Scientist Phone: (33) 476 82 49 43 (IRAM) IRAM FAX: (33) 476 51 59 38 (IRAM) 300 Rue de la Piscine Phone: (49) 893 200 6589 (ESO) F-38406 Saint Martin d'Heres France E-Mail: guillote at iram.fr (IRAM) sguillot at eso.org (ESO) From awootten at NRAO.EDU Tue Mar 7 11:39:04 2000 From: awootten at NRAO.EDU (Al Wootten) Date: Tue, 7 Mar 2000 11:39:04 -0500 (EST) Subject: [asac] Propositions for Japan participation to an enhanced ALMA project. Message-ID: <200003071639.LAA05343@polaris.cv.nrao.edu> Propositions for Japan participation to an enhanced ALMA project. Draft by S.Guilloteau From the December 4, 1999 meeting of the ALMA Liaison Group in Grenoble. Revised after the February 16, 2000 meeting of the ALG in Mitaka. This document present a number of possible contribution of Japan to an enhanced ALMA project. The enhancements resulting from the various proposition are quite varied : adding new capabilities (e.g. supra-THz capability), shortening the project duration, increasing the sensitivity, simplifying the maintenance or reducing global cost in some areas. Each proposed contribution is presented in 4 steps : a description, the expected improvement on ALMA, the method proposed to value the contribution, and a short term consequence on Phase I activities. In addition to the specific items mentionned below, which result in visible improvements, it is of course also expected that Japan shares the basic infrastructure and running costs of ALMA. 1) An addition of a number of 12-m antennas. Description : Involvement of Japanese industry is essential for Japan participation in the project. Japanese industry could build up to 1/3rd of the total number of 12-m antennas. These antennas would be built to the same or better specifications than the US-European antennas, with a possibly different design, but plug-in compatible in the same stations. ALMA benefit : Antennas of similar performances would increase the ALMA sensitivity (or speed). Antennas of better performances would significantly increase the highest frequency capabilities of ALMA. ALMA value : Unless antennas performance are significantly increased, it is proposed to evaluate this contribution on an equal value for each antenna, unrelated to its origin. Phase I implication : Coordinate antenna specifications and interfaces, including foundation specifications. 2) An addition of a number of smaller, high accuracy, antennas in a compact array. Description : Japan could built a compact array of small, but high surface accuracy, antennas. This array could take the form of e.g. a compact hexagon with 7 antennas of 6 to 8 m in diameter. The outer antennas could be moveable on rails to allow fast reconfiguration in order to tailor the shape of the array to the source declination, to avoid shadowing effects. The antenna mount and receiver cabin could be identical to those of the 12-m antennas, allowing them to move on larger configurations also, and to have the highest compatibility for receiver interfaces. Expected antenna surface accuracy is of the order of 15 microns rms or better. ALMA benefit : Such a compact array would enhanced ALMA capabilities for short spacing measurements, specially at the highest frequencies where the 12-m antenna performances and the atmospheric properties make the problem most difficult. It would also allow to operate the 12-m antennas with an under-illumination pattern at the highest frequencies to select a better compromise field-of-view and pointing performance versus sensitivity. The reconfigurable option would open the possibility for ALMA to explore the highest frequencies, perhaps even above 1 THz, with appropriate field of view and angular resolution. ALMA value : Assuming the same complement of receivers as the 12-m antennas, we would give each small antenna the same value as a 12-m antenna. The increased complexity of the proposed ? quick ? repositioning system compensates the savings expected from the smaller dish diameter. Phase I implication : Watch out mount and foundation design to allow close packing of 6 to 8 m antennas. 3) A participation to the junction effort Description : A new facility for SIS junction production is being developed in Mitaka. This facility could be used for ALMA junction production. Japan is also developing innovative technologies for junctions (e.g. the distributed junction scheme from Dr. Noguchi) ALMA benefit : This contribution could alleviate a possible bottleneck in the ALMA project ALMA value : This contribution would be evaluated on the basis of US-EU agreement for junction production value. Phase I impact : Developments should be integrated in ALMA Phase I, to be incorporated in receiver design and production plans. 4) Fabrication of one/several receiver frequency channels Description : Japan could provide e.g. the 490 GHz receivers for ALMA (or other bands). ALMA benefit : This could speed up the completion of the ALMA project, and avoid excessive retrofit actions to the receiver packages. ALMA value : This contribution would be evaluated on the basis of US-EU agreement for receiver band value. Phase I impact : Japan receiver experts should become involved in the Joint Receiver Design activity if they are going to be suppliers of some frequency modules. 5) Cryogenics Description : Japan is probably the biggest supplier of cryocoolers, with proven reliability. ALMA benefit Basic contribution ALMA value : Market prices Phase I impact : Implementation of a real scale test at 5000 m is very valuable. 6) Photonics Description : NTT is actively developping high frequency photomixers, which may be suitable for the full photonic LO system for ALMA ALMA benefit : Photonic LO system is simpler than the photonic reference approach. ALMA value : Value based on estimated cost of the photonic reference + multiplier solution and purely photonic approach, whichever is the most expensive. The gain in simplicity justifies extra cost (if any). Phase I impact : Evaluation of photodetectors needs to be performed actively. Good liaison between the Tucson group and NAOJ is important here. 7) Correlator Description : Japan is developping a wideband, 128 000 channels FX correlator. The goal is ultimately to cover the full 2 GHz bandwidth (perhaps even 4 GHz) with this number of channels. This removes the extra complexity of input filtering (analog or digital). ALMA benefit : Line surveys could be carried out more effectively. Serendipitous discoveries are to be expected (e.g. molecular masers in stars). Multiple-line observation and continuum subtraction from narrow lines could be executed more precisely and effectively. Heavy molecules could be detected by pattern-matching integration of the line forests. ALMA value : The value should be based on the observing time savings that such a correlator will provide for the astronomy projects requiring narrow lines observations. The induced computing cost resulting from the huge number of channels should be evaluated and accounted accordingly. The effective sensitivity should also be asserted. Phase I impact : Invite Japanese expert to correlator PDR in January. Develop scientific evaluation based on typical observing scenarios. 8) More digital bandwidth Description : Japan could provide enhanced transmission bandwidth from antenna to central building, and from central building to Operation Support Facility in San Pedro. ALMA benefit : Current nominal bandwidth of standard fibers does not allow 3-bit samples to be transferred on single fiber from the antenna. This would become possible. Ultra high speed link with OSF would allow to relocate the correlator and all its associated computing equipment in San Pedro. ALMA value : Based on cost. Phase I impact : Maintain contacts for information. 9) Other proposals mentionned A few other options were discussed, among which a large single-dish antenna, a super-computer for enhanced data mining capabilities, etc These were felt impracticable, or to far from the scope of an ? Enhanced ALMA ? project. From awootten at NRAO.EDU Tue Mar 7 11:42:38 2000 From: awootten at NRAO.EDU (Al Wootten) Date: Tue, 7 Mar 2000 11:42:38 -0500 (EST) Subject: [asac] Implementation paths for options of Japanese contributions to "Enhanced ALMA" Message-ID: <200003071642.LAA06064@slacktide.cv.nrao.edu> Apologies for the gobbledegook my mailer encoded on the last message. I will try to defeat that here. Al (ALG meeting, March 9th, Leiden) Implementation paths for options of Japanese contributions to "Enhanced ALMA" 1.Introduction Japanese LMSA project has been proposed with special emphasis on the scientific performance of the array at submillimeter wavelengths with a collecting area and spatial resolution comparable to fifty 10m antennas and 10km maximum baseline, respectively. Large FX correlator system with very high spectral capability has also been considered as a key device in the project. The ALMA Liaison Group (ALG) had three face-to-face meetings in Grenoble, Tokyo, and Leiden in order to produce and evaluate options for Japanese contributions which would lead to the enhanced Aracama large milllimeter/submillimeter array (hereafter E-ALMA) referred to the Resolution signed in 12 November 1999. The implementation paths and their priorities are proposed for Japanese contributions to the E-ALMA project considering the best compatibility between the baseline ALMA project and the LMSA project. The Japanese contributions are classified into two parts; 1)participation to the baseline ALMA project, and 2)enhancements to the baseline ALMA project. The ALMA Liaison Group(ALG) has investigated and evaluated their scientific benifit and technical importance in order to prepare a report to the ACC and NAOJ. In the following, priorities are shown as A: high, B: medium, C: low). 2.Participation to the baseline ALMA project 1.1 One third of 12m antenna elements (priority: A) Japanese group will accelerate the design and cost estimate of 12 m antenna to be compared with the US/Europe prototype antennas. Japanese antennas will be designed with a special emphasis on the performances in submillimeter wavelengths. The antenna design should not necessarily be the same, but plug- in compatiblity with US/Europe antennas is highly recommended. European, Japanese and US groups will exchange the necessary information to coordinate the design effort of the production antennas to make their antennas as compatible as possible. Japanese group will examine the Interface Control Document and will propose necessary modifications and/or additions as soon as possible. Japanese group will design the antenna transporter to contribute one of three transporters. The ALG will determine the optimum number of antennas to be shared between Japan and US/European groups. 7.1 10km configuration (priority: A) Japanese group will estimate the cost for the 10km configuration including antenna pads, electric power transmission, signal transmission, access road. 3.1 SIS junction effort (priority: A) Japanese contribution to the junction fabrication is absolutely necessary to guarantee a stable supply of junctions in order to achieve a reliable operation of the telescope system. A plan of sharing the frequency range should be determined. 3.1 Fabrication of selected receiver frequency bands (priority: A) Japanese receiver group has already been participating in the ALMA phase 1 activities by providing a design for the band #8(385-500GHz). The effort could be extended to the band #10(787-950GHz). Japanese group will accelerate the design and cost estimate of the modules for those frequency bands. The ALG has tacit understanding that each group will provide the receiver modules for all antennas. 3.1 Cryogenics effort (priority: B) Japanese group will develop a reliable cryogenic system under the collaboration with Japanese industries. The reliability of the cyogenics will be evaluated with the actual experiences in the ASTE project. 3.1 Photonic reference system (priority: A) Japaese group is developing a high frequency photodiodes in collaboration with NTT. The Japaense baseline plan is producing a photonic reference siganal up to 300GHz. All photonic option is also within the scope of the Japanese development and should be recommended. If a photonic calibration system is proved to be very effective, Japanese group will also participate in the joint develoment. 4.1 High speed sampler (priority: A) Japaenese group is developing a high speed sampler in collaboration with OKI. These is a possibility of Japan Europe collaboration in this area. 3.Enhancements to the baseline ALMA project 3.1 Increase the total number of 12m antennas (priority: A) The total number of antennas in the E-ALMA could be between 64 and 96, possibly around 75. The number should be optimized based on the scientific rationale as well as the impact to the total cost. The same argument as in 1.1 could be also applied here. 4.1 Ultra Compact Array (UCA) with smaller antennas (priority: B) European, Japanese and US groups recognize the importance of the small array of smaller antennas to fill the gap in short spacings and will study of the design and cost estimate of UCA jointly. Considering the higher cost for a small number of antennas, the ALG would give each smaller antennas the same value as a 12m antenna. Due to the limited man power, Japanese group set a lower priority in supplying the UCA than adding 12m antennas. 5.1 More digital bandwidth (priority: C) Technology for tera-bit data link is already existing in Japan and there is a possibility of Japanese contribution in this area. Japanese group will estimate the cost of enhanced link system if US/Europe group consider this valuable contribution. 3.1 Next generation correlator (priority: A) Europe and Japanese groups will try to find a scheme of joint effort in the development of the next generation correlator system. In order to define the joint development program, the basic correlator architecture (i.e. FX vs XF) should be agreed. The reliability of a large scale digital system is also very important for the stable operation at the very high site. The expertise of Japanese correlator designer could be integrated in the design and development efforts. 5.1 Large computer system for data archiving and mining (priority: C) Although the shortage in Japanese man power will limit the software development, there will be a possibility of Japanese contribution in introducing a large computer system. (Masato Ishiguro) From awootten at NRAO.EDU Tue Mar 7 11:43:23 2000 From: awootten at NRAO.EDU (Al Wootten) Date: Tue, 7 Mar 2000 11:43:23 -0500 (EST) Subject: [asac] Propositions for Japan participation to an enhanced ALMA project. Message-ID: <200003071643.LAA06071@slacktide.cv.nrao.edu> Propositions for Japan participation to an enhanced ALMA project. Draft by S.Guilloteau From the December 4, 1999 meeting of the ALMA Liaison Group in Grenoble. Revised after the February 16, 2000 meeting of the ALG in Mitaka. This document present a number of possible contribution of Japan to an enhanced ALMA project. The enhancements resulting from the various proposition are quite varied : adding new capabilities (e.g. supra-THz capability), shortening the project duration, increasing the sensitivity, simplifying the maintenance or reducing global cost in some areas. Each proposed contribution is presented in 4 steps : a description, the expected improvement on ALMA, the method proposed to value the contribution, and a short term consequence on Phase I activities. In addition to the specific items mentionned below, which result in visible improvements, it is of course also expected that Japan shares the basic infrastructure and running costs of ALMA. 1) An addition of a number of 12-m antennas. Description : Involvement of Japanese industry is essential for Japan participation in the project. Japanese industry could build up to 1/3rd of the total number of 12-m antennas. These antennas would be built to the same or better specifications than the US-European antennas, with a possibly different design, but plug-in compatible in the same stations. ALMA benefit : Antennas of similar performances would increase the ALMA sensitivity (or speed). Antennas of better performances would significantly increase the highest frequency capabilities of ALMA. ALMA value : Unless antennas performance are significantly increased, it is proposed to evaluate this contribution on an equal value for each antenna, unrelated to its origin. Phase I implication : Coordinate antenna specifications and interfaces, including foundation specifications. 2) An addition of a number of smaller, high accuracy, antennas in a compact array. Description : Japan could built a compact array of small, but high surface accuracy, antennas. This array could take the form of e.g. a compact hexagon with 7 antennas of 6 to 8 m in diameter. The outer antennas could be moveable on rails to allow fast reconfiguration in order to tailor the shape of the array to the source declination, to avoid shadowing effects. The antenna mount and receiver cabin could be identical to those of the 12-m antennas, allowing them to move on larger configurations also, and to have the highest compatibility for receiver interfaces. Expected antenna surface accuracy is of the order of 15 microns rms or better. ALMA benefit : Such a compact array would enhanced ALMA capabilities for short spacing measurements, specially at the highest frequencies where the 12-m antenna performances and the atmospheric properties make the problem most difficult. It would also allow to operate the 12-m antennas with an under-illumination pattern at the highest frequencies to select a better compromise field-of-view and pointing performance versus sensitivity. The reconfigurable option would open the possibility for ALMA to explore the highest frequencies, perhaps even above 1 THz, with appropriate field of view and angular resolution. ALMA value : Assuming the same complement of receivers as the 12-m antennas, we would give each small antenna the same value as a 12-m antenna. The increased complexity of the proposed ? quick ? repositioning system compensates the savings expected from the smaller dish diameter. Phase I implication : Watch out mount and foundation design to allow close packing of 6 to 8 m antennas. 3) A participation to the junction effort Description : A new facility for SIS junction production is being developed in Mitaka. This facility could be used for ALMA junction production. Japan is also developing innovative technologies for junctions (e.g. the distributed junction scheme from Dr. Noguchi) ALMA benefit : This contribution could alleviate a possible bottleneck in the ALMA project ALMA value : This contribution would be evaluated on the basis of US-EU agreement for junction production value. Phase I impact : Developments should be integrated in ALMA Phase I, to be incorporated in receiver design and production plans. 4) Fabrication of one/several receiver frequency channels Description : Japan could provide e.g. the 490 GHz receivers for ALMA (or other bands). ALMA benefit : This could speed up the completion of the ALMA project, and avoid excessive retrofit actions to the receiver packages. ALMA value : This contribution would be evaluated on the basis of US-EU agreement for receiver band value. Phase I impact : Japan receiver experts should become involved in the Joint Receiver Design activity if they are going to be suppliers of some frequency modules. 5) Cryogenics Description : Japan is probably the biggest supplier of cryocoolers, with proven reliability. ALMA benefit Basic contribution ALMA value : Market prices Phase I impact : Implementation of a real scale test at 5000 m is very valuable. 6) Photonics Description : NTT is actively developping high frequency photomixers, which may be suitable for the full photonic LO system for ALMA ALMA benefit : Photonic LO system is simpler than the photonic reference approach. ALMA value : Value based on estimated cost of the photonic reference + multiplier solution and purely photonic approach, whichever is the most expensive. The gain in simplicity justifies extra cost (if any). Phase I impact : Evaluation of photodetectors needs to be performed actively. Good liaison between the Tucson group and NAOJ is important here. 7) Correlator Description : Japan is developping a wideband, 128 000 channels FX correlator. The goal is ultimately to cover the full 2 GHz bandwidth (perhaps even 4 GHz) with this number of channels. This removes the extra complexity of input filtering (analog or digital). ALMA benefit : Line surveys could be carried out more effectively. Serendipitous discoveries are to be expected (e.g. molecular masers in stars). Multiple-line observation and continuum subtraction from narrow lines could be executed more precisely and effectively. Heavy molecules could be detected by pattern-matching integration of the line forests. ALMA value : The value should be based on the observing time savings that such a correlator will provide for the astronomy projects requiring narrow lines observations. The induced computing cost resulting from the huge number of channels should be evaluated and accounted accordingly. The effective sensitivity should also be asserted. Phase I impact : Invite Japanese expert to correlator PDR in January. Develop scientific evaluation based on typical observing scenarios. 8) More digital bandwidth Description : Japan could provide enhanced transmission bandwidth from antenna to central building, and from central building to Operation Support Facility in San Pedro. ALMA benefit : Current nominal bandwidth of standard fibers does not allow 3-bit samples to be transferred on single fiber from the antenna. This would become possible. Ultra high speed link with OSF would allow to relocate the correlator and all its associated computing equipment in San Pedro. ALMA value : Based on cost. Phase I impact : Maintain contacts for information. 9) Other proposals mentionned A few other options were discussed, among which a large single-dish antenna, a super-computer for enhanced data mining capabilities, etc These were felt impracticable, or to far from the scope of an ? Enhanced ALMA ? project. From awootten at NRAO.EDU Tue Mar 7 11:43:57 2000 From: awootten at NRAO.EDU (Al Wootten) Date: Tue, 7 Mar 2000 11:43:57 -0500 (EST) Subject: [asac] DRAFT Minutes of the Alma-Japan Liaison Group, Tokyo, 16-Feb-2000 Message-ID: <200003071643.LAA06078@slacktide.cv.nrao.edu> Minutes of the Alma-Japan Liaison Group, Tokyo, 16-Feb-2000 DRAFT NOT YET REVISED BY ALG MEMBERS Present : Ishiguro, Kawabe, Kurz, Napier, Guilloteau Excused : Brown Observers : Wootten, Chikada, Nakai, Fukui The meeting started with a review of collaborations in Phase I. General : The list of forthcoming meeting was checked for attendance of Japanese representative. Because of the current meeting could not be attended by R.Brown, it was dediced to have another ALG meeting in Leiden on March 9, just before the ASAC. Reporting of the ALG to the ASAC will occur on March 10. Antennas : Proprietary rights limit the diffusion of information. Specifications have been distributed. ICD (Interface Control Documents) have been distributed. The creation of a Japanese ICD for use with Mitsubishi was suggested as action item. This may become an urgent topic if negociations are required. A formal response from Mitsubishi will be asked for. For the additional small (8-m) antennas, a pre-design study and costing experiment based on common mount re-use should be started. Ultimately, it should be confronted with a dedicated design and cost. Studies of metrology system is going in parallel in Europe (and US also to some extent). Some information may be restricted due to contractual limitations. A working group on testing procedures for the antennas should be established. Information exchange about the 10-m antenna at NRO is desirable. Sending a young scientist from IRAM to participate to the 10-m testing was suggested, but turned out to be impractical because of the short notice. Receiver : Channel 8 configuration was proposed by Sekimoto san, and is being included in the overall receiver design. It includes SSB, dual-polarisation with a modified MPI as diplexer for the local oscillator injection with low loss. Common receiver deign is trying to use mirrors (rather than lenses) as much as possible to minimize losses. Photonics : There is good progress on high frequency photodetectors. However, the timescale of 2002 is short to convince ourselves that the full photonic solution is viable. Japan suggest to use the photonic solution up to 300 GHz, with multipliers for higher frequencies. (? Option 2.7 ? as compared to the current ? Option 2.3 ? which includes photonic only up to band 3). Parallel development of photomixer and coupling system in Europe is thought valuable for at least another year. It is suggested to refocus multiplier developments on high frequency multipliers with cooling. Re-emphasis on the development of fixed-tuned tripler is needed. The development of a photonic reference system, which would improved phase calibration and perhaps remove the need for the round-trip measurement in the LO, is continuing. Correlator : Study and prototyping of the A/D converter (? sampler ?) is going on at Nobeyama. The goal is 8 GHz clock rate. Note that the test correlator planned to evaluate sampler for ALMA will only work up to 4 GHz (perhaps slightly more). It is useful to continue parallel development as planned in Europe, but information exchanges would help. There is a possibility of joining Japan and European efforts if the timescales are of order 2002- 2003. In the second part of the meeting, the draft document on the Enhanced ALMA presented by S.Guilloteau was discussed. A number of small corrections were mentionned. It was understood that construction by Japan industry of a number of 12-m antennas was essential for participation of Japan to ALMA. Such a construction is not possible on the basis of blue-prints, but requires proprietary design, This will lead to 2 different types of 12-m antennas. This is not considered as a serious problem provided specifications and interfaces are identical. The different set of tools and maintenance procedures is considered as unimportant given the large series which are considered (transporters were not discussed). Since the total number of antennas is not yet known, it will be important to see what are the ASAC recommendations on the ? enhanced ALMA ? project. A cost estimate for Japan participation to enhanced ALMA should be available by end of March (with errorbars [-20 %, +50 %]). Preliminary costing on both sides were presented, but cannot be quoted in writing yet. Finally, the list of actions and deadlines was reviewed. Action items : - Minutes of meetings (S.Guilloteau) - Revise to documents (S.Guilloteau) - Implementation plan in Japan (M.Ishiguro) - Diffusion to ASAC members of these documents After March 9 (next meeting) - Finalize recommendation to ACC and NAOJ - Write global implementation plan (R.Kurz M.Ishiguro) The final report should be made available to all ACC members no later than March 21. From nje at strw.leidenuniv.nl Mon Mar 13 12:41:21 2000 From: nje at strw.leidenuniv.nl (Neal J. Evans II) Date: Mon, 13 Mar 2000 18:41:21 +0100 Subject: [asac] ASAC-Outline Message-ID: <200003131741.SAA29541@strw.LeidenUniv.nl> Dear Colleagues, Thanks to all of you for a productive meeting in Leiden. I think we worked well, settled many issues, and set the path for dealing with others. Now I need those contributions by Friday! Here is the outline, with names attached, and some reminders about schedule and format. Best wishes, Neal Draft Outline of Report (names in CAPITALS reponsible for writing up and sending to Neal) Use LaTex2e AAS macros 1. Introduction (NEAL) Charter, Procedures, our reps to working groups 2. ALG issues (PIERRE, Roy, Karl, Mark) Scientific evaluation of contributions from Japan 3. Receivers (GEOFF, Jack, Dick, Ewine, Nick, Rafael, Chris) 4. System (RAFAEL, Arnold) 5. Configurations (KARL, Min, Roy) 6. Antennas and Total Power (MALCOLM, Jack, John) 7. WVR (JOHN, Chris, Mark) 8. Future Projects, Issues (NEAL, Karl) UC Array Configurations LO systems Software Spectrum Management Site (LEO) Outreach 10. Summary Appendices 1. Polarization - Crutcher 2. Total Power - Welch 3. WVR - Hills, Richer 4. IRWVR - Naylor (Wilson) 5. Q band receivers - Carlstrom Schedule: Mar 17 Contributions due to Neal Mar 21 Draft to committee for comments Mar 26 Comments due Mar 29 Final version to Project Scientists, who will present to AEC, ACC Each section should be about a page, explaining the issues, and giving our recommendations. Send as separate ascii files -- no attachments, MIME encoding, ... Don't leave rogue &, %, $, ... without proper latex syntax... Please! From nje at strw.leidenuniv.nl Tue Mar 21 11:20:32 2000 From: nje at strw.leidenuniv.nl (Neal J. Evans II) Date: Tue, 21 Mar 2000 17:20:32 +0100 Subject: [asac] README-ASAC report Message-ID: <200003211620.RAA09259@strw.LeidenUniv.nl> Dear Colleagues, First, many thanks to everyone who delivered their sections on time. I was delighted to see that they were in good form, requiring a minimum of editing. I have mostly assembled them, smoothed out a few things, and extracted some summary recommendations. Taking editorial privilege, I converted English into American; we can alternate for future reports... Some work still needs to be done to make things consistent and avoid repetition, and I will appreciate your comments on that. The most important things for you to check are these. 1. The summary recommendations. I tried to make these brief and hit the main points, with reference back to the sections for the full recs. The choice of these is important, so please tell me if crucial points are missing from the summary. 2. The representatives to the Working Groups (Appendix B). In consultation with Karl, I assigned our liaison members. These mostly follow the pattern of who worked on which section of the report, with some adjustments. If you are unhappy with your "assignment", feel free to complain. Our new chairman can adjudicate :-) Texnicalities: 1. The file uses the latest AAS style files, including LaTex2e. If your institution has not upgraded to this, you can use the older Latex by uncommenting the first line and commenting the second line. 2. You will need to run latex at least twice to get all the references to sections correct. 3. Appendices B-G are not included since you already have them. They will be included in the final report. If anyone has the latex source file for Carlstrom's report, I would appreciate getting it. I only have the postscript and John is out of town. 4. You can send detailed comments by extracting pieces of the text and making changes or by sending the whole file back with embedded comments and changes marked in some way that you tell me (e.g., including your initials in parentheses just before the change). 5. Comments received by next Sunday, March 26, will be incorporated in the final version, which I will deliver to the project scientists as a latex file on March 29. Thanks again, Neal From nje at strw.leidenuniv.nl Tue Mar 21 11:20:58 2000 From: nje at strw.leidenuniv.nl (Neal J. Evans II) Date: Tue, 21 Mar 2000 17:20:58 +0100 Subject: [asac] ASAC report.tex Message-ID: <200003211620.RAA09264@strw.LeidenUniv.nl> %\documentstyle[11pt,aaspp4]{article} \documentclass[preprint,10pt]{aastex} \begin{document} \title {\bf Report of the ALMA Scientific Advisory Committee: March 2000 Meeting} \author {Arnold Benz, Geoff Blake, Roy Booth, Pierre Cox, Dick Crutcher, Neal Evans, Mark Gurwell, Rafael Bachiller, Karl Menten, John Richer, Nick Scoville, Ewine van Dishoeck, Malcolm Walmsley, Jack Welch, Christine Wilson, Min Yun} \affil{ALMA Scientific Advisory Committee} \author{Leo Bronfman, Yasuo Fukui, Tetsuo Hasegawa, Masahiko Hayashi, Ryohei Kawabe, and Naomasa Nakai} \affil{Active Observers } \section{Introduction} \label{intro} The ALMA Scientific Advisory Committee (hereafter ASAC) was formed in late 1999, as requested by the ALMA Coordinating Committee (hereafter ACC). The role of the ASAC is to provide scientific advice to the ACC, the ALMA Executive Committee (hereafter AEC) and the project, via the project scientists. As requested, the ASAC developed its own charter, which we supply as Appendix \ref{charter}. The ASAC decided to hold monthly telecons and regular meetings. For the near future, we will meet before each meeting of the ACC, in order to deliver a report in time for the ACC meeting. The telecons will supply rapid responses to queries from the project management and project scientists, while the meetings will allow exploration in more depth of particular issues and will result in a written report. To ensure good communications, the ASAC will designate members to act as liaison to each of the working groups in the project; these are listed in Appendix \ref{liaison}. The ASAC members also committed themselves to helping to educate the larger community about ALMA. This document reports on the first meeting of the ASAC, held in Leiden, The Netherlands, on March 10-11, 2000. The topics covered at the meeting emerged from our telecons or from queries from the working groups. Some issues require further study and some topics were deferred to future meetings. These are listed in section \ref{future}. We summarize the overall recommendations in section \ref{summary}. \section{ALMA Liaison Group Issues} \label{alg} The possibility of a contribution of Japan in the ALMA project has received strong and positive support from the ASAC. Such a contribution will make ALMA the largest international collaboration achieved in astronomy and enhance the project in a number of important ways. It will increase the sensitivity of the array and add new technical capabilities. If this collaboration is achieved, Japan will have an equal partnership in the ALMA project with the US and Europe and share the infrastructure and running costs. The basic contribution of Japan to the ALMA project will be to add 12-meter antennas to the agreed 64 x 12-meter antennas. This greater collecting area will result in a better sensitivity (or observing speed), close to the original goal of a $\rm 10,000 \, m^2$ array. This improved sensitivity will compensate the need to share the observing time with a greater number of users. Further contributions of Japan to an enhanced ALMA project are related to specific technical developments, including the participation in the future correlator, construction of the highest frequency receivers, or the photonic LO system. It is too early for the ASAC to prioritize the importance of these; further discussion is needed. It is clear that the contribution of Japan in the ALMA project could also open new perspectives for the project. In particular, the possibility to add to the project a compact array of smaller, high accuracy dishes would be a most interesting addition. It would greatly improve the image quality for extended sources and the performance at the highest frequencies. This possibility should therefore be rediscussed when the Japanese participation is confirmed. \section{Receivers} \label{receivers} Along with the telescopes, the receiver packages largely determine the capabilities of ALMA. The Joint Receiver Development Group (JRDG) has raised a number of questions and requested clarification from the ASAC. These may be broken down into questions concerning the frequency bands and their priority, the total power stability, the WVR specs (dealt with in a separate section), polarization requirements, calibration accuracy, and receiver configurations (principally single sideband versus double sideband operation). Recommendations for each of these areas are outlined below. {\bf Frequency Bands}. The ASAC concurs that the four bands to be initially installed on the array should be (in order of increasing frequency) Band 3 (86-116 GHz), Band 6 (211-275 GHz), Band 7 (275-370 GHz), and Band 9 (602-720 GHz). While we still believe that frequency coverage should be as complete as possible, we responded to the request for prioritization of the bands as follows. \begin{itemize} \item First Priority: Bands 3, 6, 7, and 9 \item Second Priority: Bands 1, 4, and 2 (see below) \item Third Priority: Bands 5, 8, and 10 \end{itemize} We strongly urge that the JRDG study the possibility of extending the lower frequency range of Band 3 to include the SiO maser transition near 86 GHz. If this is possible, Band 2 would drop to third priority. The frequency intervals of the other bands are reasonable. Band 10 is scientifically quite interesting. It is in the third priority because the technology of THz SIS heterodyne receivers is in an early state, and it will be difficult to make ALMA work at its highest operating frequency. Some delay in the installation of this band will enable the most sensitive receivers to be installed and for the telescope performance to be optimized. Note that Band 1 is in the second priority list, and it must be considered in receiver layout. If it will not be in the main Dewar, then designs for optics that allow a second Dewar, possibly also containing the WVR, should be developed. It is not necessary for the WVR and Band 1 receivers to operate simultaneously. {\bf Total Power Stability}. For On-The-Fly (OTF) mapping capabilities, the requisite total power stability is of order 10$^{-4}$ in one second. The ASAC recommends that this level be a goal, rather than a hard specification, pending further study. The over-riding concern is the receiver sensitivity, and better performance should not be sacrificed for stability at this stringent level. However, this level of stability may allow considerable simplication (avoiding nutating subreflectors), and we encourage the JRDG to study the issue and report back to the ASAC on the prospects for achieving this level of stability and on possible tradeoffs in doing so. {\bf WVR Specs}. These are discussed at length elsewhere (Section \ref{wvr}). The main point here is that this system must be incorporated into the overall design and receiver specs. {\bf Polarization}. Polarization work will be an important part of ALMA research. Strong efforts should be made to have the polarized single-dish beams as stable as possible; consequently, the ASAC recommends that careful consideration be given to placing the 345 GHz receiver on-axis. For linear polarization work the basis state of feeds would ideally be circular polarization. If circular feeds impose important limitations on tuning range or increase significantly the noise temperature, a system for rapid, accurate calibration of linear feeds should be implemented. Obtaining zero and short spacing polarization data is essential. A nutating subreflector has a limited angular throw and introduces varying angles with respect to the optical axis of the primary mirror. The OTF technique proposed for total power observations would be ideal for polarization if the requisite gain stability can be achieved. Finally, the different polarization properties of the two prototype antennas and other polarization properties of the test interferometer and single-dish techniques should be carefully measured as they may be a consideration in procurement decisions (see Section \ref{antennas}). {\bf Calibration Accuracy}. The ALMA calibration spec of 1\%~is adequate scientifically, perhaps even a bit agressive. A cold calibration load in the primary Dewar is probably unnecessary. {\bf Receiver Modes}. The superb quality of the Chajnantor site and the non-ideal nature of any optical system means that the theoretical improvement in single sideband (SSB) versus double sideband (DSB) receivers may be difficult to realize in practice. DSB receivers are far easier and cheaper to fabricate, especially at submillimeter frequencies, and the ASAC recommends that a careful design study be undertaken that assesses the likely performance loss for DSB operation. If the loss is sufficiently small, considerable cost savings and ease of operation can be realized. The ASAC would like to revisit this question once the SSB versus DSB study is completed. It is very likely that ALMA will become operational with both SSB and DSB receivers. This change in operational characteristics has important implications for the ALMA correlator, and the ASAC also recommends that the initial and subsequent ALMA correlators be designed with both modes of operation in mind. The operating system and software environment may also be affected. {\bf Summary}. The ASAC confirms that Bands 3, 6, 7, and 9 have the top priority and should be installed first. While complete frequency coverage is important, we have divided the other bands into second and third priorities. We recommend study of extending the lower end of Band 3 to include 86 GHz. In addition, the JRDG should consider placing the Band 7 receiver on-axis. Designs that accomodate the Band 1 receiver are essential. The ``relaxed'' WVR constraints may allow the Band 1 and WVR receivers to share a Dewar, and the JRDG should consider such designs. Finally, the ASAC requests a presentation at our next meeting of a detailed plan for the mass production, integration and testing of the ALMA Phase II receivers. \section{System} \label{system} The ALMA system deals with many aspects of ALMA. We expect to revisit many of these areas in the future. We summarize below our recommendations on the issues addressed at this meeting. \begin{enumerate} \item The baseline project consists of 64 antennas of 12 m diameter. The inclusion of an ultracompact array (UCA) to fill the gap in short spacings is considered of high importance. The UCA could consist of about 7 antennas of 8 m diameter, but its design and cost should be carefully studied. The inclusion of the UCA in ALMA could be related to the possible participation of Japan in the project (see Section \ref{alg}). \item The main array should consist of a number of 4 to 6 sub-arrays, but the number of frequencies operating simultaneously will not exceed 3 or 4. At present we could envision 4+1 subarrays. Namely: \begin{enumerate} \item The main interferometric subarray \item Antennas for reconfiguration and baseline determination \item Two subarrays to simultaneously carry out two of the following functions: \begin{itemize} \item Secondary subarray at second frequency band \item Transient event monitoring \item mm-wave VLBI \item Testing, repair, receiver warm-up or cool-down, etc. \end{itemize} \item The main single-dish subarray or the UCA (if included in the final project). \end{enumerate} \item The prototype antennas should be equipped with nutators and stable receivers. The number of ALMA antennas equipped for total power measurements (nutators) should be 4, but this number will be reconsidered after the tests with the prototype antennas. The rest of the array antennas should be equipped with receivers of good gain stability ($\Delta G/G = 10^{-4}$ in 1 second). (See also section \ref{receivers} and \ref{antennas}). \item Due to its scientific interest, the option of the 30-47 GHz receivers has to be kept. The costs of including this band needs a more detailed evaluation. (See also section \ref{receivers}). \item A detailed calibration plan, including polarization issues and phase calibration, needs to be elaborated. \item Doppler tracking will be needed to provide accurate frequency calibrated data. \item Polarization observations in total power mode with ALMA will impose requirements on the system that deserve a detailed study. \end{enumerate} \section{Configurations} \label{config} Within the Configurations Working Group most of the discussion focusses on two major alternatives for the basic array layout: The spiral zoom array concept described by Conway in ALMA Memos \#216, 260, 283, and 291 and the ``doughnut'' array developed by Kogan guided by the goal of achieving minimal sidelobes (ALMA Memos \#171, 212, 226, and 247). For both concepts realistic array layouts considering topographic constraints have now been studied (ALMA Memos \#292 and 296). It appears that for both layouts comparable sidelobe levels can be achieved, which are of order 6--8\% (for snapshots!), so that a decision to adapt the one or the other design has to be based on a number of other factors such as logistics and scientific requirements. For example, guided by experience with the VLA, one might expect that the observers' demand might be highest for the most extended configuration (for maximum resolution) and the most compact one (maximizing surface brightness sensitivity). This clearly necessitates the need for optimizing logistics and planning software to optimize observations taking place in between re-configuration time. A need for model images has arisen and a total of five images will be chosen for use with all simulations. More imaging simulations are necessary for arrays involving baselines up to 20 km, where terrain considerations are the major issue. Given ALMA's excellent brightness sensitivity, imaging of thermal emission from gas and dust with such long baselines will open new vistas. Resolutions better than 10 milliarcseconds will be achieved, which are essential for studies of some of ALMA's key science goals, such as the formation of planets. As decisions on antenna pad locations have to be made by late 2000, we recommend that the Configurations Working Group report on progress to the ASAC at our next meeting, after which we can make a final recommendation. Since the large size of working group might be conducive to excessive discussions, intervention by the project scientists might be necessary to warrant a timely decision process. \section{Antennas and Total Power} \label{antennas} The prototype antenna contractors have been selected. We therefore concentrated on recommendations for testing procedures and antenna issues that impact other areas. We considered the priorities when testing the prototype antennas. For the prototype tests, we stress the following points: \begin{itemize} \item It is extremely important to test whether and under what conditions the pointing specifications (0\farcs6 ) are met. Developing observational strategies aimed at optimizing the pointing is an extremely important goal. In particular, one should look seriously at the possibility of installing optical telescopes on all antennas together with a servo system allowing real time pointing corrections. It seems likely that such systems are only effective if they are planned as part of the system and the committee recommends therefore that a system of this type is considered very seriously. \item It is also extremely important to have some method of recovering zero spacing flux using all or part of the array operated in single dish mode (see Appendix \ref{powerapp}). The committee recommends that a detailed comparison be made of the relative merits of using nutators switching rapidly (10 Hz) and On-The-Fly (OTF) mapping. A decision on the best strategy for ALMA should be made subsequent to these tests. In particular, one should test whether rapid OTF mapping (e.g 30\arcmin\ scans in 1 sec with 1 second turn-around) is feasible and whether the required gain stability ($\Delta G/G$) of order $10^{-4}$ per sec.) can be attained. Tests should also be made with the water vapor radiometer (WVR) in order to assess the ability of the WVR to monitor atmospheric emission fluctuations. Analogous studies are needed to test how effectively chopping with a simple nutator eliminates atmospheric fluctuations. This can be most efficiently done with one nutator for each prototype antenna. With this information in hand, it should be possible to decide whether nutators are or are not necessary for the array antennas. The general opinion of the ASAC was that if one could reach the scientific goals without using nutators, this was preferable. Thus one should aim at a system which could do an OTF map with all 64 antennas simultaneously. \item Polarization measurements are also sensitive to missing zero spacing flux (see Appendix \ref{polapp}), and thus it should be possible to do polarization OTF at at least 2 ALMA frequencies. The decision discussed above (OTF versus nutators) may be different if one is measuring polarized flux and thus a test of polarization OTF is desirable. \item Stabilizing the gain of the front end to $10^{-4}$ can be accomplished by selecting components with low temperature coefficients and by regulating their temperature to $\Delta T \leq 10^{-2}$K. Regulating the rest of the electronics in the laboratory to that level will be difficult, and it might be best to use a (temperature regulated) total power detector on the front end for the continuum total power measurements, rather than trying to use the correlator as the continuum detector. \end{itemize} \section{Water-Vapor Radiometry} \label{wvr} Accurate phase calibration is a critical requirement for ALMA, and the baseline design of ALMA uses a 183 GHz receiver (mounted slightly off-axis from the astronomical beam) to measure a strong atmospheric water line. Under various assumptions about the atmospheric pressure and temperature, and the location of the turbulence, the electrical path above each antenna can be derived. Richard Hills and John Richer contributed a report outlining the status of the 183 GHz systems currently in place (Appendix \ref{wvrapp}), and a series of suggestions for the requirements of a second generation system. Christine Wilson presented a report by David Naylor (Appendix \ref{irwvrapp}) on an alternative strategy, which uses a 20$\mu$m photometer to measure water vapor fluctuations in the infrared. These reports were discussed in detail. The specific recommendations of the ASAC are: \begin{enumerate} \item The water vapor radiometers are central to the scientific success of ALMA, and the project should ensure that their development is adequately resourced and integrated with all aspects of the ALMA system. \item The project should design and test preferably two (identical) prototype/pre-production 183 GHz radiometers as part of the Phase~1 project. These should, if possible, be tested on reasonable astronomical sites when completed. The possibility of putting them on the 12-m prototype antennas at the VLA site during the test interferometer work is highly attractive, and the feasibility of this option should be investigated. \item The baseline design should use a cooled 183 GHz radiometer. Whether to cool or not is, strictly speaking, an engineering problem; there was some feeling that although not absolutely required to achieve the required sensitivity, the benefits of cooling in terms of stability and noise probably outweigh the costs. \item The project should adopt a specification for the WVR system as follows: it should correct the atmospheric path above each antenna to an accuracy of 10(1+$w_v$)$\,\mu$m on a timescale of 1 second, over a period of 5 minutes and allowing for a change in zenith angle of 1 degree; $w_v$ is the precipitable water vapor in mm. \item Although it not possible to put very firm design constraints on the optics, the project should adopt as the specification that the maximum permissible offset between radiometer and astronomical beams be 10\arcmin, and (if possible) smaller for the higher frequency channels. \item The project should check that the above specifications are sensible and adequate. In particular, the short timescale behavior of the atmosphere should be quantified to ensure that correction of phase on 1 second timescales is rapid enough. \item There are scientific and productivity gains to be made by correcting the wavefront tilt across each antenna (the so-called ``anomalous'' refraction). This effect most strongly compromises mosaic observations, and those at high frequencies. However, given that there are large periods of time when this effect will not be major problem, the ASAC does not recommend adopting such a system as the baseline design at present. Further study of the loss of observing time this effect produces should be made, and this recommendation should be reassessed at future meetings. \item The baseline design for the water vapor radiometer remains a 183\,GHz system. The alternative Canadian solution using 20$\,\mu$m radiometers should be examined further, probably by the Canadians themselves, and further reports on progress should be brought to the ASAC. In particular, the correlation of the 20$\,\mu$m and 183\,GHz systems should be examined on the JCMT. The main theoretical problems of the 20$\,\mu$m technique that need to be investigated are its ability to sample the correct patch of atmosphere; its performance in differing cloud conditions; and the accuracy of the path estimation as a function of pressure, temperature and water vapor distribution. \item The project should examine the role of the system water vapor radiometers in the following: a) the amplitude calibration system, through their estimates of the atmospheric opacity above each antenna; and b) in single-dish mode observing, where they could be used to estimate the atmospheric emission. The scientific benefits of these techniques, and the extra requirements they place on the system, should be investigated. \item The project should accelerate its work on understanding the different atmospheric models used by the WVR systems to predict path errors from water line measurements. \item The location of the WVR is an engineering problem, and the solution likely depends on the degree of cooling required, and the final optical design adopted. There appear to be no show-stopping problems with locating it either in the same Dewar as the astronomical receivers, or in its own cryostat. The optimum engineering solution should be investigated. The ASAC does note that the simultaneous operation at 183 GHz and 30-45GHz (``Band 1'') is not a scientific requirement, so it is straightforward to locate these systems in the same Dewar if that makes sense. \end{enumerate} \section{Future Issues} \label{future} There are many issues that require ASAC attention in future meetings. We list here those issues that we expect to focus on in future telecons and our next meeting. \begin{itemize} \item Planning for Phase II. We would like to see a presentation on the plans for managing Phase II, including the procedures and criteria to be used to select between parallel developments. A plan for construction of the receivers (see Section \ref{receivers}) should be presented. \item Configurations. This issue received considerable discussion, summarized in section \ref{config} above, but we plan to revisit the topic after the Configuration Working Group finishes the simulations recommended above. \item Ultra-Compact Array. One very interesting enhancement that Japanese participation might add is an ultra-compact array of smaller, more accurate antennas. The scientific potential of this array will need further elaboration and study. \item Local Oscillator Systems. Developments on photonic systems are still ongoing, and we should evaluate the status of these. In addition, the implications of some of our recommendations in this report for LO systems should be evaluated. \item Software. The planning for software systems is less advanced than in other areas. We would like to hear a presentation on these plans at our next meeting. \item Spectrum Management. Since commercial broadcasting has interest in bands in the ALMA region, we would like to hear a report on the status of efforts to protect these bands. \item Site. We would like a report on the status of site arrangements. \item Outreach. Since the ALMA project still needs to be explained to the larger community, we would like a presentation on the plans for outreach. \end{itemize} \section{Summary} \label{summary} We summarize our major recommendations. These are in the order discussed in the text and not in any priority order. More detailed recommendations can be found in the section referenced by the major recommendations. \begin{itemize} \item We strongly support continued discussions aimed at including Japan in the ALMA project (Section \ref{alg}). \item We confirm that the first four bands to be implemented should be Bands 3, 6, 7, and 9. We establish priorities for the remaining bands, but emphasize that full frequency coverage is still desired (Section \ref{receivers}). \item Polarization studies will be a very important part of ALMA science. We recommend attention to polarization in all aspects, but most importantly in the receiver area (Sections \ref{receivers}, \ref{antennas}). \item The advantages of SSB operation over DSB operation of the receivers are not so clear. We recommend further study of the tradeoffs and reconsideration of the issue at a future ASAC meeting (Section \ref{receivers}). \item If an ultra-compact array of smaller, more precise antennas can result from participation of Japan, it would add important capabilities. We recommend further study of this possibility (Sections \ref{alg}, \ref{system}). \item The capability for 6 subarrays should be kept, but with no more than 4 simultaneous frequencies (Section \ref{system}). \item The Configuration Working Group should complete simulations of different array configurations and testing against a library of test images in time for an in-depth presentation at the next ASAC meeting (Section \ref{config}). \item Recovering total power is a major issue. This may be best done with OTF mapping if receivers can be built with gain stability of $\Delta G/G = 10^{-4}$ (Sections \ref{receivers}, \ref{system}, \ref{antennas}). \item Tests of total power techniques, comparing OTF with gain-stable receivers to nutating secondaries should be made on the prototype antennas. Decisions on equipping the array with nutating secondaries should be based on the outcome of these tests (Section \ref{antennas}). \item The water vapor radiometers are essential and must be integrated into all aspects of the ALMA system (Section \ref{wvr}). \end{itemize} \newpage \appendix \section{THE ASAC CHARTER} \label{charter} \begin{enumerate} \item The ALMA Scientific Advisory Committee (ASAC) was formed by the ALMA Coordinating Committee (ACC) to provide scientific advice to the ACC, to the ALMA Executive Committee (AEC), and to the Project Scientists. The ASAC will also provide communications to the wider community. \item To fulfill these goals, the ASAC will take the following steps: \begin{enumerate} \item Hold monthly telecons. \item Meet face-to-face as needed. In the current phase, we plan to meet before each meeting of the ACC. The frequency of meetings may decrease as ALMA becomes more fully defined, but we will probably meet at least once per year. \item Produce a report to the ACC, with a copy to the AEC, before each meeting of the ACC. \item Reply to questions from ALMA project staff and raise issues for their consideration via minutes or "white papers". \item Designate a member of the ASAC to act as liaison to each of the working groups in the project. \item Establish a web site where the community can learn what issues we are addressing and provide input. We will post minutes of telecons and meetings there, as well as reports to the ACC, subject to approval of the ACC. \item Announce our existence and membership in astronomical newsletters, expressing our interest in receiving questions and advice and in giving colloquia about ALMA. \end{enumerate} \item We have agreed to the following procedures. \begin{enumerate} \item We will have a Chairperson and a Vice-Chairperson at all times. At the end of each face-to-face meeting, the Vice-Chairperson will become Chairperson and we will elect a new Vice-Chairperson. We expect the role of Chairperson to rotate between North America and Europe. \item Decisions will be made by simple majority. A minority report may be included in the report to the ACC. \item Normally, we will communicate through the Project Scientists, both in receiving questions from the project technical staff and in providing answers. However, direct communication with project staff will be used for clarifications, information, etc. The liaison members are an example of this direct communication. \end{enumerate} \end{enumerate} \section{ASAC Liaison to Working Groups} \label{liaison} The liaisons to the Working Groups and other organizations are as follows. To implement this system, the Chairpersons of the working groups should incorporate these representatives of the ASAC into their email distribution lists and telecons. When possible and relevant, the ASAC representatives should attend meetings of the working groups. To facilitate this, we have usually listed a represetative from each hemisphere. \begin{itemize} \item Management: Al Wootten, Stephane Guilloteau \item ALMA Liaison Group: Pierre Cox, Neal Evans \item Antennas: Jack Welch, Malcolm Walmsley \item Receivers: Ewine van Dishoeck, Geoff Blake \item Configurations: Min Yun, Roy Booth \item Backend: Rafael Bachiller, Nick Scoville \item Software: Mark Gurwell, Arnold Benz \item Calibration, including Water Vapor: John Richer, Christine Wilson \item System Integration: Dick Crutcher, Karl Menten \item Site: Leo Bronfman \end{itemize} \section{Polarization} \label{polapp} \section{Total Power} \label{powerapp} \section{Water Vapor Radiometry} \label{wvrapp} \section{Infrared Water Vapor Radiometry} \label{irwvrapp} \section{Rationale for Band 1} \label{band1app} \end{document} From nje at strw.strw.leidenuniv.nl Tue Mar 28 05:15:11 2000 From: nje at strw.strw.leidenuniv.nl (Neal J. Evans II) Date: Tue, 28 Mar 2000 12:15:11 +0200 Subject: [asac] ASAC-README Message-ID: <200003281015.MAA10870@strw.LeidenUniv.nl> Dear Colleagues, I received only a few comments on the first draft of the ASAC report. While it would be nice to attribute the dearth of criticism to the perfection of the draft, I suspect otherwise... In any case, I have incorporated the comments I received and edited the text to make things more consistent and avoid some duplication. As suggested, I added countries behind each name in parenthesis in the list, as well as identifying myself and Karl as Chairman and Vice-Chairman (BTW, I used ...man or ...woman when speaking about a particular individual and ...person when speaking of the position in general; I am open to other solutions, but I refuse to be a piece of furniture (Chair).) Please check that your name and country are as you want them. I will send the current version in the next email. I can incorporate any comments received by the end of the day today. I will send the final version to the project scientists to forward to the AEC and ACC Wednesday at 10 AM in Leiden. Cheers, Neal From nje at strw.strw.leidenuniv.nl Tue Mar 28 05:17:10 2000 From: nje at strw.strw.leidenuniv.nl (Neal J. Evans II) Date: Tue, 28 Mar 2000 12:17:10 +0200 Subject: [asac] report.tex Message-ID: <200003281017.MAA10876@strw.LeidenUniv.nl> %\documentstyle[11pt,aaspp4]{article} \documentclass[preprint,10pt]{aastex} \begin{document} \title {\bf Report of the ALMA Scientific Advisory Committee: March 2000 Meeting} \affil{ALMA Scientific Advisory Committee} \author {Arnold Benz (Switzerland), Geoff Blake (USA), Roy Booth (Sweden), Pierre Cox (France), Dick Crutcher (USA), Neal Evans (USA, Chairman), Mark Gurwell (USA), Rafael Bachiller (Spain), Karl Menten (Germany, Vice-Chairman), John Richer (UK), Nick Scoville (USA), Ewine van Dishoeck (Netherlands), Malcolm Walmsley (Italy), Jack Welch (USA), Christine Wilson (Canada), Min Yun (USA)} \affil{Active Observers } \author{Leo Bronfman (Chile), Yasuo Fukui (Japan), Tetsuo Hasegawa (Japan), Masahiko Hayashi (Japan), Ryohei Kawabe (Japan), and Naomasa Nakai (Japan)} \section{Introduction} \label{intro} The ALMA Scientific Advisory Committee (hereafter ASAC) was formed in late 1999, as requested by the ALMA Coordinating Committee (hereafter ACC). The role of the ASAC is to provide scientific advice to the ACC, the ALMA Executive Committee (hereafter AEC) and the project, via the project scientists. As requested, the ASAC developed its own charter, which we supply as Appendix \ref{charter}. The ASAC decided to hold monthly telecons and regular meetings. For the near future, we will meet before each meeting of the ACC, in order to deliver a report in time for the ACC meeting. The telecons will supply rapid responses to queries from the project management and project scientists, and the minutes will be posted on the web. The meetings will allow exploration in more depth of particular issues and will result in a written report, such as this one. To ensure good communications, the ASAC will designate members to act as liaison to each of the working groups in the project; these are listed in Appendix \ref{liaison}. The ASAC members also committed themselves to helping to educate the larger community about ALMA. This document reports on the first meeting of the ASAC, held in Leiden, The Netherlands, on March 10-11, 2000. The topics covered at the meeting emerged from our telecons or from queries from the working groups. Some issues require further study and some topics were deferred to future meetings. These are listed in section \ref{future}. We summarize the overall recommendations in section \ref{summary}. \section{ALMA Liaison Group Issues} \label{alg} The possibility of a contribution of Japan in the ALMA project has received strong and positive support from the ASAC. Such a contribution will make ALMA the largest international collaboration in astronomy and enhance the project in a number of important ways. It will increase the sensitivity of the array and add new technical capabilities. If this collaboration is achieved, Japan will have an equal partnership in the ALMA project with America and Europe and share the infrastructure and running costs. The basic contribution of Japan to the ALMA project will be to add 12-meter antennas to the agreed 64 x 12-meter antennas. This greater collecting area will result in a better sensitivity (or observing speed), close to the original goal of a $\rm 10,000 \, m^2$ array. This improved sensitivity will compensate the need to share the observing time with a greater number of users. Further contributions of Japan to an enhanced ALMA project are related to specific technical developments, including the participation in the future correlator, construction of the highest frequency receivers, or the photonic LO system. It is too early for the ASAC to prioritize the importance of these; further discussion is needed. It is clear that the contribution of Japan in the ALMA project could also open new perspectives for the project. In particular, the possibility to add to the project a compact array of smaller, high accuracy dishes would be a most interesting addition. It would improve the image quality for extended sources and the performance at the highest frequencies. This possibility should therefore be rediscussed when the Japanese participation is confirmed. \section{Receivers} \label{receivers} Along with the telescopes, the receiver packages largely determine the capabilities of ALMA. The Joint Receiver Development Group (JRDG) has raised a number of questions and requested clarification from the ASAC. These may be broken down into questions concerning the frequency bands and their priority, the total power stability, the Water Vapor Radiometer (WVR) specs (dealt with in a separate section), polarization requirements, calibration accuracy, and receiver configurations (principally single sideband versus double sideband operation). Recommendations for each of these areas are outlined below. {\bf Frequency Bands}. The ASAC concurs that the four bands to be initially installed on the array should be (in order of increasing frequency) Band 3 (86-116 GHz), Band 6 (211-275 GHz), Band 7 (275-370 GHz), and Band 9 (602-720 GHz). The ASAC reiterates that the frequency coverage should be as complete as possible, but we respond to the request for prioritization of the bands as follows. \begin{itemize} \item First Priority: Bands 3, 6, 7, and 9 \item Second Priority: Bands 1, 4, and 2 (see below) \item Third Priority: Bands 5, 8, and 10 \end{itemize} We strongly urge that the JRDG study the possibility of extending the lower frequency range of Band 3 to include the SiO maser transition near 86 GHz. If this is possible, Band 2 would drop to third priority. The frequency intervals of the other bands are reasonable. Band 10 is scientifically quite interesting. It is in the third priority because the technology of THz SIS heterodyne receivers is in an early state, and it will be difficult to make ALMA work at its highest operating frequency. Some delay in the installation of this band will enable the most sensitive receivers to be installed and the telescope performance to be optimized. Note that Band 1 is in the second priority list, and it must be considered in receiver layout. If it will not be in the main Dewar, then designs for optics that allow a second Dewar, possibly also containing the WVR, should be developed. It is not necessary for the WVR and Band 1 receivers to operate simultaneously. {\bf Total Power Stability}. For On-The-Fly (OTF) mapping capabilities, the requisite total power stability is of order 10$^{-4}$ in one second. The ASAC recommends that this level be a goal, rather than a hard specification, pending further study. The over-riding concern is the receiver sensitivity, and better performance should not be sacrificed for stability at this stringent level. However, this level of stability may allow considerable simplication (avoiding nutating subreflectors), and we encourage the JRDG to study the issue and report back to the ASAC on the prospects for achieving this level of stability and on possible tradeoffs in doing so. {\bf WVR Specs}. These are discussed at length elsewhere (Section \ref{wvr}). The main point here is that this system must be incorporated into the overall design and receiver specs. {\bf Polarization}. Polarization work will be an important part of ALMA research. Strong efforts should be made to have the polarized single-dish beams as stable as possible; consequently, the ASAC recommends that careful consideration be given to placing the 345 GHz receiver on-axis. For linear polarization work the basis state of feeds would ideally be circular polarization. If circular feeds impose important limitations on tuning range or increase significantly the noise temperature, a system for rapid, accurate calibration of linear feeds should be implemented. Obtaining zero and short spacing polarization data is essential. A nutating subreflector has a limited angular throw and introduces varying angles with respect to the optical axis of the primary mirror. The OTF technique proposed for total power observations would be ideal for polarization if the requisite gain stability can be achieved. Finally, the different polarization properties of the two prototype antennas and other polarization properties of the test interferometer and single-dish techniques should be carefully measured as they may be a consideration in procurement decisions (see Section \ref{antennas}). {\bf Calibration Accuracy}. The ALMA calibration spec of 1\%~is adequate scientifically, perhaps even a bit agressive. A cold calibration load in the primary Dewar is probably unnecessary. {\bf Receiver Modes}. The superb quality of the Chajnantor site and the non-ideal nature of any optical system means that the theoretical improvement in single sideband (SSB) versus double sideband (DSB) receivers may be difficult to realize in practice. DSB receivers are far easier and cheaper to fabricate, especially at submillimeter frequencies, and the ASAC recommends that a careful design study be undertaken that assesses the likely performance loss for DSB operation. If the loss is sufficiently small, considerable cost savings and ease of operation can be realized. The ASAC would like to revisit this question once the SSB versus DSB study is completed. It is very likely that ALMA will become operational with both SSB and DSB receivers. This change in operational characteristics has important implications for the ALMA correlator, and the ASAC also recommends that the initial and subsequent ALMA correlators be designed with both modes of operation in mind. The operating system and software environment may also be affected. {\bf Summary}. The ASAC confirms that Bands 3, 6, 7, and 9 have the top priority and should be installed first. While complete frequency coverage is important, we have divided the other bands into second and third priorities. We recommend study of extending the lower end of Band 3 to include 86 GHz. In addition, the JRDG should consider placing the Band 7 receiver on-axis. Designs that accomodate the Band 1 receiver are essential. The ``relaxed'' WVR constraints may allow the Band 1 and WVR receivers to share a Dewar, and the JRDG should consider such designs. Finally, the ASAC requests a presentation at our next meeting of a detailed plan for the mass production, integration and testing of the ALMA Phase II receivers. \section{System} \label{system} The ALMA system deals with many aspects of ALMA. We expect to revisit many of these areas in the future. We summarize below our recommendations on the issues addressed at this meeting. \begin{enumerate} \item The main array should consist of a number of 4 to 6 sub-arrays, but the number of frequencies operating simultaneously will not exceed 3 or 4. At present we could envision 4+1 subarrays. Namely: \begin{enumerate} \item The main interferometric subarray \item Antennas for reconfiguration and baseline determination \item Two subarrays to simultaneously carry out two of the following functions: \begin{itemize} \item Secondary subarray at second frequency band \item Transient event monitoring \item mm-wave VLBI \item Testing, repair, receiver warm-up or cool-down, etc. \end{itemize} \item The single-dish subarray or an ultra-compact array (if included in the final project). \end{enumerate} \item The prototype antennas should be equipped with nutators and stable receivers. The number of ALMA antennas equipped for total power measurements (nutators) should be 4, but this number will be reconsidered after the tests with the prototype antennas. If feasible, the rest of the array antennas should be equipped with receivers of good gain stability ($\Delta G/G = 10^{-4}$ in 1 second). (See also section \ref{receivers} and \ref{antennas}). \item Due to its scientific interest, the option of the 30-47 GHz receivers has to be kept. The costs of including this band needs a more detailed evaluation (See also section \ref{receivers}). \item A detailed calibration plan, including polarization issues and phase calibration, needs to be elaborated. \item Doppler tracking will be needed to provide accurate frequency calibrated data. \item Polarization observations in total power mode with ALMA will impose requirements on the system that deserve a detailed study. \end{enumerate} \section{Configurations} \label{config} Within the Configurations Working Group most of the discussion focusses on two major alternatives for the basic array layout: the spiral zoom array concept described by Conway (ALMA Memos \#216, 260, 283, and 291); and the ``doughnut'' array developed by Kogan guided by the goal of achieving minimal sidelobes (ALMA Memos \#171, 212, 226, and 247). For both concepts realistic array layouts considering topographic constraints have now been studied (ALMA Memos \#292 and 296). Both layouts appear to achieve comparable sidelobe levels, which are of order 6--8\% (for snapshots!); consequently, a decision to adopt one or the other design has to be based on a number of other factors, such as logistics and scientific requirements. For example, guided by experience with the VLA, one might expect that the observers' demand will be highest for the most extended configuration (for maximum resolution) and the most compact one (maximizing surface brightness sensitivity). Such considerations should be included in the choice of array concepts. A need for model images has arisen and a total of five images will be chosen for use with all simulations. More imaging simulations are necessary for arrays involving baselines up to 20 km, where terrain considerations are the major issue. Given ALMA's excellent brightness sensitivity, imaging of thermal emission from gas and dust with such long baselines will open new vistas. Resolutions better than 10 milliarcseconds will be achieved, which are essential for studies of some of ALMA's key science goals, such as the formation of planets. As decisions on antenna pad locations have to be made by late 2000, we recommend that the Configurations Working Group report on progress to the ASAC at our next meeting, after which we can make a final recommendation. Since the large size of the working group might be conducive to excessive discussions, intervention by the project scientists might be necessary to warrant a timely decision process. \section{Antennas and Total Power} \label{antennas} The prototype antenna contractors have been selected. We therefore concentrated on recommendations for testing procedures and antenna issues that impact other areas. We considered the priorities when testing the prototype antennas. For the prototype tests, we stress the following points. \begin{itemize} \item It is extremely important to test whether and under what conditions the pointing specifications (0\farcs6) are met. Developing observational strategies aimed at optimizing the pointing is an extremely important goal. In particular, one should examine the possibility of installing optical telescopes on all antennas, together with a servo system allowing real time pointing corrections. It seems likely that such systems are only effective if they are planned as part of the system and the committee recommends therefore that a system of this type is considered soon. \item It is also very important to have some method of recovering zero spacing flux using all or part of the array operated in single dish mode (see Appendix \ref{powerapp}). The committee recommends that a detailed comparison be made of the relative merits of using nutators switching rapidly (10 Hz) and On-The-Fly (OTF) mapping. A decision on the best strategy for ALMA should be made subsequent to these tests. In particular, one should test whether rapid OTF mapping (e.g 30\arcmin\ scans in 1 sec with 1 second turn-around) is feasible and whether gain stability ($\Delta G/G$) of order $10^{-4}$ per second can be attained. Tests should also be made with the water vapor radiometer (WVR) in order to assess the ability of the WVR to monitor atmospheric emission fluctuations. Analogous studies are needed to test how effectively chopping with a simple nutator eliminates atmospheric fluctuations. Equipping each prototype antenna with a nutator will facilitate these studies. With this information in hand, it should be possible to decide whether nutators are or are not necessary for the array antennas. The general opinion of the ASAC was that if one could reach the scientific goals without using nutators, this was preferable. Thus one should aim at a system that could do an OTF map with all 64 antennas simultaneously. \item Polarization measurements are also sensitive to missing zero spacing flux (see Appendix \ref{polapp}), and thus it should be possible to do polarization OTF at at least 2 ALMA frequencies. The decision discussed above (OTF versus nutators) may be different if one is measuring polarized flux and thus a test of polarization OTF is desirable. \item Stabilizing the gain of the front end to $10^{-4}$ can be accomplished by selecting components with low temperature coefficients and by regulating their temperature to $\Delta T \leq 10^{-2}$K. Regulating the rest of the electronics in the laboratory to that level will be difficult, and it might be best to use a (temperature regulated) total power detector on the front end for the continuum total power measurements, rather than trying to use the correlator as the continuum detector. Because these are engineering issues, we welcome iteration between the ASAC and the JRDG. \end{itemize} \section{Water-Vapor Radiometry} \label{wvr} Accurate phase calibration is a critical requirement for ALMA, and the baseline design of ALMA uses a 183 GHz receiver (mounted slightly off-axis from the astronomical beam) to measure a strong atmospheric water line. Under various assumptions about the atmospheric pressure and temperature, and the location of the turbulence, the electrical path above each antenna can be derived. Richard Hills and John Richer contributed a report outlining the status of the 183 GHz systems currently in place (Appendix \ref{wvrapp}), and a series of suggestions for the requirements of a second generation system. Christine Wilson presented a report by David Naylor (Appendix \ref{irwvrapp}) on an alternative strategy that uses a 20$\mu$m photometer to measure water vapor fluctuations in the infrared. These reports were discussed in detail. The specific recommendations of the ASAC are: \begin{enumerate} \item The water vapor radiometers are central to the scientific success of ALMA, and the project should ensure that their development is adequately resourced and integrated with all aspects of the ALMA system. \item The project should design and test preferably two (identical) prototype/pre-production 183 GHz radiometers as part of the Phase~1 project. These should be tested on reasonable astronomical sites when completed. The possibility of putting them on the 12-m prototype antennas at the VLA site during the test interferometer work is highly attractive, and the feasibility of this option should be investigated. \item The project should adopt a specification for the WVR system as follows: it should correct the atmospheric path above each antenna to an accuracy of 10(1+$w_v$)$\,\mu$m on a timescale of 1 second, over a period of 5 minutes and allowing for a change in zenith angle of 1 degree; $w_v$ is the precipitable water vapor in mm. \item Although it is not possible to put very firm design constraints on the optics, the project should adopt as the specification that the maximum permissible offset between radiometer and astronomical beams be 10\arcmin, and (if possible) smaller for the higher frequency channels. \item The project should check that the above specifications are sensible and adequate. In particular, the short timescale behavior of the atmosphere should be quantified to ensure that correction of phase on 1 second timescales is rapid enough. \item There are scientific and productivity gains to be made by correcting the wavefront tilt across each antenna (the so-called ``anomalous'' refraction). This effect most strongly compromises mosaic observations, and those at high frequencies. However, given that there are large periods of time when this effect will not be a major problem, the ASAC does not recommend adopting such a system as the baseline design at present. Further study of the loss of observing time this effect produces should be made, and this recommendation should be reassessed at future meetings. \item The baseline design for the water vapor radiometer remains a 183\,GHz system. The alternative Canadian solution using 20$\,\mu$m radiometers should be examined further, probably by the Canadians themselves, and further reports on progress should be brought to the ASAC. In particular, the correlation of the 20$\,\mu$m and 183\,GHz systems should be examined on the JCMT. The main theoretical problems of the 20$\,\mu$m technique that need to be investigated are its ability to sample the correct patch of atmosphere; its performance in differing cloud conditions; and the accuracy of the path estimation as a function of pressure, temperature, and water vapor distribution. \item The baseline design should use a cooled 183 GHz radiometer. Whether to cool or not is, strictly speaking, an engineering problem; there was some feeling that although not absolutely required to achieve the required sensitivity, the benefits of cooling in terms of stability and noise probably outweigh the costs. \item The project should examine the role of the system water vapor radiometers in the following: a) the amplitude calibration system, through their estimates of the atmospheric opacity above each antenna; and b) in single-dish mode observing, where they could be used to estimate the atmospheric emission. The scientific benefits of these techniques, and the extra requirements they place on the system, should be investigated. \item The project should accelerate its work on understanding the different atmospheric models used by the WVR systems to predict path errors from water line measurements. \item The location of the WVR is an engineering problem, and the solution likely depends on the degree of cooling required, and the final optical design adopted. There appear to be no show-stopping problems with locating it either in the same Dewar as the astronomical receivers, or in its own cryostat. The optimum engineering solution should be investigated. The ASAC does note that the simultaneous operation at 183 GHz and 30-45GHz (``Band 1'') is not a scientific requirement, so it is straightforward to locate these systems in the same Dewar if that makes sense. \end{enumerate} \section{Future Issues} \label{future} There are many issues that require ASAC attention in future meetings. We list here those issues that we expect to focus on in future telecons and our next meeting. \begin{itemize} \item Planning for Phase II. We would like to see a presentation on the plans for managing Phase II, including the procedures and criteria to be used to select between parallel developments. A plan for construction of the receivers (see Section \ref{receivers}) should be presented. \item Configurations. This issue received considerable discussion, summarized in section \ref{config} above, but we plan to revisit the topic after the Configuration Working Group finishes the simulations recommended above. \item Ultra-Compact Array. One very interesting enhancement that Japanese participation might add is an ultra-compact array of smaller, more accurate antennas. The scientific potential of this array will need further elaboration and study. \item Local Oscillator Systems. Developments on photonic systems are still ongoing, and we should evaluate the status of these. In addition, the implications of some of our recommendations in this report for LO systems should be evaluated. \item Software. The planning for software systems is less advanced than in other areas. We would like to hear a presentation on these plans at our next meeting. \item Spectrum Management. Since commercial broadcasting has interest in bands in the ALMA region, we would like to hear a report on the status of efforts to protect these bands. \item Site. We would like a report on the status of site arrangements. \item Outreach. Since the ALMA project still needs to be explained to the larger community, we would like a presentation on the plans for outreach. \end{itemize} \section{Summary} \label{summary} We summarize our major recommendations. These are in the order discussed in the text and not in any priority order. More detailed recommendations can be found in the section referenced by the major recommendations. \begin{itemize} \item We strongly support continued discussions aimed at including Japan in the ALMA project (Section \ref{alg}). \item We confirm that the first four bands to be implemented should be Bands 3, 6, 7, and 9. We establish priorities for the remaining bands, but emphasize that full frequency coverage is still desired (Section \ref{receivers}). \item Polarization studies will be a very important part of ALMA science. We recommend attention to polarization in all aspects, but most importantly in the receiver area (Sections \ref{receivers}, \ref{antennas}). \item The advantages of SSB operation over DSB operation of the receivers are not so clear. We recommend further study of the tradeoffs and reconsideration of the issue at a future ASAC meeting (Section \ref{receivers}). \item If an ultra-compact array of smaller, more precise antennas can result from participation of Japan, it would add important capabilities. We recommend further study of this possibility (Sections \ref{alg}, \ref{system}). \item The capability for 6 subarrays should be kept, but with no more than 4 simultaneous frequencies (Section \ref{system}). \item The Configuration Working Group should complete simulations of different array configurations and testing against a library of test images in time for an in-depth presentation at the next ASAC meeting (Section \ref{config}). \item Recovering total power is a major issue for continuum observations of extended sources. This may be best done with OTF mapping if receivers can be built with gain stability of $\Delta G/G = 10^{-4}$ (Sections \ref{receivers}, \ref{system}, \ref{antennas}). \item Tests of total power techniques, comparing OTF with gain-stable receivers to nutating secondaries should be made on the prototype antennas. Decisions on equipping the array with nutating secondaries should be based on the outcome of these tests (Section \ref{antennas}). \item The water vapor radiometers are essential and must be integrated into all aspects of the ALMA system (Section \ref{wvr}). \end{itemize} \newpage \appendix \section{THE ASAC CHARTER} \label{charter} \begin{enumerate} \item The ALMA Scientific Advisory Committee (ASAC) was formed by the ALMA Coordinating Committee (ACC) to provide scientific advice to the ACC, to the ALMA Executive Committee (AEC), and to the Project Scientists. The ASAC will also provide communications to the wider community. \item To fulfill these goals, the ASAC will take the following steps: \begin{enumerate} \item Hold monthly telecons. \item Meet face-to-face as needed. In the current phase, we plan to meet before each meeting of the ACC. The frequency of meetings may decrease as ALMA becomes more fully defined, but we will probably meet at least once per year. \item Produce a report to the ACC, with a copy to the AEC, before each meeting of the ACC. \item Reply to questions from ALMA project staff and raise issues for their consideration via minutes or ``white papers". \item Designate a member of the ASAC to act as liaison to each of the working groups in the project. \item Establish a web site where the community can learn what issues we are addressing and provide input. We will post minutes of telecons and meetings there, as well as reports to the ACC, subject to approval of the ACC. \item Announce our existence and membership in astronomical newsletters, expressing our interest in receiving questions and advice and in giving colloquia about ALMA. \end{enumerate} \item We have agreed to the following procedures. \begin{enumerate} \item We will have a Chairperson and a Vice-Chairperson at all times. At the end of each face-to-face meeting, the Vice-Chairperson will become Chairperson and we will elect a new Vice-Chairperson. We expect the role of Chairperson to rotate between North America and Europe. \item Decisions will be made by simple majority. A minority report may be included in the report to the ACC. \item Normally, we will communicate through the Project Scientists, both in receiving questions from the project technical staff and in providing answers. However, direct communication with project staff will be used for clarifications, information, etc. The liaison members are an example of this direct communication. \end{enumerate} \end{enumerate} \newpage \section{ASAC Liaison to Working Groups} \label{liaison} The liaisons to the Working Groups and other organizations are as follows. To implement this system, the Chairpersons of the working groups should incorporate these representatives of the ASAC into their email distribution lists and telecons. When possible and relevant, the ASAC representatives should attend meetings of the working groups. To facilitate this, we have usually listed a representative from each hemisphere. \begin{itemize} \item Management: Al Wootten, Stephane Guilloteau \item ALMA Liaison Group: Pierre Cox, Neal Evans \item Antennas: Jack Welch, Malcolm Walmsley \item Receivers: Ewine van Dishoeck, Geoff Blake \item Configurations: Min Yun, Roy Booth \item Backend: Rafael Bachiller, Nick Scoville \item Software: Mark Gurwell, Arnold Benz \item Calibration, including Water Vapor: John Richer, Christine Wilson \item System Integration: Dick Crutcher, Karl Menten \item Site: Leo Bronfman \end{itemize} \section{Polarization} \label{polapp} \section{Total Power} \label{powerapp} \section{Water Vapor Radiometry} \label{wvrapp} \section{Infrared Water Vapor Radiometry} \label{irwvrapp} \section{Rationale for Band 1} \label{band1app} \end{document} From nje at strw.strw.leidenuniv.nl Wed Mar 29 05:26:19 2000 From: nje at strw.strw.leidenuniv.nl (Neal J. Evans II) Date: Wed, 29 Mar 2000 12:26:19 +0200 Subject: [asac] ASAC Final Report Message-ID: <200003291026.MAA16904@strw.LeidenUniv.nl> Dear Colleagues, The ASAC report is now in the (virtual) hands of Al and Stephane, meaning it is on an ftp site where they can get it. There were a number of last minute suggestions, which I have now incorporated. Also, all the appendices are now part of the document. If you want a copy of the really final version, here is how to get it. ftp lithium.strw.leidenuniv.nl login as anonymous and give your email as password cd pub/nje/asac Then you can mget whatever you need. If you want the finished report, all you need is report.ps. If you want to reformat, break it up into pieces, etc., take report.tex and the figures that it calls in. The following applies if you take the latex files. Texnicalities: 1. The file uses the latest AAS style files, including LaTex2e. If your institution has not upgraded to this, you can use the older Latex by uncommenting the first line and commenting the second line. 2. You will need to run latex at least twice to get all the references to sections correct. Thanks again for the prompt submissions and timely suggestions. See you at the next ASAC meeting, Neal