[mmaimcal] Major accomplishments August 2000

[Al Wootten]

I'll send this off shortly unless I receive comments...
Thanks for your input to this.

Al

10.     Calibration, Imaging and Science       August 2000 Report

        Major accomplishments August 2000
 
I. Imaging - Configuration Studies, Site and Water Vapor Radiometry

The imaging and calibration group held several phone meetings during August,
with the agenda, indeces and linked documents to be found at:
http://www.cv.nrao.edu/~awootten/mmaimcal/

A. Configuration Studies

The simulation efforts for the primary ALMA array have reached a point 
where the basic simulation procedure is agreed upon, and 
results exist for parameters agreed upon at
the March face to face meeting for presentation to the ASAC.
Steve Heddle in UK is making good progress on the imaging simulation.
John Conway, Min Yun and Brian Butler advised him on this effort during August.
A telecon for the configuration
working group was held on 23 August to discuss the results thus far
and how to proceed.  At month's end a good library of results was available,
and the simulations were proceeding to study of differences in images 
formed by different array designs.

 The strawperson configuration memo, including the complete layouts 
and initial examples of imaging
performance, was finished by month's end as ALMA Memo No. 320 by Yun and
Kogan.  Min Yun left the project at month's end to take up a position at
the University of Massachusetts at Amherst; he plans to continue his
ALMA activities as he can.

B. Atacama Compact Array

The most pressing issue for now is the definition of and imaging test for 
the Atacama Compact Array (ACA).  During August, the definition of the array
converged on several similar strawman designs.  A protomemo by Guilloteau
suggested 8m antennas, owing to similarity in receiver cabin design 
possibilities and good calibration sensitivity, particularly in the 
submillimeter, while u,v coverage was not so suitable as might be with a
6m telescope.  A standalone array of 10, 8m telescopes resulted from these
considerations.  Another point of view stresses getting good u,v coverage
by employing 6m telescopes.  A memo by Welch suggested a standalone array
of 12 x 6m antennas by month's end.  Imaging tests commenced during 
August for the ACA, using arrays of 6m and 8m antennas to judge relative
performance.   Yun's single pointing simulations to evaluate the
imaging performance of the 6m versus 8m antennas for the ACA have been
indeterminate so far.  These simulations and other discussions
within the configuration group suggest that simulated mosaic observations
are truly needed.  Yun pursued these efforts within the aips++ and
Classic AIPS programs, and Morita also pursued studies of the Compact Array.
  By month's end a few things were clear:  simulations with reasonable
errors are needed, but that in imaging simulations dynamic range is limited
by the pointing errors of the larger antennas in the array while fidelity
index can be improved considerably by including the compact array.

C. Site

Radford updated web pages and data files for radiosonde and tipper data through
July.  Radford and Butler also worked on reduction and analysis of 
radiosonde data, and production of a paper describing these data.
 Hydrodynamic airflow simulations with Dave DeYoung (NOAO) continue.

Radford also began testing the new Inmarsat M4 terminal for delivery to the
site later this Fall.

Butler revisited a memo in preparation describing possible locations for the
compact array on Chajnantor, and Radford and Butler coauthored Sakamoto's ALMA
Memo No. 322 on weather at Pampa La Bola and Chajnantor, which appeared in
late August.

II. Calibration 
A. Interferometer/Antenna Amplitude Calibration

Mangum finished and issued a memo on amplitude calibration as No. 318 in the
Memo Series.  A two-load chopping scheme was found to have the potential
for reaching 1% accuracy in all bands, as specified by the ALMA Science
Advisory Committee.  A study by Plambeck (No. 321) reached similar conclusions.
These memos formed the basis for a recommendation by the Imaging and
Calibration Group that ALMA pursue studies of two load chopping schemes of
amplitude calibration, with particular emphasis on a location behind the
secondary.  Several concerns about this system need to be addressed, however.
It appears that the 1% accuracy sought by the ASAC will challenge any scheme.

B. Pointing Calibration

Mangum continued work on his memo on pointing of the ALMA antennas.

c. Other
 Mangum pursued the thorny question of "how
   small a field do we want to image at the maximum scanning rate of
   the ALMA antennas".  The current Vertex design indicates a turn
   around time of 0.8 seconds assuming a 30'/sec scan rate and a scan
   time of >2 seconds in OTF mode.  Note that this implies a >1 degree
   scan length at the maximum rate. In an unpublished but oft-cited work,
Holdaway found that OTF total power observing is as good as or better than beam
   switching for optimal sky subtraction when one could OTF map at the
   maximum slew rate (1 deg/s) and a 0.2 second turn around time.  So,
   Vertex is within a factor of <~6 of meeting Holdaway's simulated
   break-even point.

III. Science

A. ALMA studies, including the ASAC
 Butler and Wootten presented a paper on ALMA capabilities for the 
detection of Extrasolar Planets at the IAU symposium No. 202 on this subject.  
ALMA can directly image young systems including the star, disk and large
planets, imaging their dynamics and chemistry on few-AU scales for nearby
systems.  ALMA will directly image the gas and dust in more evolved systems,
revealing the presence of planets through gaps in the structure.  Very
young proto-Jupiters, free floating or tied to a planetary system, could
be detected with reasonable integration times out to the distances of the
nearest star-forming clouds.  As an evolved planet orbits its parent star,
the latter will undergo reflex motions measureable by ALMA; these studies
favor discovery of planets at large orbital radii and complement radial 
velocity searches, recently productive at optical wavelengths.

Wootten presented a report on ALMA at the Observatory Reports session of 
Division X, and represented the project at other IAU venues.

Final work continued on the volume summarizing ALMA Science from the meeting
last October.  A paper on ALMA observations of distant galaxies was written
for publication in the proceedings of the 'Gas and Galaxy Evolution' conference
of late Spring.

Plans for the face to face ASAC meeting in Berkeley, including the agenda
and reading material were finalized by Wootten. 

B. Imaging and Calibration

        Plans for next month

Imaging and Calibration will sponsor the ASAC face-to-face meeting in
Berkeley early in the month, bringing issues of concern to the project
to the attention of its scientific committee for advice and resolution.
As the ASAC Report is developed, we will convey it to the rest of the project
and react to the recommendations of the committee.  

Configuration work will continue, with a focus on finishing a set of the
simulations and distilling the results into a recommendation on array design.
Incorporation of various sorts of errors into the simulations can begin,
and the appropriate nature of these will be determined as we assess the
results so far and the capabilities of simulation software.

Work also continues on the characterization of the ACA.

        Issues and concerns

During September we will continue to seek a replacement for Min Yun, though it
will be difficult to match his insight and productivity.  US resources in
millimeter astronomy have shrunk, and there are few new students available.