[mmaimcal] ALG Issues--comments? No meeting Tuesday...

Mon Jun 19 22:47:15 EDT 2000

Comments on 'Plan for an Enhanced ALMA' presented 12 June 2000 by
Japanese members of the ASAC.  I have incorporated comments from
presentations by S. Guilloteau and Koh-Ichiro Morita at the ALMA
Technical Workshop, 17 Feb 2000 in Tokyo, the ASAC meeting in Leiden,
and other venues.

I. Antennas:

A. Additional 12m antennas

As Stephane noted, increasing the number of 12m antennas from 64 to 78
results in an ALMA in which sensitivity per partner is conserved, and
overall sensitivity is increased.  This seems like a reasonable goal.

B. The Terahertz Array

A compact array of seven 6-8m antennas is added, with each antenna
bringing an equal value to that of one 12m antenna.  The exact number
and size are poorly determined.
An alternative which poses sizable problems for pointing and surface
accuracy might be to build one submillimeter telescope of 24m diameter.

Scientific Merits of a Compact Array:

I believe that the compact array brings a very good enhancement to ALMA
as it promises production of science otherwise not addressed by ALMA, at
the highest frequency windows.  The compact array could operate at
supraTerahertz frequencies for the 15% of the time at which
transparencies in the supraTerahertz windows exceeds 30%, for instance. 
As Guilloteau noted, this Terahertz Array could operate independently
of, or in addition to the main array, with the antennas on the latter
underilluminated to improve efficiency, field of view, and effective
pointing performance.  At 1 THz, the resolution could be 3-4" over a 10"
field of view (FOV) for an 8m telescope, somewhat larger for 6m
telescopes (but see below), operating as a stand-alone array.  Issues
affecting number and diameter of antennas includes
        a. As antennas become smaller, i.e. 6m, it may be difficult to
accommodate a standard suite of receivers, and standard cabin,
compromising the economy of scale.  As the array becomes more
specialized, it becomes more difficult to operate and maintain.  
        b. As number of antennas increases, it becomes more difficult to pack
them together to obtain the smallest baselines.  The array need not be
concentric with the large array, but should perhaps be located nearby
for operation on medium baselines with underilluminated 12m antennas. 
The number of antennas is limited to perhaps 16.
        16 x 6m antennas has the same collecting area as one 24m antenna.
        c. As antennas become larger, it is easier to accommodate the standard
suite of receivers.  As the compact array may be operated as a
standalone Terahertz Array, and may require additional receivers to
operate at supraTHz frequencies, it seems unwise to restrict the
receiver cabin.  Some space may be recovered if the THz Array need not
operate at the longest wavelengths.  This suggests perhaps 8m antennas. 
However, existing implementations of inhomogeneous arrays have employed
antennas with diameters varying by a factor of two.  Nine 8m telescopes
have the same collecting area as one 24m antenna.  Seven 8m antennas
provides adequate sensitivity for matching with an ALMA mosaicking 25%
of its observing time.

A. Imaging quality should be very much improved, through provision of 
        a. short baseline information (6-10m) for imaging with the larger
array.  Although        simulations are just beginning, ALMA is expected to
proved excellent imaging at millimeter wavelengths, with some degree of
quality loss at submillimeter frequencies.  Especially at high
frequencies, direct measurement of 8m spacings provides much better
accuracy than recovery of this information through combination of
interferometric plus on-the-fly mapping data.
        b. improved cross calibration between interferometer and single antenna
performance
        c. improved performance for the combined array, particularly in bands 9
and 10.
        d. For a main array performing wide field imaging 25% of available
time, the Terahertz Array spends essentially all its time collecting
short spacing information for the wide field imaging experiments.  This
will pose some potential logistical problems--different atmospheric
conditions, calibrator fluxes, scheduling conflicts but this difficulty
is probably only at the annoyance level.
        e. The compact array may be constructed in a fixed array, with long
baselines provided through baselines with underilluminated 12m antennas.

B. Science with the Terahertz Array
  To some extent, the compact array would bring the most identifiable
new capability to ALMA.  However, it is not clear that the technology is
yet ready for construction of a Terahertz Array.  However, that
technology is being developed for the FIRST mission (2007), and may be
available during ALMA construction.
  a. Frequency.  The frequency extent addressed by ALMA would be
extended to include the three supraTHz windows accessible from the
Earth's surface with reasonable transparency.
                1. 1-1.06 THz window.  Recently, the HHT detected CO 9-8 in this
window, which also contains the CS 21-20 line.  Transmission is about
20% when transmission in the 450 micron window is about 70%.  This will
be referred to as the 1.03 THz window.
                2. 1.25 - 1.37 THz window.  This window is somewhat diced by
atmospheric lines, rather like the Band 8 window, but contains the CO
11-10 line and reaches a transparency nearly that of the 1.03 THz window
mentioned above.  Of the three supraTHz windows, this is probably of
least interest.
                3. 1.5 THz window.  This window contains the lower frequency of the
two [N II] lines, the CO 13-12 transition, and the HCN 17-16 line. 
Owing to the uniqueness of the [N II] line as a probe of the ISM, and
its strength in the Milky Way (measured by COBE) and other galaxies
(estimated from ISO measurements of the higher frequency line in other
galaxies using simple CLOUDY models), this is the primary interest of
the three supraTHz windows.  Transmission reaches 20% but the window is
somewhat broader than the 1.03 THz window.
                4. Dust continuum emission increases from most objects through these
windows, and would provide an interesting target.  Spectral baseline,
for determination of spectral energy distributions, for example,
suggests that the 1.5 THz window is of most interest.
                5. [C II] emission at 2.2 THz enters the 1.5 THz window at z=0.47,
providing a window on this line in galaxies at a time between the
present epoch and that of peak star formation.
                6. However, receivers to cover these bands would come at some future
time when technology improves.  In the interim, receivers using this
future technology might be tested on the THz array antennas. Enhanced
frequency coverage would not be part of ALMA construction.
        b. Sensitivity  - 
         1.  Sensitivity can be calculated for the proposed 7 x 8m array. 
These antennas should achieve better surface accuracy than the 12m
antennas; an assumption of 10 microns might be reasonable, better might
be achievable.  This accuracy should provide an efficiency near 60%. 
With more baselines, and a diameter better targeted to filling the gap
in spacings available to ALMA, a 16 x 6m array sensitivity is also
calculated.  
        2. Goals of a THz Array, and a compact array operating to provide short
spacings for ALMA at high frequencies, conflict somewhat in that both
goals can only be achieved in the best weather.
        3. Estimates - Proposed 7 x 8m array.
        For operation in the 1.5THz band, a receiver achieving 25 h nu/k is
assumed (SSB).  20% zenith transmission through the atmosphere received
by an 8m antenna with 55% efficiency and a main beam size of 7" are also
assumed for a source at 1.3 AM.  In one minute, the array could achieve
0.3 Jy sensitivity, of .01K in brightness temperature.  In a single
channel 1 km/s wide, this is 12 Jy km/s, or 0.5K with shortest baseline
1.5D=12m.
        4. Estimates - Proposed 16 x 6m array.
        For operation in the 1.5THz band, a receiver achieving 25 h nu/k is
assumed (SSB).  20% zenith transmission through the atmosphere received
by an 6m antenna with 55% efficiency and a main beam size of 9" are also
assumed for a source at 1.3 AM.  In one minute, the array could achieve
0.23 Jy sensitivity, of .006K in brightness temperature.  In a single
channel 1 km/s wide, this is 9 Jy km/s, or 0.2K on baselines of 1.5D=9m.
  c. Implementation
        1. Japan has proposed providing these antennas over the period
FY2002-2008.
        2. Some correlator redesign would be needed for an ALMA with an
increased number of antennas.  Escoffier has offered the opinion that
the basic design could accommodate 88 antennas, sufficient for 78 12m
antennas plus 10 smaller ones.  However, since the THz Array antennas
may not often be operated with the 12m antennas, some additional
capacity may be available.

II. Receivers and LO

 A. Receiver Bands
    The ASAC voiced interest in Band 4 (2mm) since CO at redshifts of
0.45 to 0.8 for the 2-1 line will fall in this range.  Band 3 covers
redshifts of 0. to 0.35 or so, depending upon its lower frequency, for
the 1-0 line.  Thus these two lines give good sensitivity to redshifted
CO for the lowest excitation lines over the redshift range in which most
evolution in the star formation rate has occurred, if one is to believe
published estimates.  For higher redshifts, the compaction of the
spectrum provides readier access to a range of redshifts.  I believe
that this should be a priority also.  It would be useful to lower the
lower edge of band 3, in my opinion, from 86 GHz to 84 GHz (which allows
complete overlap with the band covered by the 3mm VLBA receivers).  This
would have to reach 80 GHz for complete coverage of the two lowest CO
lines for redshifts between 0.35 and 0.45.
                The weather at Chajnantor is not always submillimeter; I believe that
Band 4 should be a priority, in place of Band 10, which I believe would
benefit by waiting for the technology to mature.

        B. LO
          This was not really developed in the memo of 12 June.  Does this
include expansion of the photonic LO system to frequencies below 300
GHz?

III. Correlator (from 12 June note plus Okamura presentation 17 Feb
2000)

  A. The proposal is that NRAO provide the first quarter of the
correlator now planned, to be replaced by a second generation correlator
producing 125 kch/IF from 85 antennas.  Architecture to be determined by
EU/Japan.  Advantages cited, especially by Okamura in February for FX
design,  include:
        a. Increased number of baselines addressed
        b. Increased sensitivity by increasing the number of bits (not
specified); easier with FX
        c. Increased bandwidths: advantages
                1. Simultaneous wide bandwidths and high spectral resolution
                   Advantages: multi line imaging not restricted by having only n IFs
                2. Imaging line surveys possible with resolution matched to source
                3. Wide velocity coverage with high resolution may be important where
a range of conditions are encountered, such as protostellar disks with
warm kinematically active centers and cold exteriors with, e.g. slow
infall.
                4. At 1.5 THz, 2 GHz corresponds to 400 km/s, suggesting the existing
design might place some restrictions on the THz Array.
                5. Wideband high resolution imaging is useful for studies of radio
loud quasars such as 3C84.  High accuracy continuum subtraction needed
                6. Recombination line studies suggest broad coverage at good
resolution
                7. Absorption line studies of distant galaxies
                8. Serendipity, such as water masers in NGC4258.
        d. Disadvantages of increased bandwidths
                1. Huge throughput may mean additional computing costs.                         These
should be assessed.
                2. FX design can result in higher development costs
                3. More cabling needed for FX design.

Clarity and light,
Al