Re: Subarrays

[Al Wootten <awootten@NRAO.EDU>]

The Imaging and Calibration Group had a discussion via email on this subject
in September when D'Addario proposed a strawman two subarray concept.
This has evolved into more complexity since then.  Michael Rupen summarized
science projects which need subarrays:

* Simultaneous multi-freq. capability, useful for
   - solar/stellar flares
   - atmospheric studies
   - cases (as at the VLA currently for 7mm) where only some of the antennas
     are outfitted with certain types of receivers or bands. I suspect this
     will be much more common for an array as huge as ALMA.
   - Barry adds pulsars

* Calibration.  If we're at all thinking about simultaneous calibration using
  part of the array, to monitor opacity and/or gain on short timescales
  through tipping scans or other measurements at one or more wavelengths, that
  suggests subarrays to do so.

* Mapping large areas. With M subarrays you do shorten
  the time to map a given area by a factor M, if sensitivity is not an issue.
  This is useful e.g. for finding interesting "weather" on the Sun and other 
  solar system objects (this can change VERY fast), or simply for doing big
  surveys.

* Fast-response observations.  If something nifty goes off you may want to
  quickly dash over to have a look, without stealing every antenna in the
  array.  Similarly ALMA will be so absurdly sensitive that monitoring obs.
  even of faint sources might easily be done with a subset of the antennas.

* Lower data rates.  Experiments which require lots of channels or very high
  time resolution will probably have to cut down on the number of baselines;
   this will undoubtedly be true initially.

* VLBI.  I don't imagine people will want to give up all or even half of ALMA
  on a regular basis for VLBI, but it will certainly be in enormous demand
  for high-freq. experiments.

* Inhomogeneous arrays.  If ALMA does go this route (cf. Wright's memos),
  there are probably reasons to split into homog. subarrays every now and then.

* Single-dish observations.  ALMA will probably do its own single dish work,
  but it's not clear whether one wants (or will be able) to use the entire
  array for this all at once.  So we'll want to split off bunches of single
  dishes, which you may or may not want to call subarrays.  As Barry points
  out this may be true especially for unique receivers (1.5 THz anyone?).

* Some folks have suggested faking big dish diameters by phasing up lots
  of little (well, moderate-sized) antennas (cf. MMA Memo 165).

Which of these require shuffling of antennas from one subarray to another?
It may be desirable to shuffle n antennas out of one subarray for a tipping
scan, then admit it back, rather than use all antennas.  For the simultaneous
SEDs one might conceivably divide into 10 subarrays, one for each frequency
band.  Single antenna operations may also require shuffling into and out of
subarrays.  VLBI may need shuffling though here it is presumably an operator
rather than the observer who controls the operation.

Barry Clark writes:
 > Come on people, I know it's a dull subject, but it's our job.  Is there a
 > scientific requirement for the software to enable the observer to 
 > shuffle antennas flexibly from one subarray to another?
 > 
 > 
 > > We seem to be agreed that we want some sort of scripting language for
 > > the direct interface to the real-time system.  In designing this language,
 > > a pivotal question is whether we want to give the observer the power to 
 > > shuffle antennas from one subarray to another via script.  To do so complicates
 > > things considerably, so I would like to close that door unless it is really
 > > needed.
 > > 
 > > 1.) It might be desired for transient phenomena.
 > > One might want to record an instantaneous SED.  For phenomena with 
 > > timescales greater than tens of seconds, it is more effcient to just 
 > > cycle the array through the frequencies, rather than splitting into 
 > > subarrays.  The faster timescale phenomena that come to mind are 
 > > a). pulsars, b).  solar flares, and c). maybe flares on other stars
 > > (the fine time-structure stuff may have such a steep spectrum that 
 > > ALMA won't see it).  (Am I missing something?)
 > > 
 > > Note also that subarraying capability has to be provided for other reasons.
 > > So, giving the observer the capability to rearrange his subarrays by 
 > > script is called for only if this subarraying is pretty dynamic.  For
 > > instance, the observer might want to occasionally recombine his subarrays
 > > for calibration purposes (could use a weaker calibrator that way).
 > > 
 > > 2.) It might be useful for instrumental reasons.
 > > For instance, if the observer is switching between two frequencies, and
 > > not all antennas are equipped with the receiver for one of these, 
 > > he would like all antennas to join in when he is at the frequency that
 > > they all have, but would have the incomplete antennas go off and do
 > > something else possibly useful when he is observing at the other.
 > > 
 > > Am I missing other scenarios?
 > > 
 > > Do these two justify the complexity of including this in the scripting
 > > language?
 > > 
 > > 
 > 
 > 
Date: Thu, 27 Jan 2000 11:40:50 +0100 (MET)
To: lucas@iram.fr
From: Majordomo@web3.hq.eso.org
Subject: Majordomo file: list 'alma-sw-ssr' file 'alma-sw-ssr.991122'

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