[mmaimcal]Re: [Almasci] Comment in ALMA memo 489

[J. Pety]

Dear Mark,

I fully support Koh-Ichiro's and Tak's remarks. Here are a few precisions:

1. The different acquisition schemes of SD data between ALMA+SD and
   ALMA+ACA+SD come from the fact that the SD have the same diameter than
   ALMA antennas while a different diameter compared to ACA antennas.

   The processing of the SD data is to try to compute the visibilities an
   interferometer like ALMA would observe at spacings smaller than 12m in a
   thought world. When the SD and ALMA antenna are identical, I claim that
   this is possible only for the zero-spacing (for other spacings, SD is
   not giving enough information). Thus there is no need to make an OTF
   map. Indeed, making an OTF map in such a case would dilute SNR over
   large area when exactly only the same small amount of information would
   be used.

   Now, this use of the SD may not be the cleverest one. From what I
   understand of MEM, people are using the ALMA dirty map and the SD map as
   boundary conditions to find the best solution according to some
   criterion. In this case, the SD information (from 0 to 12m) is maybe
   better used. In principle, CLEAN could use a similar mechanism, ie try
   to find CLEAN components both on a SD image and the ALMA dirty
   map. However, I do not know about such an algorithm but I know that
   similar algorithms are very difficult to stabilize (see the algorithm we
   use for deconvolving ALMA+ACA+SD).

   In other term, we priviliged a very robust algorithm compared to an
   hypothetical, still to be demonstrated better use of the SD time. We
   must remember that we want a solution which is reliable (ie we do not
   want have any doubt that in some cases our algorithms may fail).

2. Band guard: my recollection is that simulations show that observing a
   band (either SD or interferometry) around the field-of-view we want to
   image always improve fidelities. Depending on the processing of the SD,
   it may be mandatory (ie to avoid aliasing effects). For the
   interferometric map, I don't think it is mandatory but it
   helps. However, I doubt that program committees will allow us to observe
   around the field-of-view of interest (which may be empty!): it will be
   seen as a lost of time.

3. Declination range: To optimize the use of ALMA, it will probably be used
   as much as possible when sources are high and for high sources. In
   addition, what will be the smallest elevation authorized at high
   frequency where the atmosphere plays such an important role? So this
   limitate the effect of baseline projection. But anyway, declination
   effects must obviously be studied in more details.

4. Simultaneous observations are always desireable. However, I expect that 
   PIs will merge ALMA configurations taken at different times. Is that as
   much different than taking ALMA and ACA observations at different times?
   What must be carefully planed is that the ALMA and ACA observations are
   done in similar weather conditions.

Best regards,
   Jérôme.