[mmaimcal] Summary of case for OSF interferometry

[Jeff Mangum]

Hi Al,

Alwyn Wootten wrote:

>Folks,
>Truth time is here--comments on the following?
>Holdaway’s simple holography simulation package in AIPS++/glish
>See https://wikio.nrao.edu/bin/view/ALMA/HoloGraphy
>Using two-element interferometric holography and the brightest compact
>celestial radio sources available, we will have enough sensitivity to
>accurately set the panels near the center of the dish, and not at the edge
>of the dish. Alternatively, using a larger cell size (0.4 m) which won't
>permit panel setting, we can very accurately confirm the surface accuracy
>of the dishes.
>Discussion: The basic question: is it worthwhile to set up a correlator at
>the OSF site to do two element holography on astronomical sources?
>  
>
No. The logical sequence of antenna measurements, which has been 
confirmed with measurements on the prototype antennas, is as follows:

   1. Photogrammetry: ALMA and/or the contractor do this, perhaps both
      before and after transmitter holography. Measures elevation
      dependence of surface quite accurately.
   2. Transmitter Holography: Set and verify the surface to better than
      20 micron.
   3. OOF: If a single dish radiometry system is available, then do this
      as a check of (1) and (2).

Interferometric holography is only needed later, and plays no logical 
role in the large number of tasks that need to be done at the OSF.

>Note that second antenna is followed one month later by third—time for
>interferometry at the OSF is very limited.
>NOTE: if we want to do panel settings from astronomical holography at the
>AOS, we will have to wait until we have 10 antennas.
>Alternatives:
>	Tower holography provides setting at one elevation.
>       Photogrammetry provides measure of surface variation as f(el); has
>worked better than expected at ATF (numbers secret?)
>Out of focus beam maps have given good results at ATF (numbers secret?)
>Out of focus holography has given poor results at ATF owing to limitations
>of the evaluation receivers.  (Numbers secret?)
>  
>
OOF beam maps and OOF holography are the same thing.

>Need estimate of accuracy at OSF with ALMA receivers.
>I think that the project might not establish an interferometer at the OSF
>and be just as well off.
>  
>
I agree completely.

Cheers,

Jeff