[mmaimcal] Polarization Teleconference

[Steven T. Myers]

For those of us in Socorro, I have reserved the Mid-Level conference
room for 10:30 MST.

We should look over the Receiver Optics chapter of the Project Book, 
this will make the complexities of the optical design clearer.

In particular, as Larry points out and I mentioned yesterday, the fact
that we need the dual-mirror refocusing optics to minimize the dewar   
window size makes putting a band on the telescope optical axis a moot
point - the symmetry is broken and thus the problem then becomes
optimizing the mirror figures for performance.  I presume this has
been taken into account.

Also as Larry points out, the optics requires that any rotation of the
polarizations be done inside the dewar as the cartridge must maintain
orientation of the dewar optics to the telescope optics.  One could  
in principle do this, but this would have to be done on assembly and thus
could not be undone easily.  My guess is that we would be much better off
with a homogeneous system, and focus our efforts on characterization, 
calibration, and incorporating the polarization information into the   
imaging algorithms (as Holdaway has pointed out, variations over the 
primary beam cannot be corrected for so much as must be dealt with in
the imaging step, a sort of extra deconvolution).

For the test interferometer, we can play with some of the aspects but as
it will not have the production dewar optics and will not be the same
focal plane layout, we will not be able to fully test things.  We should 
try to get the prototype cartridges for each band (would need two of
course!) on the TI as soon as is possible down the line.  Perhaps we could
designate one of the bands (350GHz?) as the "primary polarization" band
and aim to get two of the production model proptypes asap and do
exhaustive testing?

I think the design gang has done their best in trading off the options.
The ImCal group has been harping on them regarding these issues pretty  
much throught he process.  I felt we all pretty much agreed that circular
polarization was untenable (due to bandwidth, losses, and the need to    
put stuff in the beam), though we left the possibility open for perhaps
some optical device for a single band, and if the system were calibrated  
there is also the option of synthesizing circular in the electronics   
before the correlator (do the quarter-wave delay digitally?).   

The ImCal group and the ASAC should definitely keep in touch as the optics,
receiver, front-end, and calibration designs evolve.  In particular, the
sooner we get some quantification of the distortions to be expected the
sooner we can start working out what needs to be done.  Perhaps we should
aim to do this first for the 350 GHz band suggested by the ASAC as the
prime polarization band.  Is it possible to derive map of the expected
polarization response over the primary beam given the proposed designs?
Are there possibilites for testing (observations with VLA or other arrays)
we should consider?

    -Steve Myers

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|:| Steven T. Myers                      |:|  Associate Scientist      |:|
|:| National Radio Astronomy Observatory |:|                           |:|
|:| P.O. Box O                           |:|  1003 Lopezville Rd.      |:|
|:| Socorro, NM 87801                    |:|  Ph:  (505) 835-7294      |:|
|:| smyers at nrao.edu                      |:|  FAX: (505) 835-7027      |:|
|:| http://www.aoc.nrao.edu/~smyers      |:|                           |:|
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