[mmaimcal] Text of AI on antenna quardupod design from Beasley

[Robert Laing]

But doesn't this issue fit into what Stephane called "common mode errors"? 
If all antennas have the same feed leg induced beam error pattern, then 
the "signature" of this error term in a polarimetry measurement would 
allow for its calibration/cancellation.  Won't dissimilar designs make 
this cancellation process much more difficult, or perhaps not doable at 
all?

I don't think this is analogous to the cancellation of a common-mode path 
error. We have to model the polarized primary beam in any case. The 
question is whether we can get away with one model for everything, two 
models (one per antenna type) or in the worst case models for individual 
antennas. At the moment, the last case is clearly necessary for WSRT at 
low frequencies (notionally identical antennas!). In contrast, the VLA 
(total intensity) primary beam calibration in AIPS uses a single 
analytical model (no legs) for every antenna. So I think this is a 
quantitative question requiring a calculation, unfortunately.

Regards

Robert

P.S. I'm not suggesting that ALMA will have anything like the problems of 
WSRT at 151 MHz - just making an extreme illustration.


-- 
Robert Laing
European ALMA Instrument Scientist

European Southern Observatory
Karl-Schwarzschild-Strasse 2
D-85748 Garching-bei-Muenchen
Germany

Telephone (direct)     (+49) 89 3200 6625
           (secretary)                6631/6234/6678
Fax                                  6611

rlaing at eso.org