[evlatests] Serious Problem with default gain corrections

[Rick Perley]

    We ran on Friday night the script for establishing the elevation 
dependency of the antenna gains.   The weather was perfect -- clear, dry 
(DP ~ 30), calm.  Temperatures were nearly steady near 40F during the 
duration. 

    Immediately apparent is that the default gain corrections being 
routinely applied to the data are spectacularly in error.  (For my 
processing, I'm using the OBIT path, but I presume the same gain 
corrections are being applied through CASA also ...)

    The amplitude gain corrections being applied show a huge elevation 
dependency.  They are different for each antenna, but all follow the 
same trend.   All appear to be normalized at the zenith (which is 
o.k.).  Below I give a short table showing the *amplitude* correction 
range (amongst antennas) at an elevation of 12 degrees.  The power 
correction is the square of this value. 

    X         1.06 -- 1.12
    Ku       1.25 -- 1.35
    K         1.45 -- 1.80
    Ka       1.40 -- 2.00
    Q         2.20 -- 5.00  (corresponding to a power correction by a 
factor as large as 25!!!!!)

    These corrections are completely wrong!    Where do they come from?  
It is not from the calculation of the opacity (provided the OBIT 
calculation is the same as the AIPS calculation) -- I regenerated the 
corrections using 'INDXR', turning off the antenna gain correction, and 
leaving in the opacity.  For the Q-band data, the resulting correction 
(at 46 GHz) is 1.5 at elevation = 8 -- a reasonable value. 
    So the problem must lie with the default antenna gain curves.   One 
might suspect the correction being applied is that required for the 
period of time where the 'subreflector trick' was turned off.  But this 
is not the case -- I checked the gain corrections from the Jan 2012 flux 
density run (when the subreflector rotation was disabled) -- at X-band, 
there are no antennas requiring a correction larger than 1%!  
   
    I should be able to generate new corrections by the end of the day.