[evlatests] More on 3bit-8bit polarization leakage and stability

[Rick Perley]

    After further review of the very high quality polarization data from 
the two 3bit/8bit tests taken on Nov 16 and 17, I summarize the major 
conclusions below. 
    The data were all calibrated in an identical manner -- I used 3C84 
alone as the polarization calibrator, with the PCAL mode that utilizes a 
known polarization -- in this case, 0.0%.  (It is actually about 0.25%, 
but tests show that this small error has no significant consequence to 
the conclusions, below).  I checked the antenna cross-polarization 
solutions from this method against those of the 'traditional' method, 
which uses the parallactic angle rotation to allow solutions for both 
the antenna and source polarizations.  The differences between these two 
solutions were trivial (and were of the order of the true linear 
polarization of 3C84). 

    Major conclusions are:

    1)  Antenna polarizations for all paths (A1C1, A2C2, and B0D0) are 
very similar for both days.  Typical differences are 0.5%, maximum 
differences are about 2%.
    2) The major differences are between A1C1 and A2C2, rather than 
between solutions of A1C1 on different days. 
    3) Temporal differences for any IF are very small, almost always 
less than 0.5%.  This is especially true for the difference between 
solutions made for the first half and second half of the Nov17 data, 
showing that stability within any one observing session (even one where 
the temperatures, elevations, and antenna gains were all changing 
significantly) is very good.  This statement holds for most antennas 
between the two separate days.  There are two significant exceptions 
here -- ea06 and ea23 both showed interesting differences between the 
two days.  ea23 might be explained by the change of hardware (sampler 
board?) made last week. 

    I attach 7 plots showing the D-terms (cross-polarization amplitudes 
and phases, referenced to ea18-RCP), for the 512 MHz of spectrum common 
to all three paths on both days.  The color coding is:
    Yellow:  A1C1 Nov 16
    Green:   A2C2 Nov 16
    Purple:   B0D0 Nov 16
    Blue:      A1C1 Nov 17
    Red:       A2C2 Nov 17
    Dark Yellow:  B0D0 Nov 17

    Note that Yellow and Blue, Green and Red, and Purple and Dark Yellow 
track each other very closely for most antennas.  (These are the same 
paths on the two different days). 
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