[evlatests] L-band Polarizer Stability

[rperley at nrao.edu]

Stable polarizers are an important characteristic in simplifying
calibration.  If the polarizers are sufficiently stable for sufficiently
long periods, then users -- or pipelines -- can do polarization
calibration much more easily and efficiently than having to go through the
routine of polarization calibration.  This is especially important for
short observations, (where there is insufficient time to employ the
parallactic rotation method, or where moving to an unpolarized source is
too time-consuming).

How stable, and for how long?  Probably better than 1% in  stability.  For
duration, periods of months or more seem reasonable, for if the polarizers
are unchanging over this period of time, specific polarization calibration
could be eliminated for most observations, and use made of polarization
tables, which would be determined on, say, a yearly basis.

Early indications from the EVLA tests showed promise for the stability. 
Here I report on the results from polarization observations taken in
February 2019 and January 2020, at L, S, and C bands.

In summary, the stability at S and C bands over the 11 month span is
extremely good.  I'll circulate some images later today to show this.

However, at L-band, the situation is more confusing.  Out of 24 antennas
measured on both dates, 11 showed no change in polarization
characteristics to < 1%.  Seven showed significant changes, and six others
showed very significant differences.  (Note that the discrimination
between 'different' and 'very different' is rather subjective).

I attach three plots showing one example from each class.  ea28 showed the
largest difference for all antennas.  In these plots, frequency runs
horizontally from 1 to 2 GHz, the D-term (voltage leakage) amplitude in
tens of percent on the vertical axis of the larger panels.  Phase,
spanning 360 degrees, is in the smaller panels.  The top pair is the
leakage from LCP to RCP, the bottom pair that from RCP to LCP.

The members of each class are:

a) Same:  ea01, 4, 5, 14 16 17 20 21 22 26 and 27
b) Different:  ea06 10 12 13 15 18 23
c) Very different:  ea2 3 9 11 19 28

No measured (out of the array on one of the two dates)  ea07, 8, 24, 25.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: l-ea04-same.pdf
Type: application/pdf
Size: 150918 bytes
Desc: not available
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20200601/6a74417d/attachment-0003.pdf>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: l-ea13-diff.pdf
Type: application/pdf
Size: 144869 bytes
Desc: not available
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20200601/6a74417d/attachment-0004.pdf>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: l-ea28-vdiff.pdf
Type: application/pdf
Size: 152649 bytes
Desc: not available
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20200601/6a74417d/attachment-0005.pdf>