[evlatests] A remarkable gain change at P-band

[Ken Sowinski]

On Mon, 28 Oct 2024, Rick Perley via evlatests wrote:

> I ran a holography test last night (to assist Tracy Clarke in her 
> characterizations of the P-band primary beam).  The SB consisted of four 
> single cuts through the primary beam.  The four cuts were:  horizontal, 
> vertical, and two cuts at 45 degree angles.
>
> Each holography cut begins and ends with a short on-source calibrator 
> observation.
>
> A most remarkable thing happened between the first two holography cuts 
> in last night's test:
>
> The amplitude changed, for all antennas.  Some amplitudes went up, 
> others went down.  For some antennas, the change was barely noticeable. 
> For others, it was as large as a factor of six.  (!).  The changes were 
> not the same in the two polarizations.
>
> The effect is due to a gain chang, and not to any antenna or 
> subreflector position error —it is seen in the switched power monitors 
> —both PSum and PDif.
>
> The PSum values, at the beginning of the run, and through to the end of 
> the first holography observation, were normal at near 14 counts.
>
> This gain change occurred only after the first holography scan — about 
> ten to fifteen minutes into the observation.  For all other beam cuts, 
> the gain remained constant (at the wrong power levels).
>
> Any idea what happened?

None at all.  If Psum remained constant during the entire observation then
it has hard to see what is happening.  If I had to blame something it 
would be some arcane side effect of diddling the subreflector.  Did gain
change for the refants as well as all the moving ants?

I will have a look at the script some time to see if there is anything
unexepcted between the first two of the beamcuts.

Ken