[evlatests] A remarkable gain change at P-band

[Rick Perley]

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?

Rick
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