[evlatests] Referenced Pointing Success Rate

[Rick Perley]

In prior 24-hour flux density observations, much effort was needed in purging the effects of bad referenced pointing determinations.   These are relatively easy to find (amplitudes from one antenna are too low by the same factor on both polarizations, and rapidly worsen with increasing observing frequency), but required much labor in inspecting and flagging affected antennas.   At least 20% of the observations at Q-band had to be removed due to this problem.  In some cases, the determination failed (and was noted as such), in other cases the solution found was incorrect, and in a few other cases, it was evident the solution was correct, but not applied for the subsequent observation.

Our recent observations show this problem seems to have almost completely vanished!  Of roughly 3200 referenced pointing solutions determined, I have found only six which are clearly wrong — all on 3C138 — four antennas at one time, and two at another.   This number of incorrect solutions will likely be a bit low —about 1/3 of the target sources are too weak for the effect of incorrect pointing solutions to be seen in the data.

There is one other failure mode which did bite us.  For the standard calibrator 3C196, I chose a nearby point source — J0808+4950 for pointing solutions.  (3C196 is too heavily resolved in A config to serve as its own referenced pointing calibrator).  The majority of the pointing solutions on J0808 failed.  The reason was easy to find by querying Lorant's flux density database —its flux at C-band has dropped below 0.25 Jy (and is likely much less than this now at X-band, where we did our pointing observations).  The calibrator manual shows a flux of 1.0 Jy at X-band.  This factor of >5 drop is most unusual, and illustrates the need for us to monitor these variable sources on a reasonable timescale —and to get this information easily available for potential observations.

Rick



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