[evlatests] X-band Observations from last evening

[Rick Perley]

    Michael and Ken set things up last evening to observe 3C273 for ~two 
hours at X-band, with a single polarization, and 12 WIDAR antennas. 

    The first mystery is that only 1 hour of data found its way into the 
archive.  The operator believes the run ran to the intended end.  So 
about an hour of data are missing ...

    All 12 antennas fringed strongly and generally well.  The overall 
phase and amplitude stability are not very good, but I believe that 
nearly all the instabilities seen can be plausibly explained by the 
thunderstorms prowling about at the time ...

    The observations were of 3C273 and a close-by calibrator.  It is 
clear that the phases connect very well between these two, and that 
there were no 180 degree antenna-based phase jumps. 

    Bandpass solutions are very good.  Some antennas shows rather strong 
(1 db) ripple in their amplitudes -- evidence for a standing wave in the 
electronics.  Antennas which notably show this are ea1 and ea2.  The 
period of the standing wave is 25 MHz -- corresponding to about 6 meters 
separation between reflectors -- about the distance between the horn and 
the subreflector ...

    I generated 'pseudo-continuum' databases (100 MHz BW in each of the 
four subbands) and imaged the calibrator and source.  For the former, I 
got a plausibly noise limited image immediately after a single 
self-cal.  For the latter (3C273), a few rounds of self-calibration were 
needed to provide a 12500:1 dynamic range image.  Although not noise 
limited, (short by a factor of a few) I think it likely that the sparse 
UV coverage (less than an hour of data, and a notable lack of short 
spacings to see the more extended emission) is responsible for 
preventing a noise limited image. 

    Antennas 18 and 24 'dropped out' for significant lengths of time 
(particularly ea18) -- these do not appear to be weather related.  I 
have no explanation for this.