Daily Image
05-08-2010
A Rogues Gallery
| Submitter: | Oleg Smirnov & Ger de Bruyn |
| Description: | In a previous image http://www.astron.nl/dailyimage/main.php?date=20100730), we demonstrated the successful detection of deliberate pointing errors at the WSRT, using no other information than the observed visibilities themselves. In a follow-up to this study, we obtained two observations of another (rather crowded) field called "QMC2". The first (July 3) observation was intended as a benchmark. It was to be done with as near to perfect pointing as possible. The follow-up (July 21) observation would then have a set of deliberate pointing errors, which we would once again need to derive from the data. To our initial puzzlement, selfcal of the benchmark observation produced some unexpected artifacts around off-axis sources (see inset on bottom right of image.) These duly went away when differential gain solutions were applied. Looking at the differential gains, we became convinced that something was very wrong with telescope 8. We alerted the Radio Observatory to this "prediction after the fact", and their testing indeed revealed a bad declination encoder on the telescope, which had caused a tracking problem. The "Rogues Gallery" plot above makes this obvious. Similar to our previous image, this now shows the 12-hour average differential gain-amplitude solutions towards 17 sources of the QMC2 field, as seen by each antenna of the WSRT. In the plot for the July 3 observation, RT8 sticks out like a sore thumb. By the time of the second observation, RT8 had been fixed, but a deliberate pointing offset was introduced on RT2, RT6 and RT8, which is also evident from the plot. (Less evident is that RTB had a time-variable pointing error; the 12-hour average plots don't show this very well, but it is clear from our other plots.) Also visible are minor pointing problems with other telescopes (e.g. RT9 on July 21) which were not deliberate. QMC2 has proven to be a great "quality monitoring" field, containing 17 moderately strong sources within the core of the WSRT primary beam (the circles on the plot are at 30' radius.) It's almost ideally located (RA=1h) for regular monitoring with the WSRT and other (Northern hemisphere) instruments. |
| Copyright: | MeqTrees Foundation |
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