Compact Array calibrators
Calibration of the Compact Array is an essential part of any observation and a calibrator catalogue is maintained to assist with this. Information and a search engine for calibrators can be found at www.narrabri.atnf.csiro.au/calibrators. There is an on-going program to monitor the flux densities, to better characterise calibrators and to add to the calibrator list (particularly at high frequencies).
The different observations with the Compact Array can span two orders of magnitude in both array compactness and frequency range. Few calibrators are "good" over all of this parameter space. Consequently observers are warned that just because a source is in the calibrator list, this does not imply that it is useful for your particular observation. Please check the web information for your prospective calibrator.
As of 12 October 2005, the calibrator list has been updated with four new sources added, and improved positions entered for 86 existing sources. Generally the positional changes have been less than 0.5 arcseconds. However there was a cluster of poor calibrator positions near RA = 23 hours, DEC = 40o.
In addition to those interested in high astrometric accuracy, the updated position can have an effect on observations where multiple epochs are involved. If the nominal position of the calibrator changes between the epochs, then this will lead to a mis-registration between the epochs. If the change in nominal calibrator position is "significant" relative to the resolution and the dynamic range requirements, then care is required. One way to avoid these issues, where positional accuracy is of secondary importance, is to continue to use the old calibrator positions. To use the old positions, in SCHED select the old calibrator catalogue as the default one with the command:
defcat old_calibrators
Note that the change in the calibrator catalogue does not affect schedule files that have already been developed.
Bob Sault
(Bob.Sault@csiro.au)