VLBA Pulsar Gate Observing and Correlating Requirements
Notes from Alan Roy Concerning Phase Referenced Observations.
1. Calibrator selection
For calibrator source lists, try:
The VLBA calibrator survey: http://magnolia.nrao.edu/vlba_calib/index.html
USNO Radio Reference Frame Image Database: http://maia.usno.navy.mil/rorf/rrfid.html
For best results, source strength should be more than a few hundred mJy. Ed Fomalont had success with a 20 mJy calibrator that was within the same primary beam as his programme source, obviating the need for driving off source for calibration. (This will probably require two correlation passes, however.)
2. Switching angle
You want a calibrator within 6d, and preferably within 2d. Test observations at 15 GHz switching between pairs of strong sources shows that the residual rms phase after phase referencing remains fairly constant out to 6d then rapidly gets worse.
3. Switching time
Recommended maximum switching time is frequency dependent. Using the requirement that the rms phase between calibrators be < lambda/4, which should allow phase connection in 95% of cases, the following switching times are derived by Beasley & Conway 1995 ("VLBI and the VLBA", ASP Conf Ser, Vol 82, p327)). The switch cycle times are given for typical and poor seeing conditions.
4. Phase check source
You should include periodic looks at a second phase referencing source. Phase transfer fails sufficiently frequently, leaving large residual phase errors, that you really want a test source included that's strong enough to see, so that when you reduce the data and get a non-detection, you can see whether it's due to phase referencing problems, and to get a measure of the decorrelation expected on your programme source.
5. Expected performance
Tests at 15 GHz with a 6-min cycle time and switching angles between 1.3 and 9.9d yielded phase referenced images with dynamic ranges of 13 to 33 (25th & 75th percentiles). These test have yet to be completed and extended to other frequencies.
VLBA Correlator Pulsar Gate Specifications
Correlator Mode Restrictions with the Pulsar Gate
Useful Links to Pulsar Webs Sites
Email comments to: John Benson