Hints and Strategies for Successful Phase Calibration

In most programs, calibrator sources are observed at least once an hour and sometimes as frequently as every 10 minutes. Calibrator observations are not only important for tracking instrumental phase and gain drifts, atmospheric and ionospheric gain and phase variations, but for monitoring the quality and sensitivity of the data and for spotting the occasional gain and phase jumps.

There are several criteria for choosing and using a calibrator. A list of guidelines, in decreasing order of importance, follows:

  1. Choose the calibrator closest to your source. If it is within 10 deg., atmospheric phase fluctuations will be somewhat better calibrated. It is better to have one calibrator per source over the entire run. If several are needed, try to bootstrap their positions together. However, in the smaller configurations and at longer wavelengths, these criteria can be considerably relaxed, so a single calibration for a group of sources is often preferable. Furthermore, if your target sources can be self-calibrated, the need for rapid switching between source and calibrator is entirely removed. Hourly observations of the calibrator are more than sufficient for this case, except at 22 GHz or higher frequencies where they should be no further apart than 30 minutes
  2. Choose a calibrator which has a P or S quality status for the desired configuration and frequency (see Section 4.1). The difference between P and S is minimal but P is preferred since fewer gain errors will result. However, a more nearby but weaker S or even W quality calibrator may well be preferable for phase calibration, but not for amplitude calibration. In this case the amplitude calibration, which is much more stable than the phase calibration, can be derived from observations of a more distant P quality calibrator that is observed less frequently. This situation may arise at high frequencies where only a small number of sources are sufficiently strong (> 0.5 Jy) for amplitude calibration, but the atmospheric phase fluctuations require a nearby calibrator source. As a general rule of thumb, at 0.7 cm the phase calibrator should be within 10 degrees in good weather and within 5 degrees in bad weather. If just solving for the phase, the calibrator can be as weak as twice the sensitivity on a single baseline, which is 0.1 Jy at 43 GHz. If no VLA calibrator is sufficiently close, it may be useful to consult the MERLIN calibrator lists of Patnaik et al. (1992, MNRAS, 254, 655) and Browne et al. (1998; MNRAS, 293, 257). And to properly remove tropospheric phase fluctuations at high frequencies requires very rapid switching with observations of the calibrator every few minutes.
  3. At frequencies of 1.8 GHz and below, the presence of moderately strong sources within the primary beam centered on the calibrator can cause significant closure errors. For this reason many calibrators have uv restrictions at L and P band and may be completely unsuitable in the smaller configurations. Observations performed in spectral line mode may encounter somewhat larger closure errors than indicated by the P or S quality flags (see the Key in section 4.1) due to the reduction in bandwidth smearing. When observing at L band in the D and C configurations it may be desirable to choose a calibrator with P quality status, even if it is more distant from the target source. Fortunately the atmosphere is quite stable at L band in the D and C configurations.
  4. Different calibrator codes are used only to distinguish the accuracy of the calibrator position. If absolute positional accuracy < 0.1 arcsec is desired, the position code should be an important consideration - use 'A' or 'B' calibrators. Most positions for sources with 'A' or 'B' PC codes are taken from the JPL or USNO astrometric lists.
  5. The flux density of the calibrator is of secondary importance. The only exceptions are when the calibrator will be used as a band-pass calibrator for spectral line observing, for high dynamic range observations where closure errors must be measured, and for very narrow-band spectral line experiments.
  6. The use of partially resolved calibrators for the determination of antenna gains and phases is possible with the added complication that the calibrator must be imaged first and the resulting model provided to CALIB. Use of partially resolved calibrators may occasionally be necessary in the larger configurations. Models are available on the web at http://www.aoc.nrao.edu/~cchandle/cal/cal.html

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