3.1. 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.

\section*{Choosing a Calibrator}

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

      A) 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.

         B) 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 (see 
         http://www.vla.nrao.edu/astro/guides/highfreq/schedule/).  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.  

         C) 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.

      D) 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.

      E) 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.  

      F) 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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