1. About the 2003 calibrator manual

This manual contains the VLA calibrator list and general information
on how to use it.  It consists of two sections: the first contains
information on amplitude, phase and polarization calibration of VLA
data.  The second part contains the list of calibrator sources with a
preface describing how to read the listings.
  
Many of the flux densities reported in the calibrator manual are now
over 10 years out-of-date.  This means that flux densities reported
herein can differ by more than a factor of 2 with current values.  To
address this concern we have implemented a flux history database for
VLA calibrators.  You should use it to obtain a better idea of the
current flux densities of potential calibrators.  It is accessible at
the URL:  http://aips2.nrao.edu/vla/calflux.html

Over the past few years the VLA calibrator manual has continued to
grow.  A major improvement has been the addition of Q band entries for
at total of 1675 calibrators.  Note however, that most of the
calibrators are flat spectrum and rapidly variable, so the flux
densities reported here may not reflect the current level for a given
source.  Be conservative when selecting calibrators at high
frequencies.

In this revision the calibrator manual contains 5523 entries for 1860
sources.  Positions for 950 sources were refined on Aug. 28 2001 using
the VLBA Calibrator Survey observations as reduced by the NASA Goddard
Space Flight Center Geodesy group with the Calc/Solve package.  These
positions have typical positional errors less than 1 milliarcsec (see
Johnston et al 1995, AJ 110, 880; and Beasley et al. 2002, ApJS, 141,
13).  This database is also the primary source of positions for the
VLBA correlator.  We stress that all users who desire accurate
positions should use the J2000 coordinates.
 
Updates are expected throughout the coming year so for the most
current version of this document the reader is recommended to access
the World Wide Web at
http://www.vla.nrao.edu/astro/calib/manual/index.shtml.  A search engine for
the calibrator database accessible via the WWW is available and has
seen frequent use.  The revised calibrator database is regularly
incorporated into OBSERVE.  For those who require a plain ascii text
version of the calibrator database, it is available at
ftp://ftp.aoc.nrao.edu/pub/mclausse/calman/csource.mas.
 
The VLA calibrator list has been compiled from many different sources,
so it is difficult to assess its completeness.  Certainly it is
incomplete along the galactic plane, as the majority of the finding
lists excluded this region.  Not all sources in the list have yet been
observed at all bands listed.  If no entry exists for a calibrator at
the desired frequency the user may still be able to judge its utility
based on information given for adjacent bands.  This is generally
required for selecting K band calibrators.  If no suitable calibrator
can be found near your target source then you may want to consult the
list of other resources for finding calibrators available from the
calibrator manual web page.

Comments or questions regarding this manual should be addressed to
Mark Claussen (mclausse@nrao.edu) at the AOC in Socorro, NM.  Printed
copies of this manual may be obtained by contacting Gayle Rhodes at
the AOC.

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