The FIRST survey is a long-term project whose completion is many years
in the future. The area of sky covered by the FIRST survey will
increase in increments of 
every 16 months. At the
very least, then, the FIRST catalog of sources will grow with time.
More confusing for the potential user, however, the catalog of sources
for some particular region of the sky may change with a new release.
This will be generally true for regions currently at the edge of the
survey for which a previously constructed coadded image will be
enhanced as new fields become available. Likewise, sources in the
vicinity of internal gaps will change when the gaps are subsequently
filled. In addition, source positions and flux densities can be
expected to change at the one-sigma level as new data are added. At
worst, some sources will disappear and others appear with new releases
of the catalog as coverage gaps are filled and as we develop more
sophisticated algorithms to exclude spurious sources. It is our
intention to archive each release of the FIRST catalog so that
researchers using a particular version of the catalog will have
continuing access to that version. Instructions for accessing all FIRST
data products are available via the FIRST homepage
(http://sundog.stsci.edu).
Acknowledgments
The success of the FIRST survey is in large measure due to the generous support of a number of organizations. In particular, we acknowledge support from the NRAO, the NSF (grants AST-94-19906 and AST-94-21178), IGPP/LLNL, the STScI, NATO, the National Geographic Society (grant NGS No. 5393-094), Columbia University, and Sun Microsystems. We also thank Columbia undergraduates Ali Kinkhabwala, Beth Willman, and Gabe Perez-Giz for assistance in performing the comparisons of the FIRST catalog with the data from other wavelength regimes and with the Lehar high-resolution survey. This paper is Contribution Number 601 of the Columbia Astrophysics Laboratory.
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