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Notes on the use of PCRS Guide Star Catalog, 22 August 03
Based on a document submitted by Amy Mainzer, PCRS team.
Many people are not using the PCRS Guide Star Catalog to select their PCRS
(peakup) targets. While you can of course use any peakup star you'd
like, you should be aware of the stringent requirements we levied on PCRS
Guide Star Catalog objects to ensure the success of your observation.
First, we strongly encourage people to use the PCRS Guide Star Catalog
whenever possible. With a maximum spacing between guide stars of ~0.7
degrees, there should almost always be a PCRS guide star close enough to a
science target to avoid the buildup of significant gyro error. Although
the PCRS Guide Star Catalog is not yet built into Spot, it will be in a
future release. Meanwhile, the PCRS GSC is accessible here: http://ssc.spitzer.caltech.edu/pcs/PCRSGuideStarCatalog.txt
You should check the comments in the first few lines of the file to be
sure you have version 2.4f. You should also get the Explanatory Document for the catalog
which describes units, etc.
SOME IMPORTANT NOTES ON PCRS GUIDE STARS:
- PCRS is sensitive to stars with V band magnitudes 7.0 - 10. There is
NO ADVANTAGE to choosing brighter objects - the PCRS exposure calculator
automatically adjusts the integration time to compensate for lower flux.
In all cases, the dominant error terms for PCRS are systematic, not
random. Selecting the guide star that is closest to your science target
(rather than the brightest guide star within 0.7 degrees) will result in
lower pointing error.
- With only 16 pixels, PCRS cannot tell the difference between single and
multiple stars. Do not use visible binary stars to avoid having the
centroid distorted. The PCRS Guide Star Catalog is free of binaries.
- Similarly, the PCRS cannot distinguish between the target star and
any other object with a magnitude difference less than than 5.5 magnitudes
from the target star's magnitude within 60 arcsec of the target. Targets
with neighboring objects (whether stars, galaxies, etc.) can result in a
distorted centroid. For a 10th magnitude target, that means that there
can be no stars brighter than 15.5 nearby. The completeness limit of the
HST guide star catalog (which was one of the sources we used in compiling
the PCRS GSC) is 15.5. It was not possible to ensure that there were no
close neighbors near stars fainter than magnitude 10.0, despite the fact
that the PCRS is sensitive to much fainter objects.
- Use of variable stars is not recommended, unless you know what its
magnitude is to within +/-0.25 magnitudes at the epoch of the observation.
The PCRS Guide Star Catalog is free of variable stars.
- Stars with high proper motion and/or parallax are not recommended
(because over time, their positional accuracy degrades). The PCRS
Guide Star Catalog is free of these objects.
- The PCRS-GSC contains approximately 2 million stars and was,
therefore, necessarily constructed via an automated process that relied
primarily on star catalogs. Although the best effort has been made to
ensure the validity of the objects in the PCRS-GSC, it is estimated that
<5% might be questionable. Future versions of the catalog will remove
these; in the meantime, it is advisable to check for nearby neighbors
and/or overlapping (extended) background objects. If you find a star in
the PCRS-GSC that is not a valid peak-up target, please notify the Spitzer
Helpdesk, at help@spitzer.caltech.edu.
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