[vsnet-chat 2718] Re: Supernova Magnitude Observations

[Brian Skiff <bas@lowell.edu>]

     Yamaoka-san asks about the comparison between the various "large"
star catalogues for astrometry.  Some months ago I made a simple match
between USNO-A2.0 and stars in the Sloan calibration regions, which were
done using the USNO-Flagstaff 20cm transit telescope (with CCD).  This
provides positions near the current epoch for some hundreds of thousands
of faint stars near the equator with about 50 milliarcsec (mas) accuracy,
i.e. essentially zero error when compared to the scans of photgraphic
plates.  The result was that the mean errors for fainter stars in USNO-A2.0
were about 0".5 on a star-by-star basis.  The systematic errors are quite
small.
     The GSC v1.2 and Gray's GSC-ACT are very similar.  The first is a
re-reduction of the original GSC but using the PPM catalogue as the
reference net; the second (obviously) is a re-reduction using the better
ACT catalogue as the net.  Both are very similar in that the sometimes-large
systematic errors in the old GSC are removed almost entirely.  The _internal_
errors are the same (0".2-0".4) naturally, since no improvement in the scans
themselves was made.  Given that the mean epoch of the GSC plates is now
getting close to 20 years old, current errors in these catalogues has grown
somewhat, to perhaps 0".4  on average per-star.  Some details about the
improvement of the GSC can be found at Jure Skvarc's Web site:

http://kastor.ijs.si/~jure/fitsblink/astrometry/astrometry.html

...which if nothing else should convince you that GSC v1.1 should be avoided!
     The current catalogue-of-precision for those with reasonably large
CCD fields is now Tycho-2.  This contains 2.5 million stars and has mean
errors for the fainter stars of about 50 mas (0".05).  This catalogue
supercedes all other such works (SAO, AGK3, PPM, Tycho-1, ACT, TRC), which
are now obsolete.
     The SDSS calibration regions are described in the "README" file 
at the USNO-Flagstaff ftp area starting at:

http://ftp.nofs.navy.mil/pub/outgoing/calibregion

Looking at this just now, I see that this contains 1.2 million stars down to
mag. 17 or thereabouts in 16 regions along the equator.  If you want to see
what your external errors are in reducing CCD astrometry, use these stars,
which for most observers will have vanishingly-small errors, i.e. all the
scatter you measure will be in your frames, not in the reference net.

\Brian

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