[vsnet-chat 4539] Re: [vsnet 2103] Photometric problem using USNO-A2.0

[Brian Skiff <bas@lowell.edu>]

     Bruce Sumner wrote:
>> [in re GSPC v2.1]  This dataset contains accurate V and R photometric
>> calibration
>> [for] stars for GSC-II, down to about magnitude 19.

     You'll find that the positions for many of the stars in the GSPC v2.1
are rather poor (like 8"-10" offsets)---have fun identifying them.  And for
nearly all the fields the photometry is garbage (i.e. errors > 0.05 mag.)
fainter than about 16.5-17.  Be very careful when using it.  I found it
so problematic that I have not added any of it to my reference file.
A gzip-compressed, trimmed, flat-ASCII copy is avialable at the Lowell
Web site:  http://ftp.lowell.edu/pub/bas/starcats/gcps2.trim.gz
This is 2.4Mb (expands to 12.5Mb) and contains only stars where the nominal
errors (which seem slightly underestimated) are < 0.05 mag. in all colors.
The data from the STScI site is a a funny FITS format, and practically all
the stars have large internal errors.  Oh, remember also there are large
numbers of duplicate entries in this list, another bit of bookkeeping that
wasn't taken care of in this release---and the reason one can tell the
errors are somewhat larger than advertised, since the scatter on the
mulitple entries show themselves plainly.

\Brian

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