[vsnet-chat 2622] Catalogue offset near NGC 6951

[Brian Skiff <bas@lowell.edu>]

     In re photometry near NGC 6951, and in particular the sequence provided
on the AAVSO chart for the field, I note that the only available calibration in
the region (that I know of) is the GSPC sequence P074, which lies about 2.5
degrees away.  The sequence is shown below along with GSC magnitudes and 
USNO-A2.0 (not A1.0) blue and red magnitudes for the same stars.  At least for
the sequence stars, the GSC magnitudes match V pretty well for stars of
ordinary color; similarly, the USNO-A2.0 blue and red are both fairly close to
standard B and R.  Thus until such time as a proper sequence becomes available,
CCD observers can probably get reasonably close to the standard scale using
either the GSC stars to adjust to V, _or_ USNO-A2.0 red magnitudes to adjust
to R.  Note that one should use as many stars as possible to make this
adjustment---just one or two stars will not give so good a result, since the
offset (or lack) applies only to an ensemble mean, not for a single star.
Note also that in USNO-A1.0 the blue magnitudes are too faint by a few tenths
and the red magnitudes are too bright by a similar amount.

\Brian



                                                                 USNO-A2.0
Name        RA  (2000)  Dec          GSC       V    B-V   mGSC    mb    mr
P074-B   20 56 34.7  +65 36 32    4255-0754  10.01  0.55  10.00
P074-D   20 53 55.0  +65 35 08    4255-1478  11.83  1.31  11.62
P074-E   20 56 47.5  +65 31 22    4255-1720  13.09  0.75  12.92  13.9  13.0
P074-F   20 54 53.7  +65 32 47    4255-1511  14.00  0.84  13.79  14.7  13.7
P074-G   20 55 14.1  +65 34 07    4255-0136  14.49  0.93  14.44  15.4  14.0

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