[vsnet-chat 2434] USNO-A2.0 zero-point near NGC 2986

[Brian Skiff <bas@lowell.edu>]

     I have examined magnitude offsets for USNO-A2.0 stars in the same sky
survey field as NGC 2986 (and SN 1999gh).  There are three sequences within
about 2 degrees of the supernova, covering the range 11.9 < V < 15.3 with
eleven stars.  The conclusions thus apply only to stars brighter than about
mag. 16.
     Particularly at the faint end, the zero-point offset of USNO-A2.0 'red'
magnitudes is close to zero in all three sequences.  Thus CCD observers
can use these magnitudes to adjust directly to Cousins R within 0.1 mag. or
so using stars with red magnitudes brighter than red mag. 16.
     Visual observers might do well by simply adding 0.5 to the A2.0 red
magnitudes (making them fainter by that much) and using those until a proper
sequence becomes available.
     The A2.0 'blue' magnitudes in the same Dec band are consistently too
bright by 0.5 mag. compared to Johnson B.  In the southern part of the field
(where only the UK/ESO Schmidt plates were used for USNO-A), the blue
magnitudes are off by -1.1 mag. (too bright).

\Brian

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