[vsnet-alert 4356] Re: very curious spectra of SN 2000H

[Brian Skiff <bas@lowell.edu>]

     In re Yamaoka-san's comments about a common photometric scale around
SN 2000H in the galaxy IC 454, I have examined the offsets in the USNO-A2.0
(not A1.0) 'red' magnitudes against a deep BVRI sequence by Arne Henden in
a field close to 1 degree south, which was made around GRB 991014.  From
14 stars in the range 13.7 < R < 18.1, the mean offset from Cousins R of the
A2.0 'red' magnitudes is -0.28 +/- 0.08.  In other words the A2.0 magnitudes
are too bright by this much on average; apart from a few outliers, the scatter
is commendably small.  Thus one can use:  R = mr + 0.28 (where 'mr' are A2.0
red magnitudes) to adjust to the standard R system.  Be sure to use all the
reasonably bright and isolated stars on your frames (at least a dozen, but more
than ~25 will be overkill) to determine the mean, since in a small sample just
one or two bad stars can throw your observations off.  This should do as a
stopgap until a proper sequence becomes available.

\Brian

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