[vsnet-chat 2720] Re: Supernova Magnitude Observations

[Brian Skiff <bas@lowell.edu>]

     Star-catalogue choice may be driven largely by star-density rather
than by accuracy.  As long as you can get at least 9 or 10 suitable stars
for reference, then that is enough, especially if those relative few have
far higher accuracy than a more populous but lower-accuracy catalogue like
A2.0.  
     If one can get 9-10 GSC v1.2/ACT stars in the CCD frame, these stars
should have somewhat smaller errors than USNO-A2.0 for most of the sky, and
thus the revised GSCs would be preferable in this case.  Note that the GSC
contains only about 15 million entries when accounting for the multiple
records for the same object, so ~10 stars per 10'x10' frame is typical.
     A couple of other points in re SNe astrometry:  it might be helpful
to obtain average positions from several frames instead of just one,
especially if the telescope can be shifted slightly between exposures so
that slightly different reference nets are involved, and the location of
the target varies somewhat.  This will give a better idea of measurement
errors than just the fits on the stars in a single image, which can be
misleadingly small.  Also, I'd like to recommend to those who do this to
also measure the nucleus of the host galaxy.  Accurate coordinates for the
galaxies themselves are often not available, and by providing the Central
Bureau with this information, the offsets (and possible problems) can be
determined more readily.  I have requested that galaxy positions be 
published on IAUCs with supernova announcements, but without effect.  (How
many bogus coordinates for SNe have been published?!)  Certainly one should
not rely on galaxy positions found in most catalogues.  New reliable
measurements of the galaxy nucleus with current "best" reference stars will
always be useful.

\Brian

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