[vsnet-chat 4622] Re: Astrometry from different sources [was:Re: V589 Her]

[Brian Skiff <bas@lowell.edu>]

     A good way to assess errors in one's astrometry is to measure several
images of a field, not just a single frame.  Place the target of interest
in a different location for each exposure (say halfway from the center
toward each corner of the CCD for four exposures).  Using USNO-A2.0, you'll
get a slightly different reference net for each picture, and the different
location of the stars with respect to the pixels will introduce very small
differences in the reductions, plus the optical geometric errors particular
to your telescope set-up (focus, tracking, seeing, tilt of the CCD, etc).
By then lookig at the scatter in your several measurements, you'll have
a much better estimate of what your real extenal errors are.  Almost always
the error estimate found from the residuals in the fit to the reference
stars from a single image will underestimate the "true" external errors,
typically by a factor of two or three.  Thus claimed uncertainties in 
supernova positions (for example on recent IAU Circulars) of 0".2-0".3 are
much too small, and are much larger, plainly shown when other observers
measure the same object on different images, even when there are no
problems with crowding, etc.

\Brian

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