[vsnet-chat 1202] Re: RY Ser position

[aah@nofs.navy.mil]

  Jan Manek gave an astrometric position for RY Ser, with ~0.1" residuals.  I just wanted
to insert the comment that these errors are internal, not external.  The GSC catalog he
used for the reference frame is known to have systematic errors much larger than this
(such as near plate boundaries), as does the USNO-A1.0 catalog (since it used the GSC as
its reference frame).  The USNO-A2.0 catalog will use Tycho and so will eliminate most of the 
systematic effects.
  We run into this quite often when comparing radio positions to USNO-A derived optical
positions.  The solution may be quite good, with residuals in the 0.1arcsec range, but when 
overlaying the two sets of positions you may see 0.5-1.0arcsec shifts.  It is sobering
that current astronomical technology is such that small offsets like that are important.
Arne

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