[vsnet-chat 4617] Re: Astrometry from different sources [was:Re: V589

[aah@nofs.navy.mil]

Gianluca asked:
>I was wondering if it was just a coincidence to have my position closer
>to the USNO-A possible counterpart, considering that I worked inside the
>USNO A2.0 frame of reference. In other words, how important, in
>comparing something , is the possibility to work inside the same
>framework? 

Yes, I would consider it entirely a coincidence.  The position of
the star in USNO-A2.0 is based on 1950's plate material
and represents its position in the ICRS reference
frame at that epoch +/- 0.2 arcsec.  Your astrometry
represents its position in the ICRS reference frame
for epoch 2001.  Both USNO-A2.0 and GSC-ACT are tied
to the ICRS reference frame, so if you use enough reference stars
to counteract any proper motion, then the measured
position from identical current-epoch images should be the
same (and is, within the measurement errors).  However,
I will say that the positions derived from USNO-A2.0 are less
reliable than those from GSC-ACT in general since the
GSC plate material is 30 years newer.  The best astrometric
source is Tycho2; the best faint astrometric source will
be UCAC when it is finally released in its entirety.
  In fact, with these CVs, it is *more* likely that
your position at epoch 2001 will not match the epoch 1950
position due to proper motion of the CV between the
two epochs.  They are intrinsically faint and therefore
tend to be nearby (a few hundred parsecs).
  If you want to positively identify a star between epochs,
you need both accurate positions at both epochs and some
estimate of the proper motion.  If you have some other
information that confirms the identification (similar magnitude
or color, or an outburst at the earlier epoch), then you can
go the other way:  assume the two objects are the same and
measure the proper motion.
  This seems like a trivial matter, but has become important in
modern astronomy.  There are many cases where you are trying
to match positions in different energy regimes (X-ray to radio,
for example) to see how structural features compare, or
finding where on a host galaxy an optical counterpart of
a gamma-ray burst resides (you can't see the host galaxy
until the optical counterpart fades away).  You not only need
to measure the position to the best of your ability, but also
give reliable estimates of the errors in your measurement.
Arne

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