[vsnet-chat 1248] Re: RY Ser position

[Jan Manek <jmanek@mbox.vol.cz>]

Arne Henden wrote:
>  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.  

Yes these are really internal.

>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.

I am aware that GSC versions 1.0 and 1.1 exhibit quite large error as Arne
mentions. However I have used GSC 1.2 version, which is a 'new' PPM based
re-reduction with significantly reduced plate-based position dependency and
also magnitude dependency.

>  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.

Accuracy of GSC 1.2 is much better compared to earlier versions. The stated
absolute precision  should be 0.3" (more about GSC 1.2 can be found at
http://vsnet-gsss.stsci.edu/gsc/gsc12/description.html). The position for RY
Ser should have about that absolute accuracy. My personal experience show
absolute errors (compared to ACT) not larger that 0.2". 

The prepared GSC 2.0 should also be based on HIPPARCOS/TYCHO/ACT catalogues
and should even contain proper motions for faint stars. See
http://vsnet-gsss.stsci.edu/gsc/GSC.HTML

Jan

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