[vsnet-chat 4555] (fwd) Re: [vsnet-alert 6017] Re: Possible Nova in Sco

[Taichi Kato <tkato>]

(fwd) Re: [vsnet-alert 6017] Re: Possible Nova in Sco

From: aah@nofs.navy.mil
Date: Fri, 22 Jun 2001 08:31:16 -0700
Subject: Re: [vsnet-alert 6017] Re: Possible Nova in Sco

Gianluca,
  My comment was meant to draw attention to the typical
observer that the field is crowded and normal techniques
of astrometry may not work; you have to be careful to
select stars that are clear of neighbors, not only for
your CCD astrometric extraction, but also because the
USNO-A catalog has problems with blended stars.  You
are a careful observer, and I was pretty sure that you
had taken this into account, but since there are several
thousand USNO-A stars in the field, I was wondering how
you made your selection of only 8.
  The second reason for the post was to again suggest
that people use UCAC in the galactic bulge area.  USNO-A
and GSC both have major problems with the photographic
density on the plates and give a lot of false detections
as well as poor astrometry on the blends.  UCAC has
avoided those problems with CCD astrometry and is more
modern epoch anyway.  Tycho2 is another alternative if
you have a reasonably wide field with short exposure.
If you use UCAC, I'd prefer that you use the original
distribution or Vizier, but some software packages
are not capable of doing that and so the next best
solution is the reformatted UCAC on my ftp site.  I'd
be interested in your astrometry on the field with
the UCAC catalog; I bet your residuals drop to 0.1arcsec
or so (at least mine do!).
Arne

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