[vsnet-chat 4556] (fwd) Re: [vsnet-alert 6017] Re: Possible Nova in Sco

[Taichi Kato <tkato>]

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

From: gian.masi@flashnet.it
Subject: Re: [vsnet-alert 6017] Re: Possible Nova in Sco
Date: Fri, 22 Jun 2001 19:09:17 +0200 (CEST)

Dear Arne,

> 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.

I appreciated your previous note, as I find your comments 
very useful to improve my knowledge of these topics. I used 
only 8 reference stars as I selected them manually (no 
automatic astrometry!), looking for single, bright -but not 
saturated- stars.

>   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!).

I'll let you know when I'll measure with this other 
catalogue. By the way: is there any place where I can order 
a CD with this UCAC? Also,k where I can learn more on this 
cat on the web?

Sincerely,
Gianluca

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