[vsnet-chat 3541] Re: GSC 6780-00497

[no name <crawl@zoom.co.uk>]

> Sebastián Otero wrote:
> 
> Hi, John and vsnet friends:
>                                               colleagues at the OCR
> (Observatorio Cristo Rey) in Rosario, Argentina, took CCD pictures of
> delta Scorpii field last night and they noticed that a catalogued
> "star" was missing.
> 
> The star is GSC 6780-00497
> GSC= 16h 03m 53.17s -22º 56' 49.4"    9.55p, 1976.411
> 
> 9.62p, 1976.493
> USNO= 16h 03m 53.23s -22º 56' 48.5"  9.4B, 8.8R , 1980.844  (V= 9.0)
> 
> This would imply a B-V of 0.4 and a yellow star, but it's missing in
> the Tycho and Tycho 2 catalogues.
> In this position we can find:
> 
> PGC 56899 - ESO 516-6, a galaxy (Sbc) with a 14.94 B magnitude.
> position:   16h 03m 53.4s -22º 56' 48".
> 
> That's what is shown in all the images retrieved from DSS, ESO, etc. A
> galaxy. But there is a 5-6 magnitude discrepance. How could this be
> explained?
> 
> Best wishws,
> Sebastian.

Hi Sebastian

Basically, we have a face on spiral here [quite a pretty one by the look
of things], of pretty even surface brightness.

Digitizing the plates will see this as a star, well it has done anyway:
with long exposure the galaxy "fills" in and ends up looking like a
point source exposure of a brighter object [at least to some machines].

No extra effort was put in to discerning such things, although some
objects are flagged as "non-stars" in the catalogue.

This can happen with planetary nebulae too, amongst other things.

Memory says GSC 2.0 is supposed to be putting more effort in to this
sort of thing, though when it will be completed nobody knows.

USNO people are supposedly splitting galaxies and stars in USNO B, which
may be a bit more sooner.

Bill Gray of Project Pluto who put the GSC-ACT together hopes to use
things like proper motions derived from different epoch&equinox surveys,
massive galaxy catalogues, and what have you, to create a GSC-ACT free
of these problems : ie one only containing stars.

As his GSC-ACT was the first to clean up some astrometry problems with
the GSC, he may well sort the problem first. He made the GSC-ACT freely
available via ADC centres, so it'd be accessible to all, not just users
of Guide n.

Brian and Arne may well chip in here, which'd be useful.

The GSC is that most wondrous and useful thing that is still riddle with
glorious faults.  It was only meant as a pattern recognition device for
the HST, I think, hence the title "HST Guide Star Catalogue".

Cheers

John

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp