[vsnet-chat 5870] Re: GSC vs Tyc2

[Michael Linnolt <linnolt@hawaii.edu>]

On Thu, 9 Jan 2003, Taichi Kato wrote:

> Re: [vsnet-chat 5865] Re: GSC vs Tyc2
>
>    It would be a good idea for Yoshida-san to devise an "expert system"
> (or some sort of artificial intelligence) for PIXY to automatically select
> the best catalog.

While Tycho2 is not perfect, GSC is much worse. The only use I ever made
of GSC was finding comps stars fainter than 12-13, where Tycho2 stops. And
then, I had to determine a "zero-point" offset for GSC by doing a
statistical comparison of the common GSC and Tycho2 stars near an object
of interest. Typically there would be significant systematic errors in
GSC, from 0.5 to 1 magnitude. These would vary from region to region, too.
GSC lists standard errors on their F,J,V bands typically around 0.4mag,
but they disclaim that as not meaningful in the standard statistical
sense, which is obviously the case if you see the types of major
systematic errors it has. Also, most mags in GSC are given in these
strange photographic bands, which are more different from standard V than
Tycho2. WHile Tycho2 standard errors get larger below mag 11-12, at least
they are statistically valid and using the mean of an ensemble of stars
should provide a reasonable degree of accuracy.

In summary, I cannot fathom any normal situation to favor GSC over Tycho2,
if the stars are listed in both catalogs. (Except maybe if you are doing a
photographic survey with these old red and blue emulsions!)

Mike Linnolt

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