Re: GSC and DS23

[Patrick Wils <patrickwils@yahoo.com>]

--- aah@nofs.navy.mil wrote:
>   It is not likely that a GSC star more than a few
> arcsec
> away from a TASS object is physically associated. 
> The astrometry
> from Michael R.'s pipeline is really pretty good. 
> As you
> increase such a search radius, the chances of
> coincidence
> increase.  

I did the search to 1' just to find out the radius
where real counterparts end, and pure coincidence of
finding a star in the neighbourhood, takes over.  This
is at around a distance of 24", where the number of
matches reaches a minimum.  This should be compared to
the matching radius of 12" use in collect_stars.pl,
and Tom's comments:

--- tdroege2@earthlink.net wrote
> collect_stars.pl uses Michael's subroutine with the 
> match radius set to 12 
> arc seconds.  This is actually a box of 24 arc 
> seconds square, so we could 
> get two stars 34 arc seconds apart in the worst 
> case.  As I pointed out 
> before, I settled on 12 arc seconds based on 
> plotting the number of stars 
> found using different values for the match radius. 
> At 10 there is little 
> decrease in the number of stars found.  It does not 
> start decreasing again 
> until 25 or so.  

66% of all matches with the GSC fall within a distance
of 2", 95% within 7" and 99% within 10".  

> You need to do, at minimum,
> matching
> with magnitude as well as just position.

I also compared the TASS V and GSC magnitudes (I know
these are not in the same color, but it gives some
indication as to whether the same star is concerned). 
For 85% of the objects the magnitude difference is
less than 0.5 mag.  In 98.6% of the cases it is less
than 1 mag.  So in most cases the match is genuine.

Patrick



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