Re: Minimum spanning circles and Voronoi diagrams

[Tom Droege <tdroege2@earthlink.net>]

Chris,

OK, consider the following situation.  A cosmic ray hits slightly off 
center for a star.  This increases the number of counts in the star, and 
shifts its position toward the cosmic ray.  Since the total photometry has 
more counts in it, the error computed is *smaller* than if the cosmic ray 
had not hit.  Further, the photometry is off, the star is measured as too 
bright.  You can't just use the error that comes out of the 
photometry.  You have to know the other information.

In this case, the only thing you know is that this position measurement of 
the star is different in position from most of the other measurements of 
the star.

Another example.  A little high thin cloud will spread out the star 
psf.  Now two close stars that might have been separated previously will 
appear to be one star.  Don't argue that this can't happen.  There is some 
set of positions for the two stars where a little cloud will cause stars 
that were previously separated to blend into one.  Again the blend will 
cause the position to change over the clear sky case.

I think the error is not enough.  We will have to carry some markers along 
with each measurement.

Tom Droege

At 08:38 PM 2/6/02 -0800, you wrote:
>Tom Droege wrote:
> >
> > I cannot imagine that one can do as well "deblending" as one does with a
> > clean image.  So I would want to mark such measurements as "not first 
> quality".
>
>I'm not 100% sure but I think the mark of "not first quality" is
>a larger error bar.  The neat thing about using a database is
>that you can specify a selection criteria when you take the
>data back out.  We just need to keep processing history and error
>bars for every data point.  We did this for thr Mark III data
>mostly.  I see Mk IV as just "much more of the same".