Re: brief note on astrometry of Mark IV comatic images

[Chris Albertson <calbertson@logicon.com>]

Herb,

I think the problem with your suggestion of sorting a star list
has one problem.  Tom's detection software likely throws away
the stuff you are looking for.  The software likely only reports
"nice clean uncontaminated stars".  If the stars are to close
(a pixel or two) the software reports one star.  If they are
a little farther apart it thinks it is an extended object and
reports nothing.  If they are even farther then it reports two
stars.  So I would doubt that there are any very close (within
say 2*FWHM) doubles to be found in the star lists even if there
where hundreds of these in the image.

Still, the spreadsheet trick is so easy to do, why not do it?
Sort by X then compute distance to star above and then then
count number of distances < n.  

Herbert R Johnson wrote:
> 
> On Thu, 01 Jun 2000 00:47:13 +0100, Andrew Bennett <andrew.bennett@ns.sympatico.ca> wrote:
> *>On Sun, 28 May 2000 19:34:29 -0500, you wrote:
> *>
> *>>I find that I am getting 6000 or so stars per medium star density image=20
> *>>when I set things to detect to mag 15 or so.   This is 600+ pixels per=20
> *>>star.  On these images it is hard to find star pairs where it might be a=
> *>=20
> *>>problem to separate them.   I am waiting patiently to get an image in =
> *>the=20
> *>>milky way to look at.  It looks (by eye) pretty easy to separate stars 5=
> *>=20
> *>>pixels apart.
> *>
> *>Confusion errors are insidious. They don't show
> *>up on image to image comparisons unless you change
> *>the PSF i.e buy a bigger telescope. I just got out=20
> 
> Tom, if you have a list of 6000 detections, how about sorting them by
> location and see if any of them overlap? For instance, if your software
> produces a list by pixel location, and uses an aperture of say five pixels;
> would it "find" an overlapping pair as two candidates with positions that are
> LESS than five pixels apart? (Make sure your software does not automatically
> reject such candidates.)
> 
> Er, "sort" is a MS-DOS utility program. If you do this manually remember
> to use root 2 times the aperture as the distance for each axis - in
> my example that's 7 pixels or so. A QBASIC program could read a sorted
> list and look if X (or Y) changes by less than 7, and print both lines.
> 
> If your list was in a database, or even a spreadsheet, I
> imagine a proper query would find them. No doubt one of our colleagues
> could take your list and do this work in less than a hour, given the
> proper info and your list.
> 
> Herb Johnson
> 
> Herbert R. Johnson              http://pluto.njcc.com/~hjohnson
> hjohnson@pluto.njcc.com         voice 609-771-1503, New Jersey USA
>              amateur astronomer and astro-tour guide
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-- 
  Chris Albertson

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