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RE: Minimum spanning circles and Voronoi diagrams
Why would the error in RA and DEC not be equal? The only
thing I can think of is that at non-equatorial areas an
RA degree is not the same as a DEC degree. I'll bet the
one sigma error angle as seen from the earth's center is
the same both ways. After all our pixels are square.
I think the error is constant if you use great circle
angles.
In the most general case you are right. we may want to use
more parameters like brightness and color to help
match. Now you have four dimensional "hyper blob" (or
whatever they call it.)
Try this: If you know the one sigma error in both directions
then you can compute the sigma value for any point on the
plain. The funtion would look like a little hill centered on
the star.
I was thinking that if you had two stars you could superimpose
the two hills. You'd have a double peaked hill. Now look
at the hieght of the saddle between the two peaks if > N
(with N about 2) then you have one star not two.
Finding the minima of a line connecting the two stars is
not to hard to compute
Still, I'd just compute the angle between the stars and
apply a threshold.
In the real world the problem is that in some images double
stars resolve and in others they do not. When they do resolve
they are both inside each other's one sigma error bound. Now
when the double does not resolve which of the pair is it asigned
to? None of the above address this problem and it is the
one we will see. If not for this my simple thresholding
would work
--- "Creager, Robert S" <CreagRS@LOUISVILLE.STORTEK.COM> wrote:
>
> > -----Original Message-----
> > From: Chris Albertson [mailto:chrisalbertson90278@yahoo.com]
> >
>
> You are saying just use the stddev number for the set? Yeah, but how
> do you
> intelligently combine both the ra and dec error bars into one number?
> This
> why I thought of the MEC, as using the radius to determine the amount
> of
> scatter. And, if there is a significant difference between the
> circle
> center and the mean position, then there must be outliers.
>
> > I think you want to simply define a minimum angle threshold
> > based on the error bars on the astrometry. 15 arcseconds
> > worked for the Mk III.
>
> Actually, I am. But, there may end up being lots of points, and I
> like
> doing unusual stuff in code. Besides, with 100 points, you have 4950
> computations. By computing the convex hull, I get small set of
> points. My
> ulterior motive is learning. Which is why I joined the Tass group in
> the
> first place :-)
>
> >
> > Near the equator why not just use SQRT(ra^2 + dec^2) with
> > ra and dec in degrees to compute the distance. It's fast.
> > You can cut down the search by first applying a simple box
> > function
> >
>
> I'll have to take a look at this. Thankfully, there is an SLA perl
> module
> Astro::SLA, so this would fit nicely with the rest of my stuff.
> Thanks for
> the pointer.
>
> >
> > Or you could simply link to a good library like "SLALIB"
> > and call the "DESEP" function and get an debugged, exact
> > answer. This is in FORTRAN but I have a C wrapper library
> > that makes using it easy from C. Works for me on both
> > Linux and Solaris.
> >
> > http://www.starlink.rl.ac.uk/star/docs/sun67.htx/sun67.html#xref_
> >
> > the SLA library was recommended to me by people on this list.
> > I agree now. You really need this if you are going to do
> > anything connected with astronomical coordinates. It is the
> > "standard" library that is in wide use. This problem is
> > not hard but others are
> > For example, pretty soon you may want to match catalog data
> > that is in a different referance system or you need to precess
> > the data to a diffent epoc.
> >
=====
Chris Albertson
Home: 310-376-1029 chrisalbertson90278@yahoo.com
Cell: 310-990-7550
Office: 310-336-5189 Christopher.J.Albertson@aero.org
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