# [AstroPy] Cross-Identification of stars in a collection of images

Joe Philip Ninan indiajoe@gmail....
Thu Mar 15 15:17:36 CDT 2012

Hi  Joe Harrington,
Wouldn't using the entire image to calculate the transformation matrix
using minimizing algorithms be a lot more time consuming? Especially when
no initial guess is given.
On the other hand, the triangulation method in the end requires to solve
only 2 linear equations using the brightest (say 10) of the common star.
Only sorting out the brightest stars from the daofind output will be time
consuming.
The solving for transformation coefficients by least squares is done by
SVD, and it wouldn't take much time compared to sorting time. (Note: we are
not using all the stars; only the brightest few to make all possible
combinations of triangles. Number of equations is only equal to no# of
stars used.)

Anyway xyxymatch in stsci package solves my problem right now.

-cheers
Joe

On 16 March 2012 00:56, Joe Harrington <jh@physics.ucf.edu> wrote:

> You could also try something simpler, based on image morphing.  Write
> the transformation matrix that scales, shifts, and rotates image 1 to
> mimic image 2.  Calculate the reduced chi-squared between the images,
> possibly with a background offset.  Put that into a function minimizer
> and solve for the elements of the transformation matrix as free
> parameters.  You'll need an initial guess at the parameters that is
> pretty good, which you can get from manually matching a few stars.
>
> Then, use the fitted matrix to transform all the coordinates of the
> identified stars in image 1 to the coordinates of image 2.  Clean those
> up if you need to by re-running your centering routine with those
> positions as initial guesses.  It sounds complicated, but I'm betting
> it's a lot fewer calcs than the gigatriangles talked about in the page
> you cited (though I didn't read thoroughly, maybe there was a shortcut),
> and there may already be image morphing code that does the ugly part.
> Or, use an optical distortions code and fit for the polynomial
> coefficients.  That would handle things like differential refraction, if
> these are ground-based images.
>
> A thornier problem is tracking features in pairs of mapped giant-planet
> images.  There you use a full image-morphing routine to derive a fluid
> flow field between two images.  If you have access to one of those (I
> don't), the star-matching problem seems like a relatively simple
> sub-case.
>
> Finally, some of our exoplanet transit survey friends might have a
> routine they are willing to share.
>
> --jh--
>
> Date: Thu, 15 Mar 2012 16:47:57 +0530
> From: Joe Philip Ninan <indiajoe@gmail.com>
> Subject: Re: [AstroPy] Cross-Identification of stars in a collection
>        of      images
> To: Richard P Wagner <rpwagner@sdsc.edu>, wkerzendorf@gmail.com
> Cc: astropy <astropy@scipy.org>
> Message-ID:
>        <CAJ_T+b7PK9hmedLRE_xwxrDkATbUAGZZhJq61O_LbxC-2AF-Fw@mail.gmail.com
> >
> Content-Type: text/plain; charset="iso-8859-1"
>
> Hi,
> The blog[1] Richard mentioned seems to be dealing with matching the stars
> using their RA and Dec.
> Even the code mentioned in the last comment of the page seems to be for
> that.
> I was looking for a code which matches stars from frame to frame using
> their pixel coordinates.
> The http://astrometry.net suggested by Wolfgang does what i wanted.
> But as he suggested, i think i shall write a function in python itself to
> do that, so that we can use it for general purpose.
> I am planning to use the algorithm mentioned by Peter B. Stetson in his
> 'The Techniques of Least squares and stellar Photometry with CCDs'  [2].
> Thanking you all for suggestions,
> -cheers
> Joe
>
> [2] http://ned.ipac.caltech.edu/level5/Stetson/Stetson5_2.html
>
>
> On 14 March 2012 21:49, Richard P Wagner <rpwagner@sdsc.edu> wrote:
>
> > Hi Joe,
> >
> > On Mar 14, 2012, at 7:13 AM, Joe Philip Ninan wrote:
> >
> > > Hi,
> > > Is there any python function to cross-identify the stars between
> > different frames and calculate the coefficients of coordinate
> > transformation equations?
> > > If not, i am planning to implement it by first running daofind using
> > pyraf and then trying to match triangles; starting from brightest stars.
> > > Is there any better/easier way to do this?
> > > Thanking you,
> > > Joe
> >
> > I don't tackle observational data myself, but I know that others work on
> > cross-matching regularly. You might find this AstroPython blog post a
> > useful starting point [1].
> >
> > --Rick
> >
> > [1]
> > http://www.astropython.org/blog/2010/4/Question-Cross-matching-in-python
> _______________________________________________
> AstroPy mailing list
> AstroPy@scipy.org
> http://mail.scipy.org/mailman/listinfo/astropy
>

--
/---------------------------------------------------------------
"GNU/Linux: because a PC is a terrible thing to waste" -  GNU Generation

************************************************