[gnuastro-devel] [task #14131] Utility to match catalogs of objects and find warpings

[Mohammad Akhlaghi]

URL:
  <http://savannah.gnu.org/task/?14131>

                 Summary: Utility to match catalogs of objects and find
warpings
                 Project: GNU Astronomy Utilities
            Submitted by: makhlaghi
            Submitted on: Thu 25 Aug 2016 05:13:53 PM JST
         Should Start On: Thu 25 Aug 2016 12:00:00 AM JST
   Should be Finished on: Thu 25 Aug 2016 12:00:00 AM JST
                Category: New utility
                Priority: 5 - Normal
              Item Group: None
                  Status: Postponed
                 Privacy: Public
        Percent Complete: 10%
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
                  Effort: 0.00

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Details:

Due to issues like the cosmic rays and also to increase the resolution and
cover contiguous wide regions, astronomical images are often taken in short
exposures. Each short exposure is usually "dithered" (slightly moved) compared
to the previous exposure for better coverage and resolution. 

Therefore the separate exposures do not match or align with each other. The
hardware information (like the telescope pointing) are often not enough
compared to the very high pixel resolution. One way to do the alignment is
this: 

1. Make catalogs of sufficiently bright sources that are imaged in the
separate exposures accurately (ideally stars which are the sharpest objects,
thus making their center best defined).

2. Match those catalogs (find the corresponding objects in separate images),
by finding the warping matrix which can minimize the distance between the
matches. GSL has good minimization algorithms.

3. Warping the pixels in each image with the relevant warping matrix.

In Gnuastro, MakeCatalog and ImageWarp can finely do the first and third
steps. But we don't currently have a utility for the second step.

I have been working on a small C program to match two catalogs within a given
radius, but currently it only identifies which objects of which catalog are
closest to which object in the other. It can deal with multiple objects being
present within the search radius of both catalogs. 

So the utility for the second step above would have to progressively apply
warpings to the coordinates of one catalog and also select different maximum
search distances (which would ideally shrink) such that the RMS distance
between the catalogs is minimized. So the free parameters which need to be
minimized are the warping matrix and the maximum distance.

I currently don't have time to implement this immediately, but if anyone is
interested, I am willing to help (and share that small C program to build
upon).




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