taskGNU Astronomy Utilities - Tasks: task #14131, Utility to match catalogs of...


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task #14131: Utility to match catalogs of objects and find warpings

Submitted by:  Mohammad Akhlaghi <makhlaghi>
Submitted on:  Thu 25 Aug 2016 08:13:53 AM UTC  
Should Start On:  Wed 24 Aug 2016 03:00:00 PM UTC Should be Finished on:  Wed 24 Aug 2016 03:00:00 PM UTC
Category:  New program Priority:  5 - Normal
Item Group:  None Status:  Postponed
Privacy:  Public Percent Complete:  50%
Assigned to:  None Open/Closed:  Open
Effort:  0.00

Sat 02 Dec 2017 07:49:45 PM UTC, comment #1:

The new Match program, will now match two catalogs based on positions.

The most recent tarball (which now includes Match from now on) can always be checked at http://akhlaghi.org/gnuastro-latest.tar.lz

Its matching is actually done through a library function that can later be used to find the proper warping between two images to register them.

Later, we can also implement a feature that given two images (instead of tables), it finds the warping matrix that must be applied to the second input so it comes to the same grid as the first. The actual warping can later be done by Warp.

Mohammad Akhlaghi <makhlaghi>
Project Administrator
Thu 25 Aug 2016 08:13:53 AM UTC, original submission:

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).

Mohammad Akhlaghi <makhlaghi>
Project Administrator


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