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[task #15637] Match RA and Dec catalog to X and Y catalog to find WCS

From: Mohammad Akhlaghi
Subject: [task #15637] Match RA and Dec catalog to X and Y catalog to find WCS
Date: Mon, 11 May 2020 21:15:24 -0400 (EDT)
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:76.0) Gecko/20100101 Firefox/76.0


                 Summary: Match RA and Dec catalog to X and Y catalog to find
                 Project: GNU Astronomy Utilities
            Submitted by: makhlaghi
            Submitted on: Tue 12 May 2020 02:15:22 AM BST
         Should Start On: Tue 12 May 2020 12:00:00 AM BST
   Should be Finished on: Tue 12 May 2020 12:00:00 AM BST
                Category: Match
                Priority: 5 - Normal
              Item Group: Enhancement
                  Status: In Progress
                 Privacy: Public
        Percent Complete: 0%
             Assigned to: sks_15
             Open/Closed: Open
         Discussion Lock: Any
                  Effort: 0.00



Let's assume that `reference-ra-dec.fits' is a single catalog that contains
the RA and Dec many sources from a reference source, for example from the Gaia
Archive <>. 

We also have multiple single exposure images that are named `img1.fits',
`img2.fits', `img3.fits' and etc. We assume that these images partially cover
the area of the reference catalog.

We then run NoiseChisel, Segment and MakeCatalog on each image to generate a
catalog of the clumps (which can be used to accurately define the center of
each source in the image) where the clump centers are in the image
coordinates: X and Y. Let's assume the catalogs are named `img1-x-y.fits',
`img2-x-y.fits', `img3-x-y.fits'.

This is the problem: we want to find the WCS of each image by matching the
individual X-Y catalogs to the RA-Dec catalog. With that WCS, we will be able
to align the images to a single pixel grid (task #15636) and do science with

The proposed interface and usage is like this:

astmatch --wcs-reference=reference-ra-dec.fits --wcscol=RA,DEC img*-x-y.fits

Normally Match takes two catalogs (to find the rows that match on  certain
coordinates). But when the `--wcs-reference' option is given, it can take any
number of input catalogs. `--wcs-reference' itself will take a single catalog
as value and use the columns specified with the `--wcscol' option.

Any number of X-Y catalogs are accepted and Match will simultaneously match
them with then RA-Dec catalog and produce a separate FITS file for each input
catalog (maybe called `img1-x-y-wcs.fits'). The FITS file won't have any data,
it will just be a header with the WCS written inside of it.

I am proposing to take multiple X-Y catalos to allow more accurate estimation
of the (optical/spherical) distortions when the inputs are exposures taken
with a single imager. In this scenario, each image will be dithered/off-set
compared to other images, but they all share the same distortions so the more
raw exposures we have, the better we are able to estimate the distortion
coefficients to implement in all the output files.

In the matching, beyond using the raw RA and Dec, we can also use the
magnitudes/brightnesses of each source and include that as a dimension to

Also, since some low-level structure like a k-d tree
<> may be necessary to optimally parse
the reference catalog, we can add a feature to estimate such optimized
internal structures in one run and use them later. Having this structure in a
file that can be directly used will probably greatly speed up the processing
in many scenarios. 


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