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

[Mohammad Akhlaghi]

Follow-up Comment #1, task #15637 (project gnuastro):

To help in completing this task, I just uploaded some data to play with. A
link to each uploaded file is available at the bottom of this comment.

The main datasets are 8 short exposure (5 sec) images of Castor
<https://en.wikipedia.org/wiki/Castor_(star)>, but each exposure is offset
compared to the others (this offset in individual exposures is called
dithering in astronomy). Please open these images and actually see how
Castor's position in the image changes in each of them.

A catalog of sources (actually "Clumps" in Segment
<https://www.gnu.org/software/gnuastro/manual/html_node/Segment.html>) is
generated for each image using NoiseChisel+Segment+MakeCatalog using the
script below (which is also available under that directory), the catalogs are
the actual inputs into this task and they have a `-x-y.fits' suffix. Each
catalog contains roughly 1500 clumps. Finally, there is also a reference
catalog with RA and Dec of 1179744 sources near Castor from ESO's Gaia
survey.

Once this task is complete, we should be able to have an accurate WCS
(including distortions) for each image.

Script to generate a catalog of sources in each image and their X and Y
positions.

# Base name of input files.
input="1881731715 3163025249 1887227377 3746244531 2923938133 4209692437
2966790193 595260509"

# Generate the catalog for each image.
for i in $input; do

    # First run NoiseChisel to separate signal from the background.
    astnoisechisel $i.fits --tilesize=50,50 --interpnumngb=21 \
                   --output=$i-nc.fits

    # Run Segment to get a label for each clump, don't bother with
    # detecting objects, they aren't relevant here.
    astsegment $i-nc.fits --onlyclumps --output=$i-seg.fits

    # Generate a catalog with the ID, X, Y, and magnitude (assuming a
    # zeropoint of 0).
    astmkcatalog $i-seg.fits --hdu=CLUMPS --ids --x --y --magnitude \
                 --output=$i-x-y.fits

    # Clean up.
    rm $i-nc.fits $i-seg.fits
done


Reference Catalog with RA and Dec of many sources:
http://akhlaghi.org/data/astrometry/gaia-dr2-near-castor.fits

List of FITS images:
http://akhlaghi.org/data/astrometry/1881731715.fits
http://akhlaghi.org/data/astrometry/1887227377.fits
http://akhlaghi.org/data/astrometry/2923938133.fits
http://akhlaghi.org/data/astrometry/2966790193.fits
http://akhlaghi.org/data/astrometry/3163025249.fits
http://akhlaghi.org/data/astrometry/3746244531.fits
http://akhlaghi.org/data/astrometry/4209692437.fits
http://akhlaghi.org/data/astrometry/595260509.fits

List of X-Y catalogs:
http://akhlaghi.org/data/astrometry/1881731715-x-y.fits
http://akhlaghi.org/data/astrometry/1887227377-x-y.fits
http://akhlaghi.org/data/astrometry/2923938133-x-y.fits
http://akhlaghi.org/data/astrometry/2966790193-x-y.fits
http://akhlaghi.org/data/astrometry/3163025249-x-y.fits
http://akhlaghi.org/data/astrometry/3746244531-x-y.fits
http://akhlaghi.org/data/astrometry/4209692437-x-y.fits
http://akhlaghi.org/data/astrometry/595260509-x-y.fits

Script to generate catalog:
http://akhlaghi.org/data/astrometry/generate-catalog.sh

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