Using the USNO A 1.0 CatalogEven if an image has enough HST Guide Stars in them to fit a world coordinate system, it is usually better to use more stars, and the US Naval Observatory A 1.0 Catalog has almost half a billion of them. |
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This FITS image
contains a galaxy whose coordinates we want to find.
The header keywords
NAXIS = 2 NAXIS1 = 1024 NAXIS2 = 1024 RA = '16:15:56.0' DEC = '31:25:10' EPOCH = 2000.0 SECPIX = 0.652provide a nominal WCS which is used at the left by skycat to plot the catalog over the image. That WCS is also used by imwcs. |
To quickly find the 100 brightest stars in the image, Emmanuel Bertin's
SExtractor program was used:
$ sex test.fits Frame: "16139p3131R" / 1024 x 1024 / 16 bits INTEGER data Background: -0.17547 RMS: 9.83951 / Threshold: 14.7593 9bjects: detected 504 / sextracted 503 > All done $ sort -n +2.0 image.sex | head -100 > test.sex |
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$ imwcs -d test.sex -c ua1 -h 50 test.fitsThe verbose mode gives more information about how the fit is going:
$ imwcs -vd test.sex -c ua1 -h 50 test.fits