Using the USNO A 1.0 Catalog

Even 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  =                0.652

provide 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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Because there are so many UAC stars in the image, the maximum number of matches was increased from the default 25 to 50 using -h.

$ imwcs -d test.sex -c ua1 -h 50 test.fits

The verbose mode gives more information about how the fit is going:

$ imwcs -vd test.sex -c ua1 -h 50 test.fits

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The final image contains standard FITS world coordinate system keywords and a logging keyword showing what version of imwcs fit the WCS:

CRPIX1  =             512.0000
CRPIX2  =             512.0000
RA      = '16:13:59.717'
DEC     = '+31:32:29.74'
EQUINOX =                 1950
EPOCH   =                 1950
RADECSYS= 'FK4'
CTYPE1  = 'RA---TAN'
CRVAL1  =        243.498821820
CDELT1  =         -0.000178979
CROTA1  =               -0.387
CTYPE2  = 'DEC--TAN'
CRVAL2  =         31.541593517
CDELT2  =          0.000178828
CROTA2  =               -0.387
IMWCS   = 'IMWCS 1.7.1, 7 Nov 1997, Doug Mink, SAO'

Last updated 25 March 1999 by Doug Mink dmink@cfa.harvard.edu