Using the USNO-A2.0 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 A2.0 Catalog has more than half a billion of them.

Click on image for larger version

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 imua2 to plot the catalog over the image. That WCS is also used by imwcs as an initial value.

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 Objects: detected 504 / sextracted 503 > All done $ sort -n +2.0 image.sex | head -100 > test.sex

Click on image for larger version
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 ua2 -h 50 test.fits

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

$ imwcs -vd test.sex -c ua2 -h 50 test.fits

Click on image for larger version

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

CRPIX1  =             514.5489
CRPIX2  =             512.2926
RA      = '16:15:56.745'
DEC     = '+31:25:06.83'
EQUINOX =                 2000
EPOCH   =                 2000
RADECSYS= 'FK5'
CTYPE1  = 'RA---TAN'
CTYPE2  = 'DEC--TAN'
CRVAL1  =        243.986435732
CRVAL2  =         31.418563763
CD1_1   =         -0.000179103
CD1_2   =          0.000000679
CD2_1   =          0.000000291
CD2_2   =          0.000178770
IMWCS   = '2.6, 9 March 1999, Doug Mink, SAO'

Last updated 29 March 1999 by Doug Mink dmink@cfa.harvard.edu