Using the USNO-B1.0 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 B1.0 Catalog has more than 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 imub1 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

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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 ub1 -h 50 test.fits

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

$ imwcs -vd test.sex -c ub1 -h 50 test.fits

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

CRPIX1  =             514.5519
CRPIX2  =             514.0039
CD1_1   =         -0.000178918
CD1_2   =          0.000000559
CD2_1   =          0.000000285
CD2_2   =          0.000178708
WCSRFCAT= 'ub1     '
WCSIMCAT= 'testx90m.sex'
WCSMATCH=                   83
WCSNREF =                  100
WCSTOL  =               2.5000
RA      = '16:15:56.711'
DEC     = '+31:25:07.74'
WEQUINOX=                 2000
EPOCH   =                 2000
RADECSYS= 'FK5     '
CTYPE1  = 'RA---TAN'
CTYPE2  = 'DEC--TAN'
CRVAL1  =        243.986294732
CRVAL2  =         31.418816405
SECPIX1 =               0.6441
SECPIX2 =               0.6434
WCSSEP  =                0.187
EQUINOX =                 2000
IMWCS   = '3.3.0, 29 January 2003, Doug Mink (dmink@cfa.harvard.edu)'

Last updated 29 January 2003 by Doug Mink dmink@cfa.harvard.edu