Example 1. Fit Image WCS Using the HST GSC

Many images will have enough HST Guide Stars in them to fit a world coordinate system to the stars found in the image.

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This sample image contains a galaxy whose coordinates we wanted to find. The header keywords

NAXIS   =                    2
NAXIS1  =                 1024
NAXIS2  =                 1024
RA      = '16:13:54.0'
DEC     = '31:31:00'
EPOCH   =               1950.0
SECPIX  =                0.652

provide a nominal WCS which can be used by imwcs as an initial guess.

To find a number of stars in the image comparable to the number of Guide Stars, the threshold for a pixel to be checked out as a possible star was set to 5000, and the program was run.

$ > imwcs -wi 5000 testx90m.fit

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

$ > imwcs -vwi 5000 testx90m.fit

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

EQUINOX =                 2000
CTYPE1  = 'RA---TAN'
CRVAL1  =              243.987
CRPIX1  =                  512
CDELT1  =         -0.000179207
CROTA1  =           -0.0727298
CTYPE2  = 'DEC--TAN'
CRVAL2  =              31.4185
CRPIX2  =                  512
CDELT2  =          0.000179207
CROTA2  =                    0
IMWCS   = 'IMWCS 1.0, 28 June 1996, Doug Mink SAO'

Click on image for larger version

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