Using DSS Sources

Sometimes, the field covered by an image does not contain enough stars from the HST Guide Star Catalog to fit a world coordinate system. For the image at the left with this header

NAXIS1  =                  251
NAXIS2  =                  239
DEC     =         -25:15:01.90
RA      =           5:47:56.50
PLTSCALE=                 1.16
EPOCH   =                2000.

imgsc only finds two stars:

$ > imgsc -v i17h.fits
6494.1679 5:47:46.414 -25:15:18.83 12.03 243.5 104.9 0
6494.1659 5:47:58.613 -25:12:55.80 14.24 100.8 228.2 0

Click on image for larger version

In such a case, imsize can be used to get the nominal region of the sky covered by the image:

$ > imsize -d i17h.fits
i17h  5 47 56.500 -25 15 01.90  4.833 4.601

By piping this output to the Digitized Sky Survey extraction program, getimage, a FITS image covering the nominal field of the telescope image can be extracted:

$ imsize -d i17h.fits |
  rsh saturn /sat0/jshaw/getimage/getimage

Click on image for larger version

imstar is used to make a Starbase catalog of the sources in the DSS image, with positions computed using the world coordinate system in the DSS image header.
$ > imstar -vhi 7000 i17h.dss.fits
The image catalog produced, i17h.dss.fits.stars, is then used as a reference catalog to fit a world coordinate system to the telescope image.
Click on image for larger version
The following command will then find stars in the image, match them to the reference catalog which has been made from the DSS image, fit a world coordinate system and write out a new image file.

$ > imwcs -wv -i 100 -c i17h.dss.fits.stars i17h.fits

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

RA      = ' 5:47:57.212'
DEC     = '-25:14:39.86'
EQUINOX =                 2000
CTYPE1  = 'RA---TAN'
CRVAL1  =         86.988383321
CRPIX1  =             125.5000
CDELT1  =         -0.000317639
CROTA1  =                0.281
CTYPE2  = 'DEC--TAN'
CRVAL2  =        -25.244406126
CRPIX2  =             119.5000
CDELT2  =          0.000317639
CROTA2  =                0.000
IMWCS   = 'IMWCS 1.0,28 Aug 1996,Doug Mink,SAO'

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Last updated 4 December 1997 by Doug Mink dmink@cfa.harvard.edu

[WCS programs] [IMWCS]