Thus CTYPE1 applies to the default WCS, which may be named using WCSNAME. A second WCS could use keywords such as CTYPE1A and WCSNAMEA. Up to 27 (no suffix, A-Z) different sets of WCS keywords may be specified.
In the WCSTools programs, the WCS to be used is specified by adding %WCSNAME to the end of a filename specified on the command line. This is added after the extension of a multi-extension FITS image is specified:
fitsfile,extension_name_or_number%WCS_name_or_letterimwcs always sets the default (no suffix) keywords.
Sometimes getting from image pixels to world coordinates is best though of a two step process, pixel<->focal plane<->sky, for example. The keyword WCSDEPx, indicating a named WCS conversion which must be applied to pixel coordinates before WCS conversion x can be applied, could be added to indicate the name or letter of the requisite WCS. This convention is described by Doug Mink and Steve Allen in a memo, "The WCSDEP Convention", available in PDF or Postscript format.
CRVAL1 = 0.5 CRVAL2 = 89.5 CRPIX1 = 1 CRPIX2 = 1 CDELT1 = 1 CDELT2 = -1 CTYPE1 = 'TLON-CAR' / Terrestrial East Longitude CTYPE2 = 'TLAT-CAR' / Terrestrial North Latitude CUNIT1 = 'deg ' CUNIT2 = 'deg ' WCSNAMEI= 'image ' / redundant reiteration of default FITS CRPIX1I = 1. / reference pixel along FITS axis i CRPIX2I = 1. / reference pixel along FITS axis i CRVAL1I = 1. / WCS value along axis j at refpix CRVAL2I = 1. / WCS value along axis j at refpix CD1_1I = 1. / CTM j_i from pixel to WCS CD1_2I = 0. / CTM j_i from pixel to WCS CD2_1I = 0. / CTM j_i from pixel to WCS CD2_2I = 1. / CTM j_i from pixel to WCS CTYPE1I = 'IMGX-CAR' / coordinate/projection type of axis j CTYPE2I = 'IMGY-CAR' / coordinate/projection type of axis j CUNIT1I = 'pixel ' / physical unit along WCS axis j CUNIT2I = 'pixel ' / physical unit along WCS axis j