Proposed FITS Standard
Multiple World Coordinate Systems

WCSTools 2.9 implements the use of multiple world coordinate systems, with the WCS to which a keyword applies designated by a suffixed upper-case alphabetic character.

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_letter
imwcs 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.

Example of Multiple WCS in a FITS Header

Thanks to Steve Allen, Lick Observatory 
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

Last updated 27 October 2010 by Doug Mink

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