2.5. Alternate axis descriptions
In some cases, an axis of an image may be described as having more than one coordinate type. An example of this would be the frequency, velocity, and wavelength along a spectral axis (only one of which, of course, could be linear). One can also describe the position on a CCD camera chip (or photographic plate) in meters as well as in degrees on the sky. To allow up to 26 additional descriptions of each axis, we propose the addition of the following optional, but now reserved, keywords defined in Table 1, where j and i are pixel and intermediate world coordinate axis numbers, respectively, and a is a character A through Z specifying the coordinate version. The axis numbers are restricted by this convention to the range 1-99 and the coordinate parameter m is restricted to the range 0-99, both with no leading zeros. Note that the primary version of the coordinate description is that specified with a as the blank character. If an alternate coordinate description is specified, all coordinate keywords for that version must be given even if they do not differ from those of the primary version. Rules for the default values of alternate coordinate descriptions are the same as those for the primary description. The alternate coordinate descriptions are computed in the same fashion as the primary coordinates. The type of coordinate depends on the value of CTYPE i and may be linear in one of the alternate descriptions and non-linear in another.
|WCSAXESa||number of axes in WCS description|
|CRVALia||coordinate value at reference point|
|CRPIXja||pixel coordinate of the reference point|
|PCi_ja||linear transformation matrix|
|CDi_ja||linear transformation matrix (with scale)|
|CUNITia||units of CRVAL i and CDELT i|
|PVi_ma||coordinate parameter m|
|PSi_ma||coordinate parameter m|
Alternate axis descriptions are optional, but may only be specified if a primary axis description is specified. The alternate version codes are selected by the FITS writer; there is no requirement that the codes be used in alphabetic sequence and no requirement that one coordinate version differ in its parameter values from another.
An optional keyword
is also defined to name, and otherwise document, the various versions of world coordinate descriptions. This keyword can be used to give the user simple names by which to request the various versions of the coordinates. It may also be used, for example, to distinguish coordinates used during data acquisition from those determined later by astrometrically rigorous reductions. It might also be used to specify which are data pixels and which are calibration pixels in a CCD image.