3.5. Coordinate system cross-references
While a coordinate representation may be shared amongst image arrays within the same column of a binary table, it may also happen that several image arrays within the same row of a binary table must share the same coordinate representation. For example, each row of a table might store a raw optical spectrum, the corresponding sky background spectrum, a flux-calibrated spectrum derived from these, and a spectrum of the error in each channel. It would not be appropriate to coerce these into a 2-dimensional data array with a heterogeneous second axis, and in any case this would complicate the addition or removal of spectra, say, as the result of data reduction. It also may not be satisfactory simply to repeate the coordinate description for each spectrum. For example, it would be preferable to apply a wavelength calibration to one shared representation rather than several identical copies.
This situation is handled by introducing coordinate system cross-references. These apply only to binary tables containing multiple image array columns, they are not relevant to primary image arrays or to pixel lists which represent only a single data set.
Coordinate system cross-references allow an image array in one column to reference the coordinate system defined for an image array in another column. The cross-reference is specified by the keyword pair
for the referred-to (target) coordinate system, and
for the referring (cross-referencing) system, and these must have identical, case-sensitive, keyword values. WCSXna must not be combined with any Table 2 coordinate keywords that use the same alternate descriptor. With regard to the Greenbank convention (Sect. 3.4), when WCSTna and/or WCSXna are columns of the table, the scope of the keyword(s) is limited to one row of the table. Thus the same value of WCSTna and WCSXna may be reused in different rows.
On encountering WCSXna, FITS header-parsing software must resolve the reference by searching for WCSTna with the same value, extracting the column number and alternate descriptor suffix encoded in the keyword itself, and then searching for and loading the associated coordinate keywords.
To continue the example above, suppose that Col. 12 contains a raw optical spectrum for which the coordinate system is fully specified, and that WCST12B has been set to 'XREF1'. Then a sky background spectrum in Col. 13 might reference this coordinate system by setting WCSX13A = 'XREF1'. In this case, Col. 13 must not contain any of the Table 2 keywords for alternate descriptor a = A, although it might contain keywords for some other value of a. The example illustrates that the alternate descriptor suffixes for WCSTna and WCSXna need not match; the association is via the keyword value alone.