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[fitsbits] CDELTn



 
 
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Old September 21st 07, 10:21 AM posted to sci.astro.fits
Mark Calabretta
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Default [fitsbits] CDELTn


On Thu 2007/09/20 22:27:17 CST, Doug Tody wrote
in a message to: Mark Calabretta
and copied to: Thierry Forveille ,


Hi Doug,

dimensional "image" with degenerate axes. The solution (which I think we
actually adopted) was to separate the image NDIM and WCSDIM, so that the


Yes we did, though it is noteworthy that we made a subtle, though
important mistake in the definition of WCSAXESa by allowing it to take a
default value. The effect of this is to nullify both NAXIS and WCSAXESa
and force parsers to be implemented in two passes - not a catastrophe but
probably not what we had in mind! My notes for wcspih() say this:

* Use of the WCSAXESa keyword is not mandatory and with its default value
* of "the larger of NAXIS and the largest index of these keywords [CRPIXj,
* PCi_j or CDi_j, CDELTi, CTYPEi, CRVALi, and CUNITi] found in the FITS
* header" it effectively invalidates the use of NAXIS for determining the
* number of coordinate axes and forces a preliminary pass through the header
* to determine the "largest index" in headers where WCSAXESa was omitted.
* Also since the use of WCSAXESa is optional there is no way to determine
* the number of coordinate representations (the "a" value) other than by
* parsing all of the WCS keywords in the header; even if WCSAXESa is
* specified for some representations it cannot be known in advance whether
* it was specified for all of those present in the header. Hence the
* scanner must be implemented in two passes.

Note how WCSAXESa nullifies itself and takes out NAXIS for good measure!
It actually becomes a liability if its value is smaller than the largest
index. Pretty nifty eh?!

Cheers, Mark

 




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