[fitsbits] CDELTn

On Thu 2007/09/20 11:51:24 -0400, Phil Hodge wrote
in a message to: Mark Calabretta

I presume you meant CDELTia rather than CRVALia. I've never understood
why, but WCS Paper I actually specifies that the default value for any
CDi_ja keyword is zero (although if all of them are absent the PC form
is assumed).

This was the de facto interpretation of CDi_j before it was made
standard by Paper I. If I remember correctly (not wanting to search
through 1000s of old emails) it was supplied by Frank Valdes and
Lindsay Davis by interpretation of the IRAF code which was considered
to be the defining instrument. So effectively it's for historical
reasons.

Cheers, Mark