[fitsbits] CDELTn

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On Thu, 20 Sep 2007, Mark Calabretta wrote:

> It's perfectly illegal, Sect. 2.1.2 of WCS Paper I leaves no room for
> doubt:
> 
>   "The PCi_j matrix must not be singular; it must have an inverse.
>    Furthermore, all CDELTi must be non-zero.  In other words,

To repeat what I said in a previous post, WCS papers will be 
"incorporated by reference" in the draft FITS standard 3.0, so there 
will be no doubt they are applicable. But maybe it is worth adding 
ranges of legal values to table 8.2 of thereabout (Bill are you keeping 
track of this for when you return from the ADASS ?)

The only other point is that CDELTn were around before WCS, and we have 
in 3.0 the rule that a change of a standard cannot make invalid what 
done before ...

> 3) If CDELTia isn't meaningful, as for a degenerate axis, then simply 
>    omit it.  It defaults to 1.0.

Looks a sensible suggestion. Not using degenerate axes is perhaps even 
more sensibile :-) 

Lucio Chiappetti

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