[fitsbits] CDELTn

On Thu 2007/09/20 20:42:46 CST, Doug Tody wrote
in a message to: Thierry Forveille
and copied to: ,
"LC's NoSpam Newsreading account"

The degenerate axis view might make sense for a very few things which
are often sampled (polarization, frequency/velocity, possibly time),
where a common model can be used whether or not a given physical
"axis" is sampled (Characterization in VO is similar). But in the
general case, applied to any "image" attribute, this is a poor model.

I agree that it's overkill to use a degenerate axis when a header
keyword could do the job (except that no keywords are currently
defined for that job).

However, the real power and importance of degenerate axes is as
illustrated in the header construction example given in Sect. 7.4.3 of
WCS Paper II. This shows how a degenerate axis may be used to define
varying (ra,dec) along the length of a long-slit spectrograph.

This clever idiom predates the WCS papers by a long way - originally it
was based on CROTAn. However, it was never formally documented and so
apparently never caught on outside the radio community. Paper II now
firmly establishes it as the way that such coordinate problems are
handled.

Cheers, Mark