[fitsbits] CRPIX clarification

[Mark Calabretta]

On Mon 2008/06/16 08:20:03 CST, Eric Greisen wrote
in a message to: Mark Calabretta <mcalabre at atnf.csiro.au>
and copied to: William Pence <William.Pence at nasa.gov>, FITSBITS <fitsbits at nrao.
     edu>

>I did not like the former wording since random groups does have a 
>regular lattice with WCS under the same rules as images along with the 
>random parameters which need not have rules.

For those not familiar with random groups, Eric refers to the regular
array that follows the random parameters, these have world coordinates
described in the conventional way using CRPIXn, CRVALn, CDELTn, and
CTYPEn.  On the other hand, my comment refers to the random parameters
themselves.  These store the world coordinate directly in PTYPEn and do
not form a regular structure.

To clarify that, the statement could be changed to "(e.g. as is done
for the random parameters using PTYPEn in random groups data)".  The
whole point is that they are *random*, i.e. don't form a regular
structure, and so provide an excellent contrast to the regular lattice
structure associated with images.

Regards, Mark