[fitswcs] Status of WCS negotiations

[Mark Calabretta]

On Wed 1998/08/05 00:02:39 +0100, Patrick Wallace wrote
in a message to: fitswcs at fits.cv.nrao.edu
and copied to: Don Wells <dwells at fits.cv.nrao.edu>,
      Mark Calabretta <mcalabre at atnf.csiro.au>

>I would still, myself, prefer to separate the three steps:  pixels
>to mm (say), rotate (and skew if required), mm to world units.  However,
>I accept Mark's parsimony argument, except that I'd go all the way and
>combine the CDELTn and PC into a CD matrix.  Multiple CD matrices would
>accomplish a range of effects including different axis scalings and
>different celestial frames, and at the same time sweep away all the
>confusion.

Thank you for your very generous concession.  I believe CDELTn/PC versus CD is
now the only point of general disagreement.  I've given my arguments in favour
of CDELTn/PC but certain syntactic and semantic issues need to be resolved
regarding multiple CDs before people can make an informed choice between
them.  One question is whether keywords for the CD matrix elements would be
in fixed format such as Cmiiijjj (i.e. C\d\d{3}\d{3} as a perl regular
expression) or variable such as CDmi_j (i.e. CD\d\d{1,3}_\d{1,3} with the 8
character limitation).  How would they be handled in binary tables headers
where the 8 character limitation really starts to bite?  Another question
concerns what needs to be duplicated in a multiple representation -
everything?  Hanish & Wells (1988) suggested writing CDELTn and CROTAn
together with the CD matrix for the benefit of old interpreters.  Is that
still a good idea?  The equations there have generated as much argument as
section 5 of G&C, appendix D in earlier drafts of G&C (e.g. Jun/93) tried to
sort it out.  This would need to be resolved.  Presumably the CDELTn/CROTAn
are only written with the first of multiple CDs?

These questions should not be difficult to resolve but a concrete counter-
proposal for multiple CD versus CDELTn/PC does need to be made.  A for/against
case should then be presented for each in dot-summary form.  Assuming it came
to a vote I expect I'd still be for CDELTn/PC (assuming I get a vote) but I'm
coming to think that it would be worth getting rolled if only we could have an
end to it!

Cheers, Mark