Apologies for the delay in replying...

I'll just reply here on your first point, and comment on the
"vector" idea 
separately:

On Mon, 17 Mar 2008, Mark Calabretta wrote:

>
> On Fri 2008/03/14 02:39:27 -0000, Paddy Leahy wrote
> in a message to: Mark Calabretta <mcalabreatnf.csiro.au>
> and copied to: fitswcsnrao.edu
>
> Dear Paddy,
>
>>> (I'm not sure what you have in mind for
"undefined").
>>
>> Intended for archives which write a standard list
of header keywords, but
>> may contain data for which the polarization is not
known, too messy to
>> specify (eg. response varies substantially over the
FOV) or not defined
>> (eg. polarization-independent beams, maps of UHECR
arrival directions).
>
> OK, but why not simply omit the polarization axis?
>

Obviously that would be done for a typical file, as now. But
having an 
"undefined" code reduces the complexity of readers
and writers 
transferring the content of a fixed format internal header
to/from fits.
Sometimes it's useful to know for sure you don't have some
info
especially if the alternative is an extensive search through
the header 
for other possible locations of polarization info.

regards,
 		Paddy

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