Utypes 0.4 comments

[Rob Seaman]

On Nov 25, 2009, at 8:58 AM, Norman Gray wrote:

> For example, we might imagine utypes appearing in a FITS file like:
>
> PFX01SSA= http://www.ivoa.net/...
> PFX02STC= http://www.ivoa.net/...
> UTYPE001= ssa:blah
> ...
>
> (see also Jonathan's suggestions in <http://www.ivoa.net/forum/dm/0610/1257.htm 
> >).  But something like that would impose length limits on the sizes  
> of prefixes and utypes as a whole: 'ssa:blah' could be at most 72  
> characters.

Presumably these would be represented as string valued keywords, in  
which case the limit is 68 characters.  An 80 character (ASCII 20-7E)  
card image total, minus 8 characters for the keyword, minus 2  
characters for the value indicator (blank-equals), minus the 2 single  
quotes around the string literal.

There have been various discussions and local conventions for keyword  
continuation, but this could also be a good opportunity to consider a  
generic VO utility extension for FITS.  Presumably UTYPEs are not the  
only VO-related FITS issues.  We could collect all such into a  
cafeteria extension - likely a purpose-built bintable - for tagging  
files and/or individual extensions (or collections of same).

Rob