[fitsbits] BINTABLE convention for >999 columns

[William Pence]

Ok. I had forgotten about those complications so I agree it's less confusing to keep the domain name

> On Jul 31, 2017, at 6:50 PM, Mark Taylor <m.b.taylor at bristol.ac.uk> wrote:
> 
>> On Mon, 31 Jul 2017, William Pence wrote:
>> 
>> None the less, CFITSIO continues to support this more general convention.  If
>> a user attempts to write a keyword longer than 8 characters the CFITSIO
>> routines will silently encode it in the FITS header using the HIERARCH
>> convention, and similarly will look for the appropriate HIERARCH keyword when
>> reading a long keyword.
>> 
>> Getting back to Mark's BINTABLE convention, I agree with François-Xavier that
>> eliminating the 'XT' domain name would simplify the BINTABLE convention a
>> little bit.  Using CFITSIO, one could then simply read or write the
>> "TFORM1234" keyword, instead of "XT TFORM1234".
> 
> But not all the keywords defined by this convention have more than
> 8 characters.
> 
> Using Francois-Xavier's suggestion, we have
> 
>   ...
>   TTYPE998= 'var_min_s_2'        /  label for column 998
>   TFORM998= 'D       '           /  format for column 998
>   TUNIT998= 'counts/s'           /  units for column 998
>   TTYPE999= 'XT_MORECOLS'        /  label for column 999
>   TFORM999= '813I    '           /  format for column 999
>   HIERARCH TTYPE999         = 'var_min_u_2' / label for column 999
>   HIERARCH TFORM999         = 'D' / format for column 999
>   HIERARCH TUNIT999         = 'counts/s' / units for column 999
>   HIERARCH TTYPE1000        = 'var_prob_h_2' / label for column 1000
>   HIERARCH TFORM1000        = 'D' / format for column 1000
>   ...
> 
> Just using CFITSIO's transparent long-keyword-means-HIERARCH rule
> to get the extended keywords would do the right thing for TFORM1000,
> TFORM1001 etc, but if you do it for TFORM999 you will get the
> wrong thing, namely the TFORM999 'container' column rather than
> the HIERARCH TFORM999 'extended' column.  And if you look for
> TDIM1234 presumably CFITSIO will attempt to retrieve the
> non-existent and illegal TDIM1234 rather than the intended
> HIERARCH TDIM1234 keyword.
> 
>> On Mon, 31 Jul 2017, Francois-Xavier PINEAU wrote:
>> 
>> I support an "overloading" behaviour (like e.g. in CSS): same keywords
>> (TFIELDS, TXXXX999) in a more specialized context (HIERARCH) should be able to
>> overload "regular" keywords that ensure the legality of the FITS
>> file (again, waiting for an updated standard).
> 
> Francois-Xavier suggests that the HIERARCH values take precedence
> over the non-HIERARCH ones in these cases.  That is possible but
> if I understand correctly would require a change of the CFITSIO
> behaviour that Bill describes, and in any case sounds rather opaque
> (it's not obvious whether the HIERARCH or non-HIERARCH form
> should take precedence).  The possibility of that kind of confusion
> looks to me like a good reason to stick to the namespace-required
> rule - then you know that the set of HIERARCH-derived keywords
> is the same as the set of keywords with embedded "." characters.
> 
> In general I prefer the idea that software wanting to use this
> convention has to explicitly 'opt in' by recognising the convention
> in use, than that it manages to work with no client code changes
> given that the underlying library code works in a particular
> (non-standard) way.
> 
> I am also a bit uncomfortable about using the HIERARCH convention
> in a way that deviates from the registered text of that convention
> (no namespace).  Since it's all outside the FITS standard anyway
> that's not a showstopper, but it makes the documentation a little
> bit more complicated (it's using something like the HIERARCH
> convention registered on the FITS web pages, but with no namespace).
> 
> Mark
> 
> --
> Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
> m.b.taylor at bris.ac.uk +44-117-9288776  http://www.star.bris.ac.uk/~mbt/