[fitsbits] once FITS always FITS

[Doug Tody]

On Wed, 22 Aug 2007, William Pence wrote:

> Here are a few comments on several related recent FITS issues:
> 
> 1.  The EXTEND keyword:  Since the STEREO mission has started producing 
> FITS files with EXTEND = F, it is too late to prohibit this usage in the 
> FITS Standard (because of the "once FITS, always FITS" rule).  The 
> technical panel was not aware of this usage when it drafted the change 
> to the EXTEND keyword definition (which was modeled after the definition 
> of the BLOCKED keyword which can only have a T value, never F).  The 
> definition of the EXTEND keyword should probably be revised to explain 
> what is meant by EXTEND = F, e.g., something perhaps like this:
> 
>    "If the EXTEND keyword value is the logical constant F, then this
>    suggests that there are no extensions following the primary array.
>    This keyword is only advisory, however, so extensions may
>    follow the primary array even if EXTEND = F, but this is not
>    recommended."

Ok; but logically, if EXTEND = T is defined, then EXTEND = F should be
as well, and what the STEREO mission did was correct.  If EXTEND=F is
explict this should probably disallow any extensions (they should be
ignored or at least suspect if present).  If EXTEND=T or the EXTEND
keyword is missing, there *may* be extensions, and the reader should
check for extensions.

> 2. Repeated keywords:  Since the recent discussions have made it clear 
> that it is not a rare occurrence for existing FITS files to contain the 
> same keyword multiple times (the technical panel had hoped this was not 
> the case), it appears not feasible to simply declare that this practice 
> is prohibited in a conforming FITS file (again because of the "Once FITS 
> always FITS" rule).   It seems that the most we can do at this point is 
> to strongly discourage this practice, because it is indeterminate which 
> of the keywords will read by software systems when retrieving the value.

Right; repeated keywords are inherently ambiguous and should be discouraged
(theoretically this need not be the case, and they could have meaning, but
this is *not* the case with FITS, and they should be discouraged).

	- Doug