[fitsbits] [Fwd: Re: Proposed Changes to the FITS Standard]

[Maren Purves]

sorry, didn't mean to send this to Bill (Pence) only.

Maren Purves wrote:
> Bill,
> 
> I read this like Bill Thompson. The fact that the standard requires
> EXTEND=T if there may be extensions doesn't prohibit putting EXTEND=F
> (never mind the formatting) if there may (must) not be.
> 
> Of course if one wants to ignore either and read the whole file
> either way that's up to the software used (but that's a very inclusive
> way of seeing this).
> 
> ?
> 
> Maren
> 
> William Pence wrote:
>> The difference here is that I was quoting the revised definition in 
>> the new draft standard, which doesn't allow for the possibility that 
>> EXTEND = F.  Given that there are existing FITS files with EXTEND = F, 
>> the definition should probably be revised to at least not prohibit 
>> this usage.
>>
>>
>> William Thompson wrote:
>>> It's not clear to me that the FITS standard forbids setting EXTEND=F.
>>>
>>>> EXTEND Keyword The use of extensions necessitates a single 
>>>> additional keyword in the
>>>> primary header of the FITS file. If the FITS file may contain 
>>>> extensions, a card image
>>>> with the keyword EXTEND and the value field containing the logical 
>>>> value T must appear
>>>> in the primary header immediately after the last NAXISn card image, 
>>>> or, if NAXIS=0,
>>>> the NAXIS card image. The presence of this keyword with the value T 
>>>> in the primary
>>>> header does not require that extensions be present.
>>> To me, having EXTEND=F when one knows that no extensions are present 
>>> is one of those "bedrock of logic" that Rob Seaman refers to.
>>>
>>> Bill Thompson
>>>
>>>
>>> William Pence wrote:
>>>> William Thompson wrote:
>>>>
>>>>> The SECCHI instrument on the STEREO mission produces some files 
>>>>> with table extensions, and other files which don't.  The value of 
>>>>> the EXTEND keyword is set to either T or F to account for this.
>>>> Strictly speaking, this does not conform to the definition of the 
>>>> EXTEND keyword in the FITS Standard, which states that the value 
>>>> field *shall* contain a logical constant with the value T, and thus 
>>>> must never be set to F.    All the more reason I think to demote 
>>>> this keyword by making it optional instead of required if the file 
>>>> contains extensions.
>>>>
>>>> Bill Pence
>>
>>
>