[fitsbits] Abuse of EXTEND keyword

[Arnold Rots]

And so:
The inescapable conclusion appears to be that EXTEND and NEXTEND are
keywords that have outlived their usefulness, should be deprecated and
ignored when present, right?

  - Arnold


William Pence wrote:
> This mention of the NEXTEND keyword caused me to rethink what is really 
> meant by EXTEND = F:  EXTEND = T is defined to mean that the FITS file 
> *is permitted* to have extensions following the primary array (the 
> actually wording in the Standard is "may contain extensions" ); it does 
> not mean that the file actually has any extensions.
> 
> If EXTEND = F, then this logically means the opposite, i.e., that the 
> FITS file is *not permitted* to have any extensions.  But this is not 
> the meaning that the STEREO mission folks really intended by setting 
> EXTEND = F since they surely don't care if users were to add extensions 
> to the files at a later date.  In fact, I can't think of any case where 
> it would really be appropriate to set EXTEND = F.  Instead, I think the 
> desired meaning can be expressed by this pair of keywords:
> 
> EXTEND  = T
> NEXTEND = 0
> 
> which means that the file currently has no extensions following the 
> primary array, but it is permitted to add extensions if desired.  I'm 
> not really recommending this, however, because as previously mentioned 
> by Preben Grosbol, it could become a burden to keep the NEXTEND keyword 
> up to date as extensions get added or removed from the file.  Instead, 
> if software needs to know if there are extensions in the file then it 
> can just do an inventory of the file in real time.  This is not a time 
> consuming operation since it does not require reading the whole file; 
> the software can directly jump to the header of each subsequent 
> extension, at least on random access storage devices.
> 
> Bill Pence
> 
> Randall Thompson wrote:
> > I agree with Tim, defining EXTEND=F to mean the same thing as
> > EXTEND=T makes it worthless, and at that point maybe it should
> > just be deprecated. You are basically saying that EXTEND=F,
> > EXTEND=T, or leaving it out completely all mean the same thing;
> > namely, extensions may or may not exist.
> > 
> > By the way, we get many files delivered for archiving within MAST
> > that use both the EXTEND and the NEXTEND keyword. Although not a
> > reserved keyword, NEXTEND is commonly used to describe the
> > number of included extensions. I guess we are in the minority on this,
> > but when these keywords disagree with the file contents we either
> > correct them or ask the contributors to do it. I don't know if either
> > is really useful for a FITS reader, but they can be useful for getting
> > an idea of the file structure when just reading the primary header.
> -- 
> ____________________________________________________________________
> Dr. William Pence                       pence at milkyway.gsfc.nasa.gov
> NASA/GSFC Code 662       HEASARC        +1-301-286-4599 (voice)
> Greenbelt MD 20771                      +1-301-286-1684 (fax)
> 
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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