Re: [fitsbits] Abuse of EXTEND keyword
- From: Arnold Rots <arots@xxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 24 Aug 2007 14:57:07 -0400 (EDT)
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@xxxxxxxxxxxxxxxxxxxxxx
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
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Cambridge, MA 02138 arots@xxxxxxxxxxxxxxxxxxxx
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