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[fitsbits] Abuse of EXTEND keyword



 
 
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  #1  
Old August 23rd 07, 10:39 AM posted to sci.astro.fits
Preben Grosbol
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Posts: 11
Default [fitsbits] Abuse of EXTEND keyword

On Thursday 23 August 2007 03:25, Tim Pearson wrote:
My guess (it's a long time ago) is that there were FITS files out *
there that had special records that did not conform to the rules for *
standard extensions, so the EXTEND keyword was needed. Or perhaps *
Grosbøl et al wanted to leave open the possibility of other uses for *
the special records, which would be indicated by different header *
keywords.


At the time, special records were used (most noticeable by random groups).
Thus, it was important to have a flag (EXTEND=T) to indicate that the file
conformed to the new rules for FITS extensions. Due to the wide usage
of random groups, special rules were introduced to allow coexistence but
other less used types of special records existed.

This flagging function (as used with GROUPS=T in random groups) indicates
that all special records conform, as stated in the last part of the sentence.
Thus, EXTEND=F could mean one of two things: 1) the writer knew about
the new rules for extensions but did not care/conform, or 2) there are no
extensions (which however is a subcase of EXTEND=T). As I said previously,
it also visually showed the user, by listing the primary header, that
information could be present in extensions.

During the introduction of the extension rules, the lack of EXTEND=T
showed clearly that the writer did not know about the new rules. Now
almost 20 years later, this is not an issues. We may ask if anybody
know of FITS files (possibly archival) which use their own 'special'
special records. Even so I see no major problem in making the keyword
optional butit's not a significant issue.

I prefer such flags (GROUPS, BLOCKED, EXTEND) as they, by nature,
just indicated an additional convention. If one introduces an explicit
version, it may be too easy to make (too) strong changes.

Preben

PS: I do notice and appreciate the 'ø'.

  #2  
Old August 23rd 07, 01:59 PM posted to sci.astro.fits
LC's NoSpam Newsreading account[_2_]
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Posts: 23
Default [fitsbits] Abuse of EXTEND keyword

On Thu, 23 Aug 2007, Preben Grosbol wrote:

I prefer such flags (GROUPS, BLOCKED, EXTEND) as they, by nature,
just indicated an additional convention.


I share this preference, provided the convention is useful and actually
used ... anyhow this does not imply the presence of the flag be
mandatory.

On Wed, 22 Aug 2007, Tim Pearson wrote:

The Grosbøl et al. paper (1988 A&A) said:

"Note that the presence of EXTEND=T in a primary FITS header ...
indicates that the file _may_ have extensions records and that any
special records will conform to the rules below." The important part
of this statement is the last bit, "any special records will conform
to the rules below"


Thanks Tim for reminding us of the second part of the statement.

However I wonder about the actual situation. I consider three cases
for the EXTEND keyword, and four things it may signal (the possibility
to have extensions, the presence of actual extensions, the presence of
special records, their conformance to the standard). The answer to each
thing can be YES NO or MAYBE.

But what does "conformance of special records to the standard" mean ?
Am I correct in assuming that in 1988 parliance "conforming extensions"
were one possible kind of "generalized special records" while in 2007
parliance "conforming extensions" are no longer "special records" ...
or at least the (deprecated) "special records" are all other possible
kinds of "generalized special records" (mutually exclusive with
"conforming extensions") ?

I note that EXTEND=F was never *explicitly* defined

I try to make this truth table ...

may have ext's HAS ext's uses spec rec they conform
---------------------------------------------------------------------
no EXTEND yes maybe maybe maybe
EXTEND=T yes maybe maybe yes if present
EXTEND=F ?? ?? maybe no if present

I can't figure out if the "??" shall be NO or "maybe". But definitely
the "maybe" in the first two lines are "maybe", so they are equivalent

.... which seems to indicate that a mandatory EXTEND=T is not needed
(which apparently is just the change in the wording of FITS 3.0 ... but
no practical change in usage ?)

Lucio Chiappetti

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