On Aug 23, 2007, at 12:18 PM, William Pence wrote:

Tim Pearson wrote:
Perhaps I am misunderstanding or misremembering something here, but I
think EXTEND=T does actually have a purpose, and to suggest that both
EXTEND=T and EXTEND=F are merely advisory is crazy. We should not add
the words "nor does the absence of this keyword necessarily imply
that the file does not contain extensions" to the FITS standard.

Can you explain why this should not be done? Also, the EXTEND = T
keyword has always been advisory, so it seems consistent to declare
that
EXTEND = F should also be advisory.

EXTEND=T when introduced was not advisory: it was an explicit
indication that all the "special records" (if any were present) after
the primary HDU contained only standard extensions. It would not have
been introduced if it was not necessary to distinguish such well-
formed files from those that contain arbitrary (unspecified) data in
the special records.

Sometime since the Grosbøl paper, it appears that two things crept
into the standard:

1. Special records that are not standard extensions can appear after
the last standard extension, even if EXTEND=T. I don't like this: it
is unnecessary and ugly and makes it harder to detect corrupted
files. Perhaps it is too late to undo it (although it is now
"deprecated").

2. Special records that are not standard extensions are not allowed
to start with 'XTENSION'. This is stated in section 3.5 of the
standard. It is this change that allows a reader to ignore the EXTEND
keyword and just look for standard extensions in the data stream.
This change would not have been necessary if change 1 had not been
made. I suppose that the chance that this change invalidated an
existing FITS file is negligible. When I wrote the above, I had not
noticed or had forgotten this statement in Section 3.5.

With these two changes, I agree that EXTEND is no longer needed. I
see no point in attaching "advisory" meaning to its presence,
absence, or value (what should a program do based on this advisory
information?); just say it conveys no information and should be ignored.

- Tim Pearson