[fitsbits] EXTEND/NEXTEND reality check

[Doug Tody]

Hi All -

There is nothing in the FITS standard which prevents a primary header
from containing information describing the remainder of the file, and
in fact this is often done with multi-extension files (the INHERIT
convention for example).  Likewise there is nothing in FITS which
prevents one from adding a keyword such as NEXTEND.  It can even be
useful to do these things.  Of course such usage does require that a
FITS writer be prepared to modify the primary header, but that is not
such a big deal (perhaps easier than hopping along sequentially through
50 extensions), and existing software already does this kind of thing.
Since it is legal FITS, already current practice, and possibly even
useful, there is no reason to deprecate such usage.

MEF files are an important part of FITS, but unfortunately this issue
of describing the overall MEF as a structured object has never been
adequately addressed.  While it may be possible for a MEF to be a random
collection of FITS extensions that are concatenated together, this is
not the only model nor necessarily even the best one.  In many applications
one would like to consider the MEF to be a structured object, and be able
to add high level information to describe the object, indendent from
what is stored in the individual extensions.  The most logical place to
put this information in FITS currently is in the primary header.

One could easily argue that we need *more* keywords in the primary
header to properly describe a MEF, not less as seems to be the trend
of this discussion.

	- Doug