A question on keywords for 3-D images

[Arnold Rots]

I agree that adopting the convention in the context of HST is
reasonably safe, but would argue that its adoption outside that
context is dangerous.  I certainly appreciated your use of the word
"convention", but although most conventions are fairly harmless if not
understood, this one can bite the unwary user: software will either
support this convention or its opposite - there is no "neutral" position.

Hence, the safe thing is not to assume inheritance and repeat all
relevant keywords in all headers.

  - Arnold

Frank Valdes wrote:
> Hello Arnold,
> 
> I appreciate your concern.  However, the usage of this is larger than you
> may think.  The inheritance approach began, I believe, with the needs at
> HST for the new instruments (NICMOS/STIS).  Years of software development
> has gone into this and it has been discussed and status papers have been
> presented at ADASS for many years.  The IRAF FITS kernel support for
> inheritance began at ST.  I expect HST to continue to use this
> approach.  NOAO adopted this because it made sense for mosaic instruments
> but it is a smaller effort.  One thing I did not say in my posting is that
> for the NOAO Mosaic the header design is such that critical keywords are
> included in all extensions even though they are the same in all extensions.
> For example, exposure time appears in all extensions.  Thus any software
> which may not merge the global keywords will still have a useful minimal
> header for interpreting the data.
> 
> I purposely phrased my comment that this was a "convention" since, as
> you say, it is not a FITS standard.  However the idea of conventions is
> a known practice in the community.  There is no "assumption" that this
> is supported by all FITS readers (though all readers should be able to
> ingest both the primary (global) and extension headers.  For the
> specific formats, NICMOS, STIS, NOAO/MOSAIC, other similar mosaics,
> people will be advised of the format.  It is not very hard to merge
> headers if needed.
> 
> Frank Valdes
> 
> My role in this has been in the design of the NOAO Mosaic format and I had
> little to do with the development of the convention.
> 
> > From arots at head-cfa.harvard.edu Thu Nov  5 08:25:03 1998
> > 
> > I would strongly warn against the use of this convention.  If all you
> > want is to use your images in IRAF and if you know others will do the
> > same, in perpetuity, it's fine.  But most other systems explicitly
> > assume the opposite: each HDU stands by itself and there is no
> > inheritance.  Previous discussions on the subject concluded that the
> > notion of inheritance is not supported by the FITS standard and may
> > not be assumed.
> > 
> >   - Arnold Rots

--------------------------------------------------------------------------
Arnold H. Rots                                         AXAF Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 81                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head-cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
--------------------------------------------------------------------------