A question on keywords for 3-D images

[Arnold Rots]

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

Frank Valdes wrote:
> Hello Bob,
> ...
> 
> Another aspect of the multiextension format, adopted as a convention by
> HST and NOAO, is the use of a global header.  The primary FITS unit consists
> only of keywords.  These keywords are common to all the image extensions
> in the file.  Thus the image extension headers can contain just the
> keywords which are different.  The IRAF software supports this concept
> of keyword inheritance.  Any task that access an image extension, as in
> the above example, sees a single header which is the merger of the global
> keywords and the separate extension keywords.  It does not need to know
> about the inheritance and two headers as this is taken care of by the
> I/O routines.
> 
> So the summary is use multiextension FITS, possibly with the inheritance
> convention, which is a solution adopted by a number of projects and is
> well supported at least in IRAF.
> 
> Cheers,
> Frank Valdes
> NOAO/IRAF Group

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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/
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