[fitsbits] XTENSION = 'FITS' proposal

[William Pence]

Stepping back a minute to look at the broader questions raised by this
proposal, I think the biggest issue is that it would basically transform the
FITS file structure from a simple linear sequence of HDUs into a
hierarchical structure with HDUs within HDUs (within HDUs (within
HDUs...)).  I worry about the impact that this fundamental change could have
on existing software and existing applications libraries (like CFITSIO).  

This would not be a problem if only a very few specialized applications
would ever need to be able to directly interpret the contents of this new
'FITS' extension, and if most general FITS software could just skip over it.
The implication in Perry's first message, however, was that it would be
desirable for general FITS software to be able to directly access HDUs that
are embedded within this hierarchical structure.  This would probably
require significant changes to the syntax of the user interface in order to
specify the location of the desired HDU within the hierarchical structure.

This could also have a significant impact on software libraries like
CFITSIO;  the data model supported by CFITSIO assumes that the HDUs in the
FITS file are linearly arranged, and a subroutine is provided to allow
applications programs to move forward or backward in the file and open the
next or previous HDU.  But under this 'FITS' proposal, CFITSIO would need to
support a whole new dimension, to be able to move up and down the hierarchy
as well as forward and back in the file.  Applications would somehow need to
be able to tell CFITSIO that they either want to a) skip over the whole
'FITS' HDU to the next HDU at that same level in the file, or b) that they
want to move down into the next level of the hierarchy and open an HDU that
is contained within the 'FITS' HDU.  At the moment, it is not clear to me
how this change could be implemented in CFITSIO and what impact it might
have on existing software.   

The other alternative schemes that have been mentioned for supporting
'groups' of HDUs (i.e., the Jennings et al. proposal, or the idea of having
'GROUP_BEGIN' and 'GROUP_END' HDUs to mark the group, or some new EXTNAME
keyword convention to define groups) do not have this problem because under
any of these schemes the FITS files themselves remain a linear sequence of
HDUs.

Bill
-- 
____________________________________________________________________
Dr. William Pence                          pence at tetra.gsfc.nasa.gov
NASA/GSFC Code 662         HEASARC         +1-301-286-4599 (voice)     
Greenbelt MD 20771                         +1-301-286-1684 (fax)