[fitsbits] Spectral FITS -- encoding extraction area/continuum

[Tom Jarrett]

Arnold Rots wrote:
> I can recommend the FITS Region convention that is being used by
> Chandra for spatial region definitions.  It would have the added
> advantage of not increasing the number of conventions while fostering
> interoperability.
> 
> See:
> 
> 	http://cxc.harvard.edu/contrib/arots/fits/region.ps
> 

This looks very promising.  We have some (naive) questions:

1. we need a real example to understand where/how to
encode this into FITS header.  can you point us to a fits file
that uses this spatial region convention?

2. regarding *multiple* regions.  How do you indicate
more than one region for a single file (here we are thinking
of N-dim spectral cubes)?

How do we indicate that the region of extraction is given in
REGION extension names "WHATEVER", and what should "WHATEVER" be?  We
get to decide this essentially, but I suppose it depends somewhat on
what some mythical ds9 of the future, or super-VOBS tool, would want to
see.  We bet the ds9-of-the-future would be happy to have lots of
different regions, distinguished by the extension name they occur in
(1 region=1 FITS extension).

3. last question: regarding the X-Y dimension, how to encode this
with pure celestial coordinates? We think Chandra uses
pixel coordinates + WCS, but which makes less sense for 1D Spitzer spectra.

Thanks again for help out,
-tom jarrett & J.D. Smith