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

[Arnold Rots]

Tom,

1. Attached are two files.  One shows the FOV of a Chandra observation,
the other a series of elliptical source regions, produced by a source
detection program (though it is a somewhat convoluted format).

2. You can use the COMPONENT column to indicate multiple regions - or
any additional column of you choice. But note that the Region
definition is purely positional and 2-D.
We use HDUCLASS='ASC' and HDUCLAS1='REGION' to identify these
extensions (though admittedly, the second example has these keywords
overridden).
But we definitely put multiple regions in one extension.
DS9, btw, can read FITS region files - it just doesn't write them.

3. We express our coordinates in terms of pixels, but as far as I can
see there is no reason why they could not be celestial coordinates.
You might have to stipulate that "lines" are to be interpreted as
great circles - which is fine for tangent point projections.
Also note that the next generation region are specifically defined in
terms of celestial coordinates.  I just posted a new version of the
VO Space-Time Coordinate Metadata document that contains the region
description and schema.
See http://hea-www.harvard.edu/~arots/nvometa/v1.2

Hope this helps,

  - Arnold

Tom Jarrett wrote:
> 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
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              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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