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

[Arnold Rots]

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

We will be happy to provide more details.
And, btw, it is nicely compatible with the VO region standard.

  - Arnold

Tom Jarrett wrote:
> FITS experts,
> I have two questions that pertain to spectral
> 2-D data that is derived from the
> Spitzer Space Telescope "IRS" spectrometer.
> The Spitzer Science Center (of which I am the
> archive scientist) is now considering how to
> package this data for public consumption.
> So we now turn to the FITS experts for advice.
> Many thanks ahead of time (and thanks to
> Bill Pence for pointing me to this group). -tom jarrett
> 
> Spectral "maps" are constructed from IRS long-slit
> scans across a source (which provide both spatial
> and spectral coverage).  Coming this spring, Spitzer
> will be releasing its first spectral maps (courtesy of
> the SINGS Legacy Project).  The FITS will either be
> binary tables (for 1-D spectra) or the new
> "table lookup" for 2-D maps and 3-D cubes.
> But there are two FITS header issues that have not been
> settled yet:
> 
> 1. How to encode the the wavelength range(s)
> where the background (continuum) was determined?
> How to encode the polynomial fitting parameters?
> 
> This info could of course be written as a COMMENT in the
> header.  But we would like to know if there is a way
> to properly encode it using FITS keywords.
> 
> 2. How to encode the 'extraction area' into the header?
> The WCS rectangle may not be appropriate for complex
> extractions (where multiple pointings with a long slit are combined)
> One of the SINGS members, J.D. Smith (Univ. Arizona)
> has a proposal that I've included below.
> 
> Has this problem already been solved?  I have heard (from a source of a
> source) that the NVO is now developing a standard for describing
> areas using multiple circles.  Can anyone point me to some
> of this work?
> 
> J.D. Smith (U of A):
> For spectral maps, unlike for staring mode observations, the extraction
> aperture on the sky is not a fixed size.  For our SINGS releases, we're
> typically delivering rectangular extractions (in the tangent-plane space
> of our small spectral maps).  But to be general, I thought allowing
> arbitrary polygonal extraction regions would be valuable, and multiple
> disjoint regions as well.  Something like:
> 
> RAPi_j = 1.2345  / RA of Polygonal Aperture [deg]
> DAPi_j = 2.3448  / RA of Polygonal Aperture [deg]
> 
> where i ranges over different polygons, and j ranges over the points in
> a single polygon `i'.  Not terribly transparent naming, but that would
> allow for i,j=1..99 to fit under 8 characters.  For SINGS, then, we'd
> have something like:
> 
> RAP1_1 = 339.27008
> DAP1_1 = 34.407056
> RAP1_2 = 339.25979
> DAP1_2 = 34.421139
> RAP1_3 = 339.26458
> DAP1_3 = 34.423528
> RAP1_4 = 339.27487
> DAP1_4 = 34.409444
> 
> with the option of adding, e.g.
> 
> RAP2_1
> DAP2_1
> ...
> 
> for a future release with disjointed extraction apertures.  This same
> specification could describe photometric apertures on images as well.
> I'm not sure if the standard numbered keywords, e.g. PCi_j, include up
> front a count of how far i and j will range.  It's possible this could
> be generalized even further, to allow for arbitrary dimensional units
> (not just RA/DEC, e.g. pixels, wavelength, etc.).  Perhaps one of the
> experts could weight in on this.
> 
> Note from Jarrett:
>  >Do we care about the multitude of slit angles that were
>  > used to create the map?  I guess not since this information
>  > is encoded in the map itself.
> 
> J.D.SMith:
> Anyway, rectangles are probably fine, but the point is that
> this pertains to 1D spectral FITS data, and can have no relation to some
> FITS cube or image of a given size.  Just like specifying WCS has
> nothing to do with the detector used to observed an image.  I could
> probably do with a rectangle, i.e. specifying, center, width, height,
> and roll angle of the long axis, but I thought polygons would be more
> general.  In particular, averaging together 3 disjoint polygonal
> extractions.  In the ideal world, other shapes like circles would be
> specifiable as well.
> 
> 
> -- 
> ********************
> Dr. Thomas Jarrett                              phone: (626)395-1844
> IPAC/Caltech, Pasadena, CA                       fax: (626) 397-7018
> jarrett at ipac.caltech.edu   http://spider.ipac.caltech.edu/staff/jarrett
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> 
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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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