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

[David Berry]

Tom,

> 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?

See

http://hea-www.harvard.edu/~arots/nvometa/

although I know of no FITS serialisation as yet, just XML.

David


>
> 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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