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

[Mark Calabretta]

On Wed 2005/02/16 16:44:12 -0800, Tom Jarrett wrote
in a message to: fitsbits at donar.cv.nrao.edu

Hi Tom,

>1. How to encode the the wavelength range(s)
>where the background (continuum) was determined?
>How to encode the polynomial fitting parameters?

I believe that various packages have methods for doing this but I'm not
aware of any attempt to standardize it (not being a WCS issue).

>Has this problem already been solved?  I have heard (from a source of a

It is recognized that WCS Paper IV will need to deal with something like
this, e.g. to allow distortion functions to be described separately for
each CCD in an image composed of a mosaic of CCDs.  However, no real
thought has been given to it.

>RAPi_j = 1.2345  / RA of Polygonal Aperture [deg]
>DAPi_j = 2.3448  / RA of Polygonal Aperture [deg]

I think this, or something similar to it, is probably about the best you
could do for now.

However, a general scheme for Paper IV would have to differ in a number
of respects:

* It could not be limited to two coordinate axes, and not just to RA &
  Dec, so the "R" and "D" in R/DAPi_j would have to become integer
  indices.

* To handle a mosaic of CCDs, for example, you'd need to tack on an
  alternate WCS specifier to associate each region with a particular
  coordinate description.

* I suspect that it would be better to specify the vertices in pixel
  coordinates rather than world coordinates.  I'm thinking of the long-
  slit example in Paper II where each point in a 2-D image (with a
  degenerate third axis) has independent ra, dec, and wavelength.

So the keyword would have to contain the following components:

   * A name part,
   * The polygon number,
   * The vertex number,
   * The axis number,
   * An alternate WCS specifier,

and, for the BINTABLE version,

   * A column number.

It seems inevitable that the eight character FITS keyword limit would
prove too restrictive, especially for the BINTABLE version of the
keywords.  A solution would be to use the record-valued keyword syntax
described in the current draft of Paper IV.  However, this is highly
experimental and it would certainly be premature to use it for your
current data.

Cheers, Mark