spectral WCS in FITS

[Markus Dolensky]

It all boils down to the question:

Is there a way to express a vector in a single cell of a FITS ASCII table, right?

If so, FITS BINTABLE structure proposed by Greisen et al. can be mapped to 
ASCII tables. And consequently XSLT can be used to transform between ASCII 
table and VOTable formats.

I don't know the answer, but at least it should clarify what we are looking for.

Markus


Robert Hanisch wrote:
> As many of you know, substantial progress has been made in the last year or
> two on standardizing the representation of world coordinate systems in FITS.
> Two papers have been published thus far, describing both a general formalism
> and specific implementation for celestial coordinates.  A third paper is in
> near-final form, describing spectral coordinate systems.  I was asked to
> read and comment on this paper, and would like to draw wider attention to it
> and raise a concern I have to see if it is shared by others in the VO
> development community.
> 
> You may find the latest draft of this paper here:
> 
>     http://www.aoc.nrao.edu/~egreisen/scs_112103.ps.gz
>     http://www.aoc.nrao.edu/~egreisen/scs_112103.ps
> 
> 
> My concern is that the paper provides no simple FITS ASCII table
> representation for spectra in which the wavelength scale is given by an
> explicit list of values, each with an associated flux.  Rather, it supports
> a wavelength scale look-up using a FITS BINTABLE extension, and only allows
> the wavelength and flux arrays to be stored as vectors in single cells.
> There are advantages to this approach in terms of efficiency and robustness.
> However, looking ahead to the imminent development of a Simple Spectral
> Access Protocol, I would expect that a basic encoding scheme for spectra
> would be a VOTable with columns for wavelength, flux, error, etc.  It seems
> to me that this VOTable should be as close in structure as a FITS
> representation as possible, to the point where we should be able to
> translate the VOTable to FITS with an XML style sheet.  Also, there is much
> extant software that reads and writes spectra in even simpler ASCII text
> files, with tab- or space-delimited columns and very simple column headers.
> Thus, in the interest of the simplest possible interoperability for spectral
> data, I have suggested to the authors of the FITS spectral WCS paper to
> consider adding a FITS ASCII table option for the tabular form of a
> spectrum.  So far I've been told that there is "no way in which it can be
> implemented."  I do not believe this statement, but would like to hear from
> others in the VO community before pressing the case further.
> 
> Thanks,
> Bob