SED Data Model: Questions and Comments

[Sebastien Derriere]

  Hi,

  My 0.02 Euros on several messages of this long thread...

"Igor V. Chilingarian" wrote:
> 
> 1/ The most important problem is handling the spectra expressed in the
> continuum-normalized fluxes. Many astronomers, especially ones working in
> stellar physics, absolutely don't care about flux calibration. They prefer
> to use continuum-normalized spectra, because it allows to measure easily
> equivalent widths of absorption lines, fluxes of emission lines, etc.
> Presently there is even no UCD (though it can be constructed) to describe
> this sort of data, though zillions of spectra (especially high-SN
> high-resolution Echelle) are available in this format. The
> continuum-normalized representation is also very useful for comparing
> spectra of different objects of different brightnesses.

  To add on Jonathan's comment:
  The UCD word "arith.ratio" means "Ratio between two quantities
described by the 
same UCD". "phot.fluxDens;arith.ratio" would therefore mean a ratio
(normalized value)
of a "Flux density (per wl/freq/energy interval)". 
  Note that the reference value of the flux can itself be described 
simply by the UCD "phot.fluxDens".

David Berry wrote:
> 
> I'm not sure it's a good principle to duplicate information in different
> forms within a DM just to make it easier to write an interpreter. For
> instance, FITS-WCS paper II expressely says this shouldn't be done in
> the context of FITS WCS headers. The unit string has all the information
> you need, in a compact human readable form. Seems good to me.

  I totally support David's point of view on this !

> It shouldn't be necessary or desirable for everyone to write their own
> code, since code sharing is presumably what we are about.

  Right, I'm sure there are many other resources (even a 'units' package
if you're
running Linux), but the one I'm most familiar with is this one:
	http://vizier.u-strasbg.fr/cgi-bin/Unit
(there's a dimensional analysis of a unit string, among others)
  And for Java developers, there is a very neat package available here:
http://cdsweb.u-strasbg.fr/cdsdevcorner/units.gml


Sebastien.

-- 
    _______
   /  ~   /, Sebastien Derriere   mailto:derriere at astro.u-strasbg.fr
  / ~~~~ //  Observatoire de Strasbourg    Phone +33 (0) 390 242 444
 /______//   11, rue de l'universite     Telefax +33 (0) 390 242 417
(______(/    F-67000 Strasbourg  France