Photometry in VO Spectrum Model

[Matteo Guainazzi]

Dear Doug, Johnathan et al.

I believe that there are two main reasons not to ban magnitudes from the 
VO realm:

  1. historical: magnitudes - like it or not - were, are and will be
     widely used by observatories and surveys, and will continue to
     appear in future VO-exposed catalogues
  2. (more important) physical: the conversion from magnitudes to flux
     (or from X-ray counts to fluxes) implies some assumptions on the
     intrinsic source spectrum and foreground absorption corrections.
     It is potentially misleading to leave the choice of this
     assumption to the data providers only, a choice which may be
     hidden in the metadata description. It is definitely better to
     leave the user a certain control on these assumptions, especially
     for non-stellar objects

 Once the filter transmission curve, and the zero point are specified, 
converting from magnitude to fluxes for a given spectral shape is a safe 
and deterministic algorithm.

 Hence, I believe that v1.0 of the Spectrum Data Model document should 
be submitted as it is, and a specific group with the mandate of writing 
a Photometry Data Model (intended in the broad sense as above) should be 
set-up and start working as soon as possible.

 Regards, Matteo

-- 
Matteo Guainazzi
XMM-Newton SOC				Matteo.Guainazzi at sciops.esa.int
European Space Astronomy Centre	of ESA	Phone:     +34 91 8131 176
VILSPA, Apartado 50727, E-28080 Madrid 	Fax:       +34 91 8131 172         

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