Re: Photometry in VO Spectrum Model

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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