Photometry in VO Spectrum Model
Matteo.Guainazzi at sciops.esa.int
Tue Oct 24 05:45:58 PDT 2006
Dear Doug, Johnathan et al.
I believe that there are two main reasons not to ban magnitudes from the
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.
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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