Photometry in VO Spectrum Model

[Pedro Osuna]

Dear all,

it was me who proposed the photometry thing. 

It is astronomers who like to do this "strange" things Doug, like liking
to share data in magnitudes rather than real fluxes. I can give you many
examples of those cases (whether "sensible" thing to do or otherwise is
another issue), but just have a look yourself at Vizier using the old
UCD system to guess how many catalogues there are there with photometric
points (for instance, Johnson J):

http://vizier.u-strasbg.fr/viz-bin/getUCD/getUCD?
tab=col&words=PHOT_JOHNSON_J

then take one of the Catalogues, and look at the explanation for the
column where the photometry is described, and you shall see how
ambiguous everything is.

You can also have a look at the paper:

Standard Photometric Systems
Annu. Rev. Astron. Astrophys. 2005.43:293-336

to see how varied and ill defined some of those systems are, and why
they have been used mainly for historical reasons.

On the Use Case side, just take a look at 2Mass catalogue for instance:

http://cdsweb.u-strasbg.fr/viz-bin/VizieR?-source=II/246

there are many other projects delivering data in magnitudes rather than
real fluxes (that's life).


I will send separately my comments to the solutions proposed by
Jonathan.

Hope this helps to clarify the _why_ of the need to have photometry
better represented in VO.

Cheers,
P.



On Sun, 2006-10-22 at 20:00 -0600, Doug Tody wrote: 
> Hi Jonathan -
> 
> I agree that this would be useful for photometry points; it is just that
> it appeared to be suggested for spectra as well, and if so it would
> be good to examine more carefully whether it is needed or desirable
> for spectra.
> 
> For filter transmission curves, one thing we should look at is the
> JHU filter profile service; see  http://www.voservices.net/filter/
> 
> It could be useful to expose filter transmission curves via an SSA
> interface, so that we don't need something separate for filter data,
> but we should first see what metadata is required to describe filters.
> I haven't looked carefully at the JHU filter database, but I see that
> it contains data for about 100 standard filters.
> 
> 	- Doug
> 
> 
> On Sun, 22 Oct 2006, Jonathan McDowell wrote:
> 
> > Doug,
> >  I don't now recall who requested this at the Tucson meeting.
> > The idea was that the Spectrum model is also very close to being
> > a model for photometry, and especially for SEDs we would like to
> > be able to mix spectra and photometry points.
> >  However, photometry observers like to report their data
> > in magnitudes, even when it is absolutely calibrated. They want
> > to say:
> >   I measured a J magnitude of 11.8, and to convert my J to Jy
> >    (ignoring color term effects) you do F = F0 * 10**(-0.4 * J)
> >    where F0 is given in the header
> > rather than calculating F themselves and putting that directly
> > in the data.
> >  Of course you'd like to point to the whole transmission curve
> > yourself; then you can calculate the flux F from the magnitude J
> > making your *own* (client side) assumptions about the object spectrum.
> > Hence the URI.
> >  I guess the real science justification is that J is a calibrated
> > value; F is more than that, it's a modelled flux making an assumption
> > about the source spectrum. So better to publish J than F as long as
> > you can get F easily.
> >  It seems a small thing to add to the model, and wins us a lot of
> > archival photometry. We discussed this around the table in Tucson
> > and you didn't seem bothered then...
> >
> >    Jonathan
> >
-- 
Pedro Osuna Alcalaya

European Space Agency (ESA)
European Space Astronomy Centre (ESAC)
Research and Scientific Support Department (RSSD)
Astronomy Science Operations Division (SCI-SD)
e-mail: Pedro.Osuna at esa.int
Tel + 34 91 813 13 14    Fax: +34 91 813 11 72
-------------------------------------------------                                                                                
European Space Astronomy Centre (ESAC)
P.O. Box 50727
E-28080 Villafranca del Castillo
MADRID - SPAIN

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