Photometry in VO Spectrum Model

[Andreas Wicenec]

Dear Pedro and all the others,

we here at ESO were actually preparing a request like this as well  
and I can
only fully support Pedro in his argumentation. I think this is not  
just a
nice-to-have add-on, but a real-world requirement, since most of the  
data we
have at hand is actually calibrated in magnitudes as well, but we are  
working
on a service which converts these magnitudes into flux using the full  
response
curves of the optical path. In order to make such a server really  
work for the VO
we would like to describe the response functions using the spectral DM.

Cheers,
Andreas

On 23.10.2006, at 11:55, Pedro Osuna wrote:

> 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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