Photometry in VO Spectrum Model

[Ed Shaya]

Jon,
	Oh, yes! So, let's cover this point as well.  The topic began as a 
discussion on spectra.  If it is relative or uncalibrated, each pixel's 
Fv in the spectrum will be an integral over the wavelengths range within 
the pixel, as you specified.  A calibration COULD remove this and give a 
true F_nu.  Whether it really does or not depends on how sophisticated 
the calibration code is, but generally the difference between the two in 
a spectrum is ignorable.
	In photometry, if one bans magnitudes, one has two types of F_nu.  In 
one, the filter's transmission curve has been theoretically removed, 
perhaps by assuming a spectral template based on the type or temperature 
of the object.  Here one has to be alert to the fact that this is always 
just a crude estimation.  The other is a band-F_nu which is actually the 
numerator in your expression below, ie what you get from 
band-F_nu(std)*10^(-m/2.5).
	The convention has been to express the former as Flux F_nu and the 
latter is just kept as a magnitude M_band.  But not always so, and there 
is no critical argument that says it must remain so.  It is after all a 
matter of taste.  More properly, it is a matter of how distasteful it is 
to provide some additional code to handle astronomers' quirks.  I think 
the two of us agree that a bit of extra code is not so terrible.

Ed
PS - I think with this, I sign off, as I am no longer vo funded.

Jonathan McDowell wrote:
> Ed, I respectfully disagree.
> The definition of mag is actually
> -2.5 log ( \int Ftarg(lambda)T(lambda)dlambda / \int Fstd(lambda)T(lambda)dlambda )
> where T is the transmission curve which can be quite broad.
> This is important, because the inversion depends on the spectrum of the source
> ("color correction") and therefore if you pick a simple conversion and just
> give the number in Jy, you have lost information. So it's more than just
> the compression efficiency that you describe.
>  - Jonathan
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