[Passband] diverting into SEDs

[Anita Richards]

> >
> > I do not know for the radio domain, but certainly in the optical I
> > would claim that the difference between an SED and a spectrum is that
> > to describe the spectrum one needs a dispersion relation; i.e. not all
> > the spectra are regularly spaced in "fravergy". A dispersion relation
> > is not available for a SED; the SED points come from different
> > catalog/images/publications etc.; the spectrum comes from dispersing
> > the light using a grating or a grism or a prism or a combination of
> > those, whose characteristics are known and described via the
> > dispersion relation.

Ah.  But even so, I think that it would not be helpful to describe each
spectral channel separately, you still want an overall band
lambda_centre = x_c
and a sensitivity function F(x)
or x_min, x_max and F(x)

plus a dispersion function which describes the position ?and shape? of
each channel G(n,x)

> So far I see an SED as being a collection of 'SED Measurements', each
> measurement being a Flux, a Passband, some kind of Frame of reference, etc.  It
> may be that an SED could include a spectrum; if someone has for example measured
> an optical spectra and wants to combine it with an X-ray count.  But I suspect
> there will be many more things required in SEDs, so I would like to leave them
> for a bit...
It is very common at least at some wavelengths to have a bandpass with
multiple channels as default and so even if you want a single flux density
measurement for an SED, F(x) is a look-up table in terms of channels.  So
maybe the simplest thing for now is that even if assume that F(x) (etc.)
will be evaluated by the data provider and supplied as a single number (or
a single number plus a way of converting from Mag or Counts to physical
units), I think that the same SED model should eventually be capable of
being extended to describing molecular (etc.) spectra.
> >
> > 8.- Atmospheric conditions; airmass and seeing are needed to
> > characterise a SED measurement; should they end up modelled in the
> > bandpass?
>
> Only in so far as they reflect passrate (throughput).  Note that later we can
> make 'atmospherisc characteristics' that include passband info.  But for now
> that is a different model.

I agree; we always have to ask ourwwelves, who will use the information?
In some cases I do not think that the VO will wrap generic tools to
compensate for atmosphere (gain_el corections as we call it in radio) any
time soon.  Either the data provider will apply it automatically, or the
user will ahve to do it.  In which case any relevant information will have
to be passed to the user as CDATA or whatever.

Having said that, I think that the way SpecView works in AVO-Aladin might
be interesting; it provides a button for applying various corrections to
ISO spectra using calibration bundled with the data.  But that is for
narrow-multi-channel molecular spectra, not SEDs.

In fact, a useful reference point for your model might be the SED tool in
AVO-Aladin; if your model could cover serving up the requirements for that
(and extending the functionality) then we would know we were solving a
problem which users wanted solved.

cheers
a