[Passband] diverting into SEDs
mch at roe.ac.uk
Mon May 31 15:33:34 PDT 2004
Alberto Micol wrote:
> 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.
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...
> 8.- Atmospheric conditions; airmass and seeing are needed to
> characterise a SED measurement; should they end up modelled in the
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.
> 9.- When playing with SEDs (a la AVO demo) it is extremely important to
> pass around the bandpass info; not only for describing the SED but to
> allow for more sophisticated analysis, like for example
> when an SED is to be compared with a library of template spectra for
> classification purposes.
> AVO and GAVO are putting together a system to actually classify SEDs of
> planetary nebulae
> by matching the SED with models of stellar atmospheres. The models are
> to be multiplied by
> the filter throughputs before the comparison can take place.
> The SEDs should hence always bring along a URI(?) defining (if not
> pointing to) the
> bandpasses of each of the SED points.
> It would be nice to have a service able to resolve those URIs to return
> all the bandpass
> characteristics and throughput.
Be careful here as you're starting to mix 'representation' with the model. When
we come to modelling SEDs we simply say that the SED must/could 'have' stellar
atmosphere/filter/etc data. Whether this 'haved' data is remotely accessed or
passed along with the rest of the SED is a representation issue we need to sort
> BTW, I remember a Jonathan's document describing a bandpass datamodel.
> What is the status of that document? Is this exercise a way to continue
> Jonathan's work?
> I should read it again now, after few years ...
It's very readable, but covers a lot more than bandpass, and I feel (Jonathon
may disagree!) that it tends to mix up all the issues of modelling SEDs. For
now, I just want to have a look at a part of it, then work up. Also other
people have also done work, and I want to try and gather it all together from
the different areas.
> That's all I can think of, after a nice free day spent with a dozen of
> screaming 7to9 years old
> kids, playing bowling and trying to make sure they get back to their
> parents safely (for the record, I almost succeeded :-).
Almost? Did you lose one or two?!
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