SED serialization

[Doug Tody]

On Tue, 13 Feb 2007, P.Didelon wrote:

> Hi,
>
> jumping in the discussion ( ::oups)
>
> Doug Tody wrote:
>
> >Hi Ivo -
> >
> >Perhaps it is better to look at it the other way around?  A simple
> >1D spectrum is a special case of a multi-segment SED with associated
> >metadata.  SED is clearly the more general case.
> >
> sorry but I feel the reverse...
> Spectra seem to me to be the more general case,
> SED beeing a "specialised" case of spectrum with absolute (or at least
> self consistent relative) photometric/flux calibration;
> spectra could be even normalised in flux with bin values between 0 and 1
> or any arbitrary intensity scale.

Yes, but that is not the SED which has been talked about here in the past!
There appear to be two views: one group views SED as a homogenized case
where flux has been normalized across the SED; the other group views
SED as a collection of primary observations which in general are not
normalized, leaving this up to the client application.  What I suggest
is that we start with the overall normalized SED object, and add the
capability (optional, for SED) to detail each individual observation.

	- Doug


> >With this approach,
> >we still have the general case of a SED as described below, plus the
> >special case of a 1D spectrum (the Spectrum model), since it is so common.
> >The most powerful applications should be able to deal with either, but
> >many simpler applications may prefer to work with Spectrum, which also
> >covers most spectral survey data.
> >
> >The SED model being proposed is not merely a multi-segment spectrum,
> >as it also needs general metadata describing the Dataset as a whole
> >(DataID, Curation, Target, Char, etc.), plus probably a table giving a
> >uniform view of the SED as a whole, with each segment reduced to a few
> >values with uniform units.  Then follows the multi-segment part as in
> >our original SED model, where an individual segment may be essentially
> >a complete observation in itself.  Since for SED the segments can come
> >from independent observations with different instruments, in the general
> >case it really is more than just a mere multi-segment spectrum.
> >
> an homogenized multi-segment spectrum?
>
> >
> >	- Doug
> >
> >
> >On Tue, 13 Feb 2007, Ivo Busko wrote:
> >
> >
> >
> >>Doug Tody wrote:
> >>
> >><snip>
> >>
> >>
> >>
> >>>Not greatly different, but the main point is that a SED may be worth
> >>>considering as a new type of astronomical data (a primary type) and not
> >>>just a fancy Spectrum.  That is, a SED could be a top level Dataset type
> >>>(like Image Spectrum, TimeSeries) which is multi-segment, with an overall
> >>>Characterization and Identification, and homogeneous units to summarize
> >>>all the segments.  The individual segments can be instances of Spectrum,
> >>>TimeSeries, or individual photometry points.  Auxiliary metadata might
> >>>be required to describe how the SED was computed, especially in the case
> >>>of dynamically computed SEDs, where one might want to include images
> >>>cutouts for photometry from multiband imagers and so forth.
> >>>
> >>>Spectral associations are slightly different, being informal
> >>>associations of primary datasets rather than a single physical dataset.
> >>>A multi-segment Spectrum could be considered an Association or a special
> >>>case of a SED; both could be valid.
> >>>
> >>>	- Doug
> >>>
> >>>
> >>Doug,
> >>
> >>I still see a benefit in having a SED to be a special case of Spectrum.
> >>Being a subclass of Spectrum means that any application code that can
> >>handle a Spectrum instance, will be able to handle a SED instance
> >>transparently. It might not be able to provide everything that a
> >>SED-aware application is, but it won't crash or refuse to handle the SED
> >>data.
> >>
> >>Maybe I am influenced by the design choices we made when writing the
> >>Specview application, but the "SED as a special type of Spectrum" design
> >>paradigm showed to be very useful in that case.
> >>
> >>-Ivo
> >>
> >>
> >>
> >>
> >>
>
> --
> Pierre
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