SED serialization

[Doug Tody]

On Mon, 12 Feb 2007, Jonathan McDowell wrote:

> Paolo,
>
>  Thanks for your posting of Feb 2; sorry for the delay in responding.
> Your questions are good ones but basically
> the answer is that they are all enhancements post version 1.0. I know that's
> not the answer you want!
>
> A new version of the doc is on my web site at
>
>  http://hea-www.harvard.edu/~jcm/vo/docs/spec102/specrc2.pdf
>  http://hea-www.harvard.edu/~jcm/vo/docs/spec102/specrc2.html
>
> I'll send it to the VO doc coordinator if it survives a couple days without
> anyone spotting horrible errors in it.
>
> - About the FITS serialization. You are right about the binary table implementation
> and that a SED could be written with multiple rows; although
> at ADASS we agreed to postpone the implementation of the multiple-row SED
> support for now. The typesetting of the data in the section 9.4 example is perhaps
> misleading; it depends how your FITS application would represent the binary data
> in the table - better to concentrate on the TTYPE/TFORM keys to see what is
> intended.
>
> - Magnitudes. Again, at ADASS we agreed to postpone the implementation of a
> full photometric model with transmission curves to a later date; it will use
> the spectrum model but add the zero point and the transmission curve.
>
> - Hence, we're postponing work on the Spectral Associations until after the
> current doc is approved, hopefully very soon.

Soon yes, but since Spectrum is driven by SSA we should conclude both
at the same time, and assume that there might be final minor changes as
a result of the initial implementations (which hopefully are progressing;
mine is done and released).

> The old draft of the doc
> is http://hea-www.harvard.edu/~jcm/vo/docs/spec96/sed96.pdf - but I know
> that Doug Tody wants to take a different approach.

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


> - Schema errors. Yep, we had already caught these and they are fixed in the
> new version.
>
>   - Jonathan
>
>
>