general comments to SSAP and SDM from outside

[Jonathan McDowell]

Dear Petr,

 I've now read your useful comments on the Spectrum document.

 The question of continuum normalized spectra is an interesting and
important one. I accept the need for Calibration = NORMALIZED.

Really what you have is two spectra:
 
 - firstly, the normalized spectrum, which I believe we correctly support 
   with 

       Spectrum.Char.FluxAxis.Calibration = NORMALIZED
       Spectrum.Char.FluxAxis.unit = ' '  (blank, i.e. dimensionless)
       Spectrum.Char.FluxAxis.ucd  = arith.ratio;phot.flux.density

    I do insist that the units are dimensionless.

 - secondly, the continuum spectrum, which often may be defined 
   using a function. We don't have a way of encoding such a function in the VO,
   but your service can just instantiate the continuum spectrum, i.e. calculate
   the flux values in each bin and make a VO Spectrum object for the continuum. 
   Then for this spectrum you have

       Spectrum.Char.FluxAxis.Calibration = ABSOLUTE
       Spectrum.Char.FluxAxis.unit = 'erg cm**-2 s**-1 Angstrom**-1' or something
       Spectrum.Char.FluxAxis.ucd  = phot.flux.density;em.wl

Given these two spectra, the user can reconstruct the full spectrum by
multiplying them together. (I don't think we are ready to add metadata to
explicitly connect them yet.) You say "the second spectrum is... fit not
data, so it is probably not correct.." - but a spectrum generated from
a fit is perfectly acceptable, we make no restriction that the VO Spectrum
has to be directly observed data.

As for the formats, this is a thorny issue. The data model proper is not
tied to the format, but we must standardize on formats that are rich
enough to store the full model. I agree with Doug that the use of FITS
images for storing spectra, although widespread, is too broken to
support in the future (nowhere good to put the error metadata etc.).
Nevertheless implementations may map between the old formats and the new
ones at the cost of losing the extra metadata. We'll have to see what
formats are most used in the future - that shouldn't affect the model itself.

I posted a revised version of the document to 

http://hea-www.harvard.edu/~jcm/vo/docs/spec101r3/SpectrumDM-20070827.pdf

 - Jonathan