Re: general comments to SSAP and SDM from outside

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