comments to SDM 1.01

[Petr Skoda]

Dear colleagues,

as I have explained in dal group, I think the IVOA spectral data model 
should pay more attention to the continuum normalized spectra and simple 1D 
FITS images with CRVAL and CDELT keywords.

So here are my comments to the proposed Spectral Data Model ver 1.01:

in sec. 3.2 Units:

As I have emphasized in dal group, the normalized spectrum is a important 
fully science ready output and thus it is not equivalent to ignore the flux 
axis by stating the units as "n/a" and telling the units to be "unitless" - 
but is is perfect quantitative and measurable information on
normalized spectra - so I propose the special notation for such a "unit" as 
a "1" (one) or if letter required as "U" (unity) or "N" (normalized)

ad 3.5 Packaging model

(1) FITS (standard BINTABLE for Spectrum, defined in this document)
-----------

What about to add the simple 1D FITS image as acceptable for simple spectra 
as well - and thus "defined in this document"

Ad 4.1 Spectral coordinate

for WAVE em.wl should be explicitely written "vacuum" wavelength
(it may be source of many errors by optical astronomers who are commonly 
using air wavelength)  - it is just counterpart of AWAV given below in 
alternative choices...

Ad 4.2 Flux (Spectral Intensity) Object:

Despite the fact the Background model is foreseen, it will not handle 
properly the continuum normalized spectra - UCD spect.continuum is a 
continuum flux but it may not describe well what I understand by the 
request to disply the shape of continuum applied to particular spectrum
(it will be mostly kind of polynomial function).

I can imagine the expression of the simple spectrum as a raw spectrum 
comming from pipeline in arbitrary (summed) ADUs and the normalized version 
produced on the fly using the description of its contiuum flux (theoretical 
function) stored separately. But again the is  problem with units for 
normalized spectrum given directly by the pipeline.
The FluxAxis.ucd arith.ratio;phot.flux.density is similar to the 
requirements, but the density is misleading and the second spectrum
(continuum) is again only a fit not data, so it is probably not correct...
We simply need ucd for dimensionless normalized  spectrum  as a kind of 
flux calibration type ;-)


at 4.6.5 Calibration

The UNCALIBRATED is not good (although it tells " they are modified by an 
unspecified coordinate-dependent correction") ABSOLUTE is not good either 
as it is not absolutely calibrtaed and RELATIVE does not fit well as 
unknown systematic error is not present - morever the units of flux are 
still expected  here, probably.
So I suggest another value of Calibration field :
"NORMALIZED"

Ad Part 9 FITS serialization

I understand that the binary table requirement in VO just comes from this 
part - "We define  a reference serialization of this data model as a FITS 
binary table"

I am afraid that the keywords here  are to  far from reality:
perhaps representing some space experiment data. I think the 1D FITS image 
should be somehow treated here and a example be given for more common 
audience.

Best regards,

Petr Skoda


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*  Petr Skoda                         Phone : +420-323-649201, ext. 361 * 
*  Stellar Department                         +420-323-620361           *
*  Astronomical Institute AS CR       Fax   : +420-323-620250           *
*  251 65 Ondrejov                    e-mail: skoda at sunstel.asu.cas.cz  *
*  Czech Republic                             pskoda at mbox.cesnet.cz     *
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