(r8) SpectrumDataModelRFC < IVOA

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Spectrum Data Model RFC

This document will act as RFC centre for the Spectrum Data Model 1.01 Proposed Recommendation.

Review period: 16 May 2007 to 12 Jun 2007 (still open)

In order to add a comment to the document, please edit this page and add your comment to the list below in the format used for the example (include your WikiName so authors can contact you for further information). When the author(s) of the document have considered the comment, they will provide a response after the comment.

Discussion about any of the comments or responses should be conducted on the data model mailing list, dm@ivoa.net.

Comments

First sample comment (by BrunoRino): ...
- Response (by authorname): ...

AndreaPreiteMartinez - 19 Jun 2007
- I have revised the UCDs in Tables 1, 2 and 3. The revised tables can be found here. Note that rows with changed UCDs are in bold+italic.
- Pag.23: erase sentence "My solution for brightness... "
- Pag.23 last line: change with "Note: we propose for the next version of the IVOA REC UCD-list to (a) change the flag of pos.eq from S to Q, (b) add two new words em.start, em.end to indicate the lower/start and upper/end boundaries of a spectral interval (as already done for the time axis)"

JonathanMcDowell -20 Jun 2007

Andrea,

I don't like the fact that you propose the same UCD for energy flux density and photon number flux density, and the same UCD for polarized flux and total flux, and the same UCD for observed flux and for source intensity per unit emission solid angle. These are different physical concepts, in some cases they have the same units and I REALLY need different UCDs to describe them to users - this is exactly what UCDs are for. I want to avoid using units to distinguish since this is of limited reliability ( e.g. source intensity as described below has the same units as a surface brightness but is a very different thing) and since it's a difference in CONCEPT not a difference in UNIT that is key here.

Concept                My new proposal                          Your proposal

Energy flux vs energy  phot.flux.density;em.energy               phot.flux.density;em.energy
Number flux vs energy  phot.flux.density;em.energy,phys.photon   phot.flux.density;em.energy
                    or phot.flux.number;em.energy

Flux density (vs wl)   phot.flux.density;em.wl                   phot.flux.density;em.wl
Polarized flux (vs wl) phot.flux.density;em.wl,phys.polarization phot.flux.density;em.wl

Flux density (vs wl)   phot.flux.density;em.wl                   phot.flux.density;em.wl
Source intensity       phys.luminosity; em.wl,phys.angArea       phot.flux.density;em.wl

Flux density           phot.flux.density                         phot.flux.density
Brightness temperature phot.flux.density; phys.temperature       phot.flux.density

Aperture area          phys.area                                 phys.area
Effective area         phys.area;phys.transmission               phys.area

Continuum-only vs wl   phot.flux.density;em.wl,spect.continuum   phot.flux.density,spect.continuum
Continuum-only vs freq phot.flux.density;em.freq,spect.continuum phot.flux.density,spect.continuum

Can you (1) explain what is wrong with my proposals (ok the first one uses a so-far-nonexistent UCD I agree, the others seem legal to me) and (2) provide alternatives that do not lose these distinctions?

I have a few other issues with your list vs mine:

Why is src.var.amplitude;arith.ratio not acceptable for the variability as fraction of mean? To say that this should have the same UCD as the actual amplitude is losing valuable discriminatory power.

What is wrong with meta.ucd and meta.unit for e.g. SpatialAxis.ucd, unit?

With e.g. StatError you suggest "stat.error;em" while I suggested "stat.error;em.wl" or "stat.error;em.freq" etc. Isn't it better to specify here, so that software can use the UCD to tell which is being given?

Why don't you like: em.wl;obs.atmos for Air Wavelength? I would really like to be able to distinguish it from vacuum wavelength. How can I tell the client (user) that air wavelengths are being used?

- thanks, Jonathan

GaryFuller - 19 Jun 2007
- Does the model allow for double sideband spectra? That is spectra for which each pixel can correspond to two different frequencies or wavelengths? This is a fairly common situation in the mm and submm.
- More generally what about the issue of describing how to convert between two different spectral axes, for example in the dsb case the upper and lower sideband frequency?
- Is there support for non-uniform spectral (and temporal) axes?
- In derived data fields signal-to-noise ratio seems a poorly defined quantity as it depends on what signal, spectral feature, is of interest to a particular user. A value for the rms noise level on the other hand would be very useful but doesn't seem to be in the model.

JonathanMcDowell -20 Jun 2007

Gary:

We discussed the double sideband spectrum case but decided that it complicated the model too much for version 1. We'll revisit this in future, and for now you just have to pick one sideband as the nominal frequency, maybe providing two Spectrum instances, one for each sideband. We acknowledge this is a limitation, as is the inability to represent two different spectra axes in a single instance.

The S/N is a bit vague, but was requested by some data providers for who a single target object is the norm. There is no rms noise, but there is a 'typical (1 sigma) error' in StatError which does much the same thing. We may add rms in a later rev but not this one.

We do, however, support non-uniform axes, since each spectral coordinate is listed explicitly.

AndreaPreiteMartinez - 22 Jun 2007

Jonathan,

1) first of all, a general consideration: if we miss an ucd-word for expressing a given quantity, then let’s create the corresponding new ucd-word.

2) as an alternative, we can agree that e.g. phot.flux.density;em.wl (or freq or energy) indicates the flux density per unit wavelength (or frequency, or energy) : no syntax problem (valid syntax ), but cumbersome if you want to add other modifiers/qualifiers (em.opt, stat.max,...). What I do not like in the form phot.flux.density;em.wl is the use of a quantity (em.wl) as a modifier/qualifier of the first quantity (phot.flux.density). In this case I prefer going to point (1) above, and re-present the proposal (see Victoria 2006) of adding:

phot.flux.density.perFreq, phot.flux.density.perWave, phot.flux.density.perEnergy, .. phot.flux.photon.per*, .. phot.flux.brTemperature, .. phot.intensity.perWave ..

So, phot.flux.density;em.wl;spect.continuum will become phot.flux.density.perWave;spect.continuum

About Polarized Flux: I don’t understand the problem. I would have added an additional attribute for the degree of polarization or the fraction of polarized flux.

the same concern applies to phys.area;phys.transmission: let’s go to phys.area.effective

src.amplitude;arith.ratio : you are right

SpatialAxis.ucd, unit: I took out meta.ucd, meta.unit for uniformity with the other axes. You can replace them, adding them also to TimeAxis .

stat.error;em : yes, if a * is legal in your context, you can write stat.error;em* (but not meaning all the em-word branch! )

em.wl;obs.atmos: I prefer em.wl.air.

3) in order to avoid confusion in multi-word ucds, I suggest also to introduce

stat.error.lower instead of stat.error;stat.min

stat.error.upper instead of stat.error;stat.max

Ciao, Andrea