relative fluxes

[Alberto Micol]

Dear SpectrumDM + SSA but also UNITs authors

As per the SpectrumDM, the flux calibration can be described to be one of:
- ABSOLUTE
- RELATIVE
- NORMALIZED
- UNCALIBRATED

Now, at ESO we have some spectra which are calibrated in a RELATIVE way
(i.e. the ratio between any two points gives the right answer) but their 
flux
is off by an undetermined constant multiplicative factor. Therefore the 
spectra
end up having "flux" values between e.g. 0.0 and 0.6.

As an example, please see this preview:
http://archive.eso.org/~amicol/tmp/relative_flux_spectrum.png

The question I have is about the UNITs of such spectrum.

The mentioned preview shows the flux label "relative flux" because this
is what the units are currently set to!
 
But obviously the string "relative flux" is not an acceptable unit string.

We could hence set the units to be "erg/cm**2/s/Angstrom"
but I would be very much afraid of the astronomers' reaction
when they would display the spectrum to see values ranging from 0.0 to 0.6.
They would normally not get to see the calibration status (RELATIVE),
or even if displayed, they wil not understand the meaning of it, will they?

Another option could be to leave the flux unit string empty,
therefore leaving entirely to the UCD (presumably: 
phot.flux.density;em.wl in my case)
the task of describing whether the spectrum is binned in wavelength,
or frequency/energy. Is this the correct way?
(But will the astronomers see the UCD? obviously not)

What is a pragmatic and coherent solution to this? Eager to get your 
suggestions.

Finally, whatever the outcome, it would be extremely nice if the 
SpetrumDM + SSA,
presumably within an associated note or tutorial,
could describe this kind of scenarios and provide suggestions to the
diligent but VO-unaware (or even VO-aware, but very much undecided, like 
me)
data providers.

The worst could happen is that different  data providers will
describe the same situation in different ways, hence hampering 
interoperability.

Many thanks in advance,

Alberto

PS:
BTW, I just noticed a little typo in the SpectrumDM document,
on the table3 "Flux Value options":
...
Spectrum.Char.SpatialAxis.ucd  meta.ucd   ucd for spectral coord  REC  
pos.eq
Spectrum.Char.SpatialAxis.unit meta.unit  Unit for spectral coord REC    
 deg
...
Obviously those are not "spectral" coordinates; keep it in mind for the next
version of the document.

 

-- 
Alberto Micol
Telephone: +49 89 32006 261
Fax:       +49 89 32006 898
Virtual Observatory Standards Group Lead
Virtual Observatory Project Office
Data Management and Operations Division
European Southern Observatory