relative fluxes

[Mireille Louys]

Hi again Alberto,
I forward this to the list too...Mireille

Hi all,

Alberto Micol wrote:
> 
> 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.
> 
I guess more metadata should be provided along the Y axis here:
title: flux...
Calibration status: relative
Offset value= 12345 or 'Unknown'
units = unitless or '   '
ucd = phot.flux.density;em.wl


> 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.
> 
Yes, I fully agree with the necessity of a User's guide covering various
  uses-cases to be represented using SSA or Spectrum Utypes .

> 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.

Thanks Alberto for pointing this .
I'll check this with Jonathan.
Cheers , Mireille

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  Mireille LOUYS          mailto: Mireille.Louys at astro.u-strasbg.fr

L S I I T                       &     CDS,
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