relative fluxes

[Arnold Rots]

RELATIVE and NORMALIZED fluxes are dimensionless quantities: they are
an ABSOLUTE flux divided by some reference flux value. Therefore the
unit should be and empty string.

An UNCALIBRATED flux could be expressed in some form of instrumental
units, e.g., count/s.

Btw, the units you are quoting are for a flux density, rather than a
flux.

Cheers,

  - Arnold

Alberto Micol wrote:
[ Charset ISO-8859-1 unsupported, converting... ]
> 
> 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
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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