relative fluxes

[Guy Rixon]

Perhaps the units legislation needs to allow the special token  
"arbitrary"?

Cheers,
Guy

On 16 Jun 2009, at 11:37, 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.
>
> 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