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[Francois Ochsenbein]

Subject: Re: relative fluxes

Hi all,

It seems that this problem could be related to the 
question of user-defined units that I tried to raise
at the DM1 session in Strasbourg -- in other terms
define a VO-wide way of writing units which are not
standard (SI or extensions) but expresses some 
well-defined calibration. 

If the terms used here for the fluxes are defined
in the SpectrumDM, the most logical choice would
be, I guess, to use a utype to express the 
calibration(s).

--Francois

>
>
>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
>
>--
>--------------------------------------------------------------
>
>  Mireille LOUYS          mailto: Mireille.Louys at astro.u-strasbg.fr
>
>L S I I T                       &     CDS,
>Ecole Nationale Superieure            Observatoire de Strasbourg
>de Physique de Strasbourg,            11, Rue de l'Universite
>Boulevard Sébastien Brant, BP 10413   67000 STRASBOURG
>67412 ILLKIRCH Cedex       	      Tel: +33 3 90 24 24 34
>---------------------------------------------------------------
>
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Francois Ochsenbein    ------   Observatoire Astronomique de Strasbourg
   11, rue de l'Universite 67000 STRASBOURG  Phone: +33-(0)390 24 24 29
Email: francois at astro.u-strasbg.fr (France)    Fax: +33-(0)390 24 24 17
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