francois at vizier.u-strasbg.fr
Wed Jun 17 11:34:59 PDT 2009
Subject: Re: relative fluxes
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
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
>Hi again Alberto,
>I forward this to the list too...Mireille
>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
>> is off by an undetermined constant multiplicative factor. Therefore the
>> end up having "flux" values between e.g. 0.0 and 0.6.
>> As an example, please see this preview:
>> The question I have is about the UNITs of such spectrum.
>I guess more metadata should be provided along the Y axis here:
>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
>> 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
>> 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
>> Many thanks in advance,
>> 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
>> Spectrum.Char.SpatialAxis.unit meta.unit Unit for spectral coord REC
>> Obviously those are not "spectral" coordinates; keep it in mind for the
>> 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
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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