relative fluxes

[Jesus Salgado]

Hi Petr,

It is true that the solution provided by Sebastien is only applicable to
"relative" calibration as described in the Spectral DM. Of course,
different issues would be if this kind of spectra is not so common (used
in TSAP but, probably, not so common for observational spectra) or if
Alberto's files are not under this category (Alberto to confirm). 

For the rest of spectra (neither absolute nor relative as per Spec. DM),
I think there is work to be done in Spec.DM (and SSAP) about a proper
description of what can be done and what not with the SSAP records (use
of continuum shape, use of spectral lines for identification, etc). On
the other hand, there are too many different kinds of spectra under the
"uncalibrated" tag...

Regards,
Jesus

On Mon, 2009-06-22 at 16:11 +0200, Petr Skoda wrote:
> 
> Hi Sebastien,
> 
> I am afraid we are getting more and more confused by the analogies.
> In case of values relative to some constant value (like mass of Sun) we 
> know the value (the same with Miguels's simulation referenced by the 
> gravitation of given mass computed for case the unit mass (that of Sun)
> so we know the dimension of the denominator. and its value.
> The operation is properly described by simple mathematics (scaling)
> 
> But here its different.
> 
> The calibration of absolute flux in spectra case is a procedure
> of using some (already known) function F(lambda) for particular comparison 
> star and forcing the unknown instrumental conversion of 
> intensities (which is in ADU and depends on the spatial width of spectra 
> and reduction procedures, sometimes on the colour and intensity of flat 
> field lamp) to this known flux function.
> And this conversion is hoped to be same for the observation of unknown 
> target and so the same function is applied (but the atmospheric extinction 
> is different and unstable etc...)
> Even you offten do not know physical flux directly (to get incident 
> photons energy from ADU is extremely difficult)
> 
> So here we have for calibrated flux already a quite complex function (not 
> constant) describing the reference values (but it is subject to black 
> magic and art of such an calibration which would comprise all the 
> observing data model and still not be correctly expressed....
> But after that you have something that could be expressed in units like 
> W/s/A/m-2
> 
> in case of NORMALIZED it is fully artificial - no reference function here 
> is not known  - the scaling is done in such a way to get the 
> pseudocontinuum to value 1.0 . The function is driven only by the visual 
> appearance but does not have any simple physical model behind so it is 
> impossible to describe this reference function in the same way as you 
> simply refer to mass of 1 Sun.
> 
> 
> So in this case is the unit realy dimensionless.
> The same is with theoretical spectra - you may compute flux theoretical 
> spectrum of given star separately in continuum and in lines. Than you fit 
> such a function through continuum points and divide into line fluxes - so 
> again the theoretical spectrum in this case s dimmensionless.
> 
> 
> I am really afraid Miguel means by theoretical spectra normalization 
> something completely different and it should be reflected in semantics and 
> ontologies as well.
> 
> 
> >
> >> 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.
> 
> I think that this is the only logical conclusion about RELATIVE and 
> NORMALIZEd spectrum flux calibration as it was meant by SSA.
> 
> For Miguel's case we have to define different term (normalized in sense of 
> contribution to general energy or so ...) Or perhaps we could return to 
> well understand term RECTIFIED (instead of NORMALIZED to unity)
> 
> > could be this reference value! With Alberto's example:
> > value=[anything from 0.0 to 0.06]
> 
> As I already explained the Alberto's case is not relative calibration - it 
> is modification by unknown function (dependent on atmosphere, instrument, 
> detector, calibration lamp and many other issues)
> 
> By relative calibration I would mean some scaling by known value (like 
> constant) or division by some known reference function ( so approaching 
> the staus of ABSOLUTE - but stil with unknown influence of something or 
> simply not having table of energy for given reference source - but still 
> useful for e.g. fitting power law)
> 
> Petr
> 
> *************************************************************************
> *  Petr Skoda                         Phone : +420-323-649201, ext. 361 * 
> *  Stellar Department                         +420-323-620361           *
> *  Astronomical Institute AS CR       Fax   : +420-323-620250           *
> *  251 65 Ondrejov                    e-mail: skoda at sunstel.asu.cas.cz  *
> *  Czech Republic                                                       *
> *************************************************************************
-- 
Jesus J. SALGADO                       Jesus.Salgado at sciops.esa.int

ESAC Science Archives Team
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European Space Agency (ESA)

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