relative fluxes
Petr Skoda
skoda at sunstel.asu.cas.cz
Mon Jun 22 07:11:29 PDT 2009
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
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* Petr Skoda Phone : +420-323-649201, ext. 361 *
* Stellar Department +420-323-620361 *
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