relative fluxes

[Jesus Salgado]

Hi Sebastien,

I am almost agree with:
> unit="1.23e-4 erg/cm**2/s/Angstrom"  (or whatever the reference value)

In my view, this solution is more useful than setting a "n/a" value to
the unit field. If we do that, we are loosing a lot of information about
the continuum shape that is still present for spectra where the relative
flux is the result to divide the spectra by a single reference value. 

A totally different case is when the spectra is normalized to the
continuum as the continuum shape information is already lost.

The problem I see with this solution is that (if I understand Alberto's
use case) the reference value looks unknown or lost. 

In order to cover this case, we were thinking in something like:

scaleSI: 	n/a 
(scale factor unknown)
dimEquation: 	ML-1T-3 
(F(lambda) flux type)

This is something we are already using in VOSpec for TSAP services.
Translated in a "non-compatible with unit string standards" string, it
would be something like:
unit="n/a erg/cm**2/s/Angstrom"

For VOSpec, this kind of spectra has a flag so the user could normalize
them to a flux value they consider, so it can be compared with absolute
normalized spectra as it is done between observed spectra and models.

Of course if the value is known both unit="1.23e-4 erg/cm**2/s/Angstrom"
or scaleSI=1.23e-4 would be, in my view, perfectly acceptable and
correct.

Regards,
Jesus

On Mon, 2009-06-22 at 13:46 +0200, Derriere Sebastien wrote:
> > 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.
> 
>    Hi all,
> 
>    My first reaction was the same as Arnold's: relative fluxes (ratio of
> the flux to some reference flux value) are dimensionless. But then how do you
> describe what reference value has been used for the scaling? Add an extra
> parameter, with its own UCD, unit, utype, etc...?
> 
>    Then it hit me: that's the same as expressing a mass in Solar masses!
> When you have a value of "1.44 solMass", you can interpret it as being
> a dimensionless quantity: "1.44 numbers of solar masses", because it is the
> ratio of the mass of the object to the mass of the sun.
>    You can also describe it as:
> value="1.44"	unit="solMass" (and if you write 1.0 solMass...)
>    or describe it as:
> value="1.44"	unit="2.0e30kg"
> 
>    So thinking of a spectra normalized to a reference, value, the unit
> could be this reference value! With Alberto's example:
> value=[anything from 0.0 to 0.06]
> unit="1.23e-4 erg/cm**2/s/Angstrom"  (or whatever the reference value)
> 
> My 2cents
> 
> Sebastien.
> 
-- 
Jesus J. SALGADO                       Jesus.Salgado at sciops.esa.int

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