relative fluxes

[Petr Skoda]

Hi Frank,

> adequate for absolute calibration and so we provided spectra that were 
> calibrated to another (lower resolution) library of spectra continua.

OK it is our reference function for RELATIVE - the other well defined 
continuum spectrum


>
> # Spectrum.Char.FluxAxis.name="Flux"
> # Spectrum.Char.FluxAxis.Unit="erg cm**(-2) s**(-1) Angstrom**(-1)"
> # Spectrum.Char.FluxAxis.UCD="phot.flux.density;em.wavelength"
> # Spectrum.Char.FluxAxis.Accuracy.Calibration="RELATIVE"


> of terms like RELATIVE and CONTINUUM NORMALIZED.

Here I was confused about continuum normalized - but I have just checked 
the library and all is clear the output spectra are in some physical flux
units and they conserve the original SED (i.e. the spectrum is falling at 
Balmer jump and and the red part as if it was absolutely calibrated just 
with different scaling factor - so it is the example of the Normalization 
mentioned by Jesus in VOSpec - in you case the reference value at given 
wavelength (550 nm or so) is put to 1.0  but at blue part it is higher and 
red part lower (e.g. continuum at 0.4).
But it is important the shape of curve mimics the real absolute flux SED.

> continuum shape was relatively correct by calibration to another lower 
> resolution library, and has numbers which include 1.0 at the tie point used, 
> the calibration is relative.  Since the continuum shape from the other 
> library was supposed to be calibrated to standard stars a flux unit seems 
> appropriate.

I would use the same description and units as you in this case.


> I don't think the particular debate being conducted is only about units but 
> about how to convey the nature of the calibration.  So people can take what I 
> considered for whatever it is worth.

I think in this special case yes - but it should not be confused with real 
continuum normalized spectrum (even you had mentioned it) - as in it the 
value of continuum EVERYWHERE is set to 1.0 and the curve is horizontal 
line at 1.0 with protruding spectral lines.
I would just to emphasize the difference still insisting that the 
NORMALIZED should be only this case with empty units.
All other should be UNCALIBRATED including pipeline products

And strictly speaking the pipeline products have to be unitless for 
echelle spectra if the blaze function is removed (otherwise the spectrum 
is not practically usable) as you divide by definition (unknown) flux of 
star (in some physical units like W/-2/s/A) by another unknown flux of 
flat field lamp. So in fact the result is really only the dimensionless 
ratio.

In the other case you divide by flat field in 2D (by so called superflat 
which has artificially set pixels near 1.0) and than you preserve counts - 
so the result is in integrated counts (or even photoelectrons if gain is 
known) but the blaze function has to be removed by process simillar to 
normalization of continua - fitting some high order polynomial....

This is necessary before joining the orders in both cases so again the 
best choice is to use the ucd and units according to real nature of the 
spectrum (and so the publisher should know the provenance of his own data 
- or check pipeline recipes)....


Petr

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*  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                                                       *
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