relative fluxes

[Alberto Micol]

Frank Valdes wrote:
> I don't think the particular debate being conducted is only about 
> units but about how to convey the nature of the calibration.

Thanks, that is exactly my point (which, as always, is the user point of 
view).


Petr Skoda wrote:
> Normalized in sense of Jesus's comment means "scaled by given VALUE"
> Normalized in sense of RECTIFIED is the modification by some 
> (practically unknown) FUNCTION
> Normalization is sense of Miguel's simulation means something like 
> estimation of influence of the given variable on whole compound result 
> (e.g. reference to unite mass of 1 Sun means what happens with my 
> model when I increase or decrease the mass)

Doug Tody wrote:

> ABSOLUTE means we have an absolute flux calibration; such data can
> be compared regardless of the source.
>
> RELATIVE means that the data was "calibrated" in some sense (we
> have no way to fully specify the details), but is not in the class
> of an absolute flux calibration.  A typical case would be an unknown
> systematic error or scale term; presumably shape analysis such as an
> equivalent width would be possible for such spectra.  Frank's example
> is typical of what was intended.
>
> NORMALIZED implies some sort of ratio, continuum normalization
> or other.  The UCD can tell more about what kind of ratio so it is
> not necessary to limit this to continuum normalization.  For full
> generality we need to include the reference spectrum flux vector,
> or possibly point to it as a separate spectrum, as Petr suggests.
> This has been considered for a future version but is not yet supported.
The SpectrumDM states:

> ABSOLUTE indicates that the values in the data are expected to be 
> correct within the given uncertainty.
>
> RELATIVE indicates that although an unknown systematic error is 
> present, the ratio of
> any two values will be correct.
>
> NORMALIZED indicates that the data have been divided by  a reference 
> spectrum; the
> flux UCD in this case should be prefixed by 'arith.ratio;' and the 
> units should be blank (dimensionless).
>
> UNCALIBRATED indicates the although the values reflect a measurement 
> of the given
> UCD, they are modified by an unspecified coordinate-dependent 
> correction. [...]

As demonstrated by this long discussion, it is clear that
all the above requires clarification in the SpectrumDM text.
This kind of confusion (in this and other standards) is killing all our 
efforts.
We have to be precise, and to provide clear and unambiguous (almost 
mathematical)
definitions, accompanied by a good set of real (as opposed to virtual) 
examples
(tutorials). Revision of the IVOA documents should take into account the
confusion that certain definitins might introduce in the (even vo-aware)
data providers. Crystal-clear documents are an art; we are good, but 
apparently
not in Arts...


I have to apologise for the confusion I created by showing you
that HARPS spectrum! I was calling it "RELATIVE"-ly calibrated.
That was not calibrated at all, as Petr correctly spotted it:
it is an UNCALIBRATED spectrum. The units should probably be
counts/s.

Still, I have VLT/UVES and GOODS (VLT/FORS2) spectra which are calibrated
in a RELATIVE sense, because the fraction of light that ended up
in the slit (for both the target and the calibration source)
is unknown. In this sense, there is a systematic _multiplicative_
unknown _factor_ (and not a systematic _error_, as per the _wrong_
SpectrumDM definition). The shape of the spectrum is correct,
and the ratio between any two values is still correct.

Net result: the units are (in my opinion) correctly set to:

BUNIT = 'erg/cm^2/s/angstrom'  /despite the IAU recommendation not to 
use angstrom
BSCALE = '10^-16'  /an indicative scaling factor, maybe to confuse the 
enemy?

but the absolute flux density remains unknown.
In such case, a normal SpectrumDM-unaware tool would display the 
spectrum as is, and the astronomer would not see that important 
calibration flag.

Did the FITS community ever tried to describe this?
Why isn't there a
BCALIB = 'RELATIVE' / or ABSOLUTE or UNCALIBRATED or RECTIFIED
FITS keyword?

Shall we reccomend one? (Haven't I just done that? ;-))

Alberto