List of Observables for Observation Core components DM
Francois Bonnarel
francois.bonnarel at astro.unistra.fr
Mon Sep 27 08:58:04 PDT 2010
Hi Petr
Petr Skoda a écrit :
>
>
> Sorry for delay (lot of things after my return from Ukraine)
> I will comment both on my previous suggestion and that of Pat:
>
> What I wanted to say is that in many places in optical spectroscopy
> there is a need to express the observable not measured as the
> instrument gives but normalized (divided) by some hypothetical
> function, that (due good reasons;-) we want to have some physical
> meaning - in our case the continuum flux (outside of spectral lines).
> The continuum is however not
> easily seen - so only some kind of approximation is introduced -
> called "pseudocontinuum" - to know the real continuum a theoretical
> model ,which we know only in simple cases :-( , must be used.
>
> In fact all the Pat's examples seems to express the same problem:
> If you start some philosophical speculations, in fact, every
> measurement - that is givin the value of observable is normalized in
> some sense to some reference function (or constant) - e.g. to measure
> distance you have to divide by constant representing the lenght of 1 m
> (the Pt-Ir rod in Sevres ;-) . If you measure SED you have to divide
> by the SED of standard star observed in same condition etc ...
>
> AS I understand the radio examples - they have some function
> representing flux out of the source, some complicated function of
> averaged intensity in given beam as a reference (e.g. antenna
> temperature).
>
>
>> You're missing "scaled counts".... You could argue that they are the
>> same
>> [as ADU] from a philosophic point of view, but to an astronomer, they
>> mean
>> something different.
>
> Scaled count again is something "normalized" by given function.
>
> Example of Francoise - the Halpha filter imaging of solar chromosphere
> is in fact the same like ordinary spectrum - in the data cube we
> (optical spectroscopists) are thinking as having one spatial point
> studied along the spectral axis - this imaging is in fact shifting
> whole plane along spectral axis - but in every point of a prominence
> you can imagine frequency shifting (if you tune the filter by changing
> its temperature - you are scanning the spectral axis).
>
Don't you have a very small range on this spectarla axis anyway ?
> So the conclusion - I think the crucial point in preparing the model
> is to define exactly the process of normalization - i.e. we have to
> describe formally the nature of the "normalizing function" - then all
> observables may be expressed as a ratio (or in general arithmetic
> function) of some "abstract value of the given variable - which we do
> not know ;-) " and the proper normalizing function . stating this I
> would be able to ask the tables (or registries) for things like
>
> flux normalized by space angle
> flux per frequency bin
> flux relative to continuum
> flux relative to radio sky background "
> etc
>
> I think there has to be first prepared such a formalismus
> it will solve the purpose of data provenance as well (reduction
> pipelines are doing some normalization as well. - in fact all spectra
> are first 2D influence by instrumental characteristics and then
> normalized by function of flat field and other functions (echelle
> blaze) to yield the resutl after extraction ...
>
Can we compare these normalization functions to some kind of "flat
field" or sensitivity function ? In that case Characterzation
contains a pointer attribute to this. This does not solve fiding a name
or ucd for the "normalized" quantity.
>
> From the philosophical point there is nothing like the "pure - ideal
> observable" - this is what we want to get using complicated theories .
> We have only observable in Bayesian sense - the value of flux in X is
> such provided we use such a method and detector .....
>
> I understand we need some practical rule to encode in XML -
> Currently I am not able to formulate this concept alone - we need to
> involve more general discussion of instrument specialists and
> theoreticians as well.
>
> Lets invite them to collaborate - not only let all on DM group
>
> P. SKODA
>
> *************************************************************************
> * 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 *
> *************************************************************************
>
--
=====================================================================
François Bonnarel Observatoire Astronomique de Strasbourg
CDS (Centre de données 11, rue de l'Université
astronomiques de Strasbourg) F--67000 Strasbourg (France)
Tel: +33-(0)3 68 85 24 11 WWW: http://cdsweb.u-strasbg.fr/people/fb.html
Fax: +33-(0)3 68 85 24 25 E-mail: francois.bonnarel at astro.unistra.fr
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