detection limit

[Arnold Rots]

There are a number of problems associated with this approach, in our
case.

As you are undoubtedly aware, we tend to measure individual photons
which means (a) that the conversion to energy-expressing units is
model-dependent, and (b) the error or resolution depends on the flux -
the result of being in the Poisson error domain.

Third, we cannot separate "signal" from flux since our PSF varies
very significantly across the field of view.

And fourth, sensitivity (however defined) varies across the FOV, partly
(but not exclusively) due to the previous point.

I'm afraid that the bottom line is that we need a more sophisticated
model, if this is to be remotely useful.

Cheers,

  - Arnold


Douglas Tody wrote:
[ Charset ISO-8859-1 unsupported, converting... ]
> Hi Francois -
> 
> I have always thought it was important to have this attribute; for 
> global/multiwave use cases it is more important than exposure time for
> example, although it serves a similar purpose.  However it can be
> complex to provide, similar to converting photometric magnitudes to
> fluxes.
> 
> Calling this the "resolution" of the observable axis is ok I guess.
> Note it is closely related to the RMS of the background and is often
> expressed that way instead.  We would want to define how these two
> are related.  Unit probably Jansky.
> 
> I think it should be a "should" attribute (i.e., optional), due to the
> difficulty of computation.  However it is highly desirable for many
> data discovery use cases.
> 
>  	- Doug
> 
> 
> On Thu, 7 Apr 2011, Fran?ois Bonnarel wrote:
> 
> > Hi all,
> > Going on with searching what kind of initial ObsTap driver use cases
> > could have been forgotten I exhumated use case 3.1 and 5.2.  which
> > requires some estimation of the detection limit. after discussing with
> > Mireille, I decided to  ask everybody:
> >
> > For datasets we must be talking of "signal"  detection limit measured on
> > the Observable axis, which cannot be confused with the detection limit on 
> > sources
> > magnitudes (or line flux) which implies flux integration on the flux area 
> > convolved
> > with the psf (lsf).
> > The signal detection limit or inverse sensitivity for a given observation is
> > defined as the minimal detectable difference of measured quantities
> > along the observable axis. In case we are measuring a flux-related
> > observable, it is the minimal detectable flux density difference,
> > limited by the noise. In term of signal theory this is exactly the flux
> > resolution.
> >
> > The utype attached to this attribute could then logically be:
> > Char.observableAxis.resolution.refval, and the name o_detection_limit
> >
> > It will be sometimes difficult for  data managers to provide this field.
> >
> > QUESTION: do we implement this field as optional ? Will be described in 
> > Annex
> > in that case
> >
> > Cheers
> > Fran?ois
> >
> >
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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