detection limit

[François Bonnarel]

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
François