What photometric accuracies can be expected

The absolute photometry AOTs PHT05 and PHT25 have been designed to obtain the
best assessment of the photometric accuracy for a given pointed observation.
Using all options offered by these AOTs, the observer can directly measure
the sky, dark and FCS straylight signals without relying on default parameters
collected at a different time and with a different observing method.

However, the observer should be aware that even very high signal accuracies
obtained from a given PHT05 or PHT25 observation cannot improve the
photometric accuracy below a limiting minumum accuracy. This minimum is
dictated by several factors of which the most important ones are listed below.

Apart from accurate signal determination, the photometric accuracy depend
also on:

- the accuracy of the calibration standards. The final calibration is
  expected to be better than 10% for the short wavelengths (lambda < 60 micron)
  and better than 15% for the longer wavelengths.
- the accuracy in the determination of the conversion from FCS heating power
  to sky flux. A calibration curve for each filter/detector combination is
  provided by the ISOPHOT instrument team. Especially for low absolute
  brightness values where reliable calibration standards are lacking the
  accuracy can be lower for a given filter/detector combination. Emphasis
  is given to consolidate the photometric calibration at low absolute
  flux levels.

It is assumed that the observer enters in PGA a "Source flux" (see
"Observing parameters")
close to the actual source flux in order to avoid possible non-linearities
in the detector responsivity. A non-linearity would cause the ratio
(sky signal)/(FCS signal) to become different from (sky flux)/(FCS flux)
when (sky signal)>>(FCS signal) or (sky signal)<<(FCS signal).

The ISOPHOT Instrument Team is currently establishing the best photometric
calibration. Many details of the instrument performance are under investigation
and calibration is constantly improving. The final calibration with the
accuracies mentioned above is therefore expected to be available not before
the end of the mission.

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