Detector performance data used for sensitivity calculations in
this document are those derived from tests on the PHT
instrument performed
in the ISOPHOT Calibration Chamber during the Acceptance
Test No. 2. The read-out noise values 
listed in Tab. 10 (constant C3) and
Equ. 29 are derived from the noise values of the
most stable dark measurements.
Refined calibration and data evaluation will actually lead
to improved S/N ratios as
compared to the ones presently quoted.
However, the sensitivity calculation does not take into account disturbances by
high energy particles.
Depending on the orbital position of the spacecraft and
the strength of the individual cosmic particle impacts, they may lead
to a S/N degradation, and hence to a loss of part of the data.
At the time of writing this version of the ISOPHOT Observer's Manual it was decided to exchange the ISOPHOT instrument by its spare unit. Although the design is identical, the individual detector assemblies behave slightly different. It was not possible to assess the detector performance data of the spare unit completely before finishing this document. First results of the spare unit Acceptance Tests, however, indicate that the performance values are the same. Any changes will be made available to proposers.