When to include a straylight measurement


The ISOPHOT instrument team performed dedicated observations to assess the
straylight level on the FCS. The straylight signal obtained by subtracting
the dark signal from the cold FCS signal was compared with the sky signal.
Observations were collected at positions without strong discrete sources
in the ISO field of view other than the zodiacal and galactic backgrounds.

The residual straylight signals indicate that the straylight contribution for
the P detectors is less then 5% of the sky background signal. In fact the
signals of the cold FCS measurement involving a P1 or P2 detector always show
a significant downward drift. This drift is due to the fact that the detector
is illuminated by the (much brighter) sky just before the beam is switched to
the cold FCS position via the chopper mirror. This is a design feature of
the instrument and cannot be avoided. Since no advanced drift corrections were
applied for the evaluation of the commissioning observations, the 5% is an
upper limit to the straylight contribution for all P filters below 25 micron.

In contrast to the P detectors, unambiguous straylight signals ranging from
3-15% of the zodiacal/galactic background signals are found in filters of the
C100 and C200 detectors.

We recommend observers aiming at an accuracy of the FCS signal better than
5% to include a straylight (dark FCS) measurement in the PHT05 observation.
For the C detectors (PHT25), in particular for the filters of the C200
detector the straylight correction is essential.

Notes:
* beware that the calibration accuracy also depends on many other factors,
  see "What photometric accuracies can be expected".
* The straylight level is filter dependent and should therefore be repeated
  for each filter if necessary.

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