Another anomaly that was found in one of the early observations
with the LWS instrument was the occurrence of straylight features
in the spectra. These show up as broad emission features, and
could easily be identified as dust emission features. However
if the spectra of different detectors are compared the features
move in wavelength from one detector to the next. The features
are observed on detectors that are close to each other in the
LWS instrument. The order of the detectors in the instrument is
not in increasing wavelength, instead the detectors are paired
with a short and a long wavelength detector in each pair.
The order of the detectors in the instrument is SW1, LW1, SW2, LW2,
SW3, LW3, SW4, LW4, SW5, LW5. The straylight features that were
observed in early observations were on detectors SW1, SW2, LW1
and LW2.
The suspicion is that the features are caused by straylight that
falls at a constant input angle onto the grating. This would
cause a straylight feature on a number of adjacent detectors,
that moves in wavelength from one detector to the next.
The cause of the straylight is not known. Removing these
features from the data is very difficult, if not impossible.
Therefore any observation that has straylight features should
be considered as failed. Unfortunately the processing software
and the quality check of the data cannot at this time identify
these observations. The observer therefore should contact the
ISO helpdesk (helpdesk@iso.vilspa.esa.es) if there is the
suspicion that there are straylight features in the data.