Dear LWS Observer,
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LWS spectra sometimes contain features which resemble broad spectral
features, but which do not occur at the same wavelength in detectors
which cover the same range. (see section 7.3 of the LWS ISO Data User's
Manual [IDUM]). The origin of these features was originally thought to
be straylight, but is now believed to be a near-infrared leak in the
blocking filters located in front of the detectors. This supposition
is based on a statistical test, which showed a correlation between
these features and all bands longwards of I. The strength of these
features is best correlated with the strength of the sources in the
H-band (1.6 um). In a small sample of post-main-sequence stars
observed in the Core Programme, all those which had H-band emission
stronger than about 2.2 magnitudes (~140 Jy) were seen to exhibit these
spurious features.
Methods for measuring the contribution from these spurious features are
being investigated. Until these are in place, it will be extremely
difficult, if not impossible, to remove these features from strongly
affected observations and we recommend that such observations be temporarily
suspended. The criterion for which observations to suspend would ideally
be based on the relative strength of the source at H and in the LWS
wavelength range (e.g., at 60 um) but at the moment we cannot quantify the
problem this precisely. Therefore, in the interim we suggest you request
your observations be BLOCKED if the sources are expected to be stronger
than about 2.5 mag at H.
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"Blocking" is the method used to temporarily inhibit scheduling of
an observation -- it is accomplished by setting the fixed time flag to
"Y" and adding the text "Do not schedule". You may request this as
an update to your programme. To make an update:
You should contact the ISO helpdesk at helpdesk@iso.vilspa.esa.es (or IPAC
at iso@ipac.caltech.edu if you have been doing your PGA work there) and
give explicit instructions for changes to made on your behalf, or request
remote access to PGA or schedule a visit (PDEC will be unavailable for
visits during October and November due to the Supplemental Call Review
Process). When the changes are completed you will be sent a final PH report
for confirmation -- you MUST respond to the PH Report with a request to
re-release your observing proposal. This procedure is necessary to
verify that the correct version of your programme is loaded into the active
Mission Database!
sincerely,
ISO Helpdesk