Dear LWS Observer, ************************************************************************* 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. ************************************************************************* "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