Dear U.S. ISO Long Wavelength Spectrometer User, With ISO's three month Performance Verification Phase finished and about 150 hours of LWS testing completed, LWS AOTs have been commissioned and routine ISO Science observations are now comencing. I have recently attended an LWS calibration review and would like to report to you on things that have been learned about the instrument and mention to you some things yet to come. Effect of Cosmic Ray Glitches ----------------------------- This topic was covered in the 7 Feb 1996 letter you received from the Vilspa ESA helpdesk. The cosmic ray hits are ocurring at the rate which is at the upper end of the rate expected before launch, and have had a greater effect on the LWS detectors than expected. As the letter stated, cosmic ray hits have required that the nominal responsivity of the detectors be reduced (by commanding a lowering the bias voltages in the integrating circuits). This reduction in sensitivity is characterized by the table provided in the letter. These cosmic ray hits have both increased the noise and have forced a loss in responsivity. When a detector is struck by a cosmic rays, there are effects in the responsivity of a detector immediatly following the hit. The responsivity fluxuates or drifts, adding low frequency noise to a spectral scan. [THE FOLLOWING WILL BE ADDED ONLY WHEN DECIDED] To help the user identify the data that has been affected by glitch-induced detector instabilities, an second SPD (Standard Product Data) file may be included in the user-deliver data products giving the un-deglitched data. Users may then be able to see where the worst hits ocurred and assess the quality of the subsequent data for themselves. Also, during a single revolution (one sidereal day in which there are 16 hours of data collection), there is a slow change in the responsivity of the detectors over time. This drift is removed by the data reduction pipeline. One consequence of the cosmic ray glitches is to make "fast mode" for the all AOTS (grating and Fabry-Perot scans) the preferred mode. Cosmic ray glitches are thought to produce correlated noise in the detectors, making the most conventional of integration schemes potentially non-optimal. The search for the best averaging scheme or median extraction scheme for data reduction is actively being sought. Note, the observer receives all scans individually that make up an observation. The pipeline processing does not average scans. Other Sensitivity Issues ------------------------ The noise in the detectors in ideal conditions should be independent of the signal strength. This unfortunately is not the case. With higher signal strength the noise level rises in all detectors (except SW1), and these rises are now being taken in to account by the PGA, PH, and time estimator logic. Fortunately, the noise rises severely only with the strongest of signals. The effect will therefore not be a problem for those measuring the flux of strong continuum or strong line sources but will be a factor for those looking at weak lines on strong continuum. Fringes ------- Fringes or systematic ripples are seen to be present on spectra of extended sources. These may be of maximal amplitude 30% of the continuum and are a fixed sine wave of the parameter 1/lambda and have a period of about 2 um at the shortest wavelength and about 10 um at the longest wavelengths. The effect is most likely caused by interference within a reflective gap somewhere along the instrument's optical path. Dispite the large magnitude of the effect for some extended sources, the fringes are believed to be almost completely removable using software developed for the post-pipeline science analysis software (see ISAP below.) Because the distance to which a source is offset or extended within the beam effects the degree of fringing, users with targets larger than 50" are advised to obtain HIRES 60um images of their targets to point if possible at the FIR peak location and also determine the source extent. See http://www.ipac.caltech.edu/ipac/iras/hires_over.html. Beamsize -------- The beam is seen to be smaller than expected: nearly 80" FWHM at all wavelengths, with 5% wings extending to 130" (Full width) at the shortest wavelengths to 200" at the longest wavelength. More exact information on the beamsize is forthcoming. Grating Range Scans ------------------- As mentioned in the 7 Feb 96 Helpdesk letter, those doing full range grating scans, LWS01, are given the following tip: "if you can do without the range 187um - 198um, and you can live with a reduction in sensitivity (about a factor of 2) in the range 170um - 187um, then specify a range of 43um to 170um." You will still get data up to 187um but the observation will take 20% less time. This may be useful for those whose observing times are driven by the short wavelength sensitivities and are beyond reaching already what they need in sensitivity at the long wavelengths. Fabry-Perot Work at Long Wavelengths ------------------------------------ The FP that operates at long wavelengths, FPL, contrary to expectations, requires variable alignment that "moves" with wavelength. The LWS AOT03 has not yet been commisioned at long wavelengths for this reason. FPL will become available when a patch to onboard software is installed. The short wavelength FPS is covering wavelengths up to about 95um and is working well. The Future ---------- Currently if you request the ESA Helpdesk to rerun your program through their PGA/PH to see the effects of the new sensitivities, they will. Further, for LWS AOT1 and AOT2, you may experiment with your program by running the LWS time estimator. To use this program at IPAC, write to iso@ipac.caltech.edu, requesting that your IPAC computer account be reactivated. You will be called and told a new password. In a matter of weeks you will also be able to make arrangements to use the latest PGA which includes the in-orbit sensitivities to adjust your programs. Alternative ways of adjusting your program will be communicated to you by email soon as well. Data Reduction -------------- The LWS pipeline will produce [WHEN THIS IS DECIDED] scans in wavelength (Jy [TBD] vs. lambda) as an AAR file (Auto Anaylsis Report). Additionally, an additional SDP file will be included with no glitch removal. Eventually (weeks from now) a post-pipeline package for science analysis called ISAP (ISO Spectroscopy Analysis Package) will become available for SWS and LWS users. It will contain software to help a user average scans, and remove bad data. Subsequent versions may have the capability of deleting glitches, and removing fringes and fitting lines. It will run under IDL and will include graphical displays.