The Long Wavelength Spectrometer (LWS)

PI: P.E. Clegg, Queen Mary and Westfield College, London, GB

The LWS is a spectrometer for medium and high resolution spectral observations in the wavelength ranges 43-196.7 and 47.0-196.7 m respectively. The instrument has a 1.65' aperture, which corresponds to the diffraction limit at 118 m. A four position aperture wheel is used to select the observing mode of the instrument. This wheel has one closed position, one open position for grating observations (43-196.7 m) and two tunable Fabry-Pérots for short (47-70 m) and long (70-196.7 m) wavelength observations. To observe the full wavelength range 10 semiconductor detectors are used covering 10 adjacent wavelength bands. The instrument has two principal observing modes, a medium resolution mode and a high resolution mode.

• In the medium resolution mode the grating is scanned to get full spectral coverage using 1st and 2nd order interference. All ten detectors will give data. The spectral resolution of the instrument in this mode is 0.3 m in the range 47.0-90.5 m and 0.6 m in the range 90.5-196.7 m; this corresponds to a velocity resolution between about 1000 and 2000 km/sec.
• In high resolution mode one of the two tunable Fabry-Pérots is used to select wavelength regions of interest. The spectral resolution in this mode is about 10000 (30 km/sec). Only one of the ten detectors will then give useful data. It should be noted that a full high resolution LWS scan will take more time that is allowed for a single observation.

To give some examples of the performance of LWS, in medium resolution grating mode an observation at 100 m of a 10 Jy object (e.g. a star) will take 4.35 sec. per spectral point, giving a S/N ratio of about 50 per spectral point. Assuming the spectrum is observed with four points per spectral resolution element (260 points in total) a full spectrum would then take 1508 sec (25m8s), including instrumental overheads. A high resolution FP observation of a 5* W/m line at 63 m on a 373 Jy continuum (e.g. a star forming region with strong continuum and line fluxes) will take 23.5 sec per spectral point, giving a S/N ratio of 21 on the total flux and 10 on the line. The total time for a line scan with 4 spectral resolution elements on either side of the line, and 4 spectral points per resolution element (36 in total) would be 1128 sec (18m48s), again including instrumental overheads.

The LWS Flight Model has been delivered to ESA and is being used for the Integrated Module Tests at DASA. The LWS instrument has been useful for detecting the dark background radiation at long wavelengths within the ISO cryostat, and determining its spectral shape. The LWS Flight Spare is currently being tested in the Cryogenic Test Facilities in the Rutherford Appleton Laboratories (Chilton, UK). Tests are going well and the overall performance of the model is satisfactory. The model is also used to test the commanding of the instrument for calibration observations.