The ISO Long Wavelength Spectrometer

The long wavelength spectrometer (LWS; Clegg et al., 1996) covered the range 43-196.9 $\mu$m with ten photoconductive detectors. The instrument could be operated in two modes; medium resolution ( $\lambda/\Delta\lambda\sim200$) using a diffraction grating and high resolution ( $\lambda/\Delta\lambda\sim8000$) using a Fabry-Pérot (FP) and grating. The operation of these two modes are described in the following sections. The LWS was designed and built by a consortium of scientists and engineers from Canada, France, Italy, the United Kingdom and the United States.

Radiation entering the LWS was folded, collimated and focused using a complex arrangement of mirrors. The optical path through the instrument is shown in Figure 1.2 before the FPs were inserted. Two FPs were used to cover the wavelength range and these were housed in a movable selection wheel (see Figure 1.3) that was placed between mirrors 5 and 6 in Figure 1.2. This wheel was driven by a specially designed cryogenic motor and could be rotated to 4 different positions. These successively placed the short wavelength FP (FPS), an open aperture, the long wavelength FP (FPL) and a blanking pad into the parallel part of the beam (although the blanking pad was never actually used in case the wheel got stuck in that position). The mechanism generally worked well throughout the mission, apart from one problem in accurately positioning the wheel which occurred in November 1996. This was subsequently fixed and no more problems occurred.

Figure 1.21.2: Photograph of the LWS before launch with an overlay of the optical path. The FP selection wheel is not shown (this was located between mirror 5 and mirror 6).

Figure 1.31.3: Development model for the FP selection wheel. The two FPs are shown on the left and right. Figure 1.6 shows the detailed structure of the FP mounting.

Figure 1.41.4: The top diagram shows a schematic of the detector block containing detectors SW1 to LW5. The spherical condensing mirror (M8) is shown at the top of the figure. The standard illuminators are shown above the detectors, numbered from 1 to 5. The lower diagram shows the transmission of an incident beam to the detector SW1 with the grating in its rest position.

To cover the full instrument wavelength range ten photoconductive detectors were used. These were cooled to cryogenic temperatures using the ISO liquid helium tank. The short wavelength range was covered by a Ge:Be and 5 unstressed Ge:Ga semiconductor detectors. Longer wavelengths (above $\sim$100 $\mu$m) were covered by 4 stressed Ge:Ga detectors. The application of mechanical uniaxial stress effectively decreased the semiconductor bandgap making the detectors sensitive to the longer wavelengths. The detector subsystem and readout electronics are described in detail by Leeks (2000). The detectors were mounted as shown in Figure 1.4. A sequence of 5 standard illuminators were placed in front of the detectors to allow calibration of the detector response through the mission.