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Testing the Long Wavelength Spectrometer for ISO

The Infrared Space Observatory (ISO) has a complement of four focal plane instruments for making a range of astronomical observations at infrared wavelengths. The telescope and instruments are operated at cryogenic temperature. Spectroscopy is shared between two of these instruments, with the Long Wavelength Spectrometer (LWS) providing for spectroscopic observation over the wavelength range 43 micrometers to 198 micrometers at two resolving powers. The flight model of the LWS has been completed, following an extensive program of performance testing and calibration at the Rutherford Appleton Laboratory. For this, a test facility has been developed to provide the necessary operating and environmental conditions, including a very low thermal background. The design and operational details of the test facility are given, followed by examples of the LWS performance values obtained. The data from these measurements will provide the initial calibration of the LWS in-orbit.