2.1 Introduction

The Long Wavelength Spectrometer (LWS, Clegg et al. 1996, [9]) was one of the four instruments on board the Infrared Space Observatory which operated between November 1995 and April 1998. The LWS covered the spectral range between 43 and 197m at medium ( 150-200) resolution using a diffraction grating and at high resolution ( 6800-9700) with either of the two Fabry-Pérots additionally placed in the beam. ISO was operated as an observatory and LWS users had the option of using any combination of four observing modes defined as standard astronomical observation templates (AOTs). These consisted of wavelength range scanning or line scanning, using either the grating alone or with the Fabry-Pérot. A sub-mode of the grating line scanning mode where the grating did not move from the rest position was also defined for narrow-band photometry. The LWS was equipped with ten photoconductive detectors overlapping in wavelength range, five operating in second order and five in first order. A Ge:Be detector was used for the shortest wavelength range (43-51m), five Ge:Ga detectors for the 50-121m range and four stressed Ge:Ga detectors for the longest wavelength range (108-197m).