Optical design of the SPIRE instrument for FIRST

The SPIRE instrument covers the 200-670 micron spectral range with a three-band, 4 x 8 diffraction limited field of view photometer, and a dual-band, 2.6 diameter field of view imaging FTS. Optimisation of the photometer optics has been given a high priority in the instrument design, allowing an all-reflecting configuration with seven miners in one plane. The design corrects for the large tilt of the telescope focal plane due to the off-axis position of the SPIRE field of view, and provides two pupil images (where a beam steering mirror and a cold stop are located) and two field images (where a pickoff mirror for the spectrometer and the final image are located). A large back-focal length allows for dichroic band separators and beam folding mirrors. The spectrometer is a Mach-Zehnder-type, dual channel FTS providing two input and two output ports. The output ports are physically separated from the input ports, and the second input port is fed from a black-body source providing compensation of the telescope background, required to minimize the effect of jitter noise. Powered mirrors are used within the interferometer arms to minimize beam diameters and to leave maximum space for the scan mechanism. The complementary output ports are filtered by band-pass filters to provide the two spectral channels required.