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The design of the Utrecht Echelle Spectrograph is identical to that of the spectrograph built at University College London for the f/36 coudé focus of the Anglo-Australian Telescope, apart from unavoidable differences arising from the different focal ratios at the two telescopes. This section summarises the basic design parameters of the spectrograph. A discussion of the design philosophy is given by Walker & Diego (Monthly Notices of the Royal Astronomical Society, vol 217, p355, 1985).
The layout of the spectrograph is shown in Fig. 1. The slit S is at the focal plane of the telescope. The light from the slit image (beam a) is collimated (beam b) by an off-axis parabolic mirror C, and traverses the three cross-dispersing quartz prisms P1, P2 and P3 before beam c is incident on the echelle E. L1, L2 and L3 are focal modifying lenses for varying the degree of overfilling of the grating. The axes of beams a, b and c are horizontal. The light is dispersed in the vertical plane (beam d) by the echelle (beam d) and is finally re-imaged by the camera.
The components of the spectrograph are assembled on an optical bench, which in turn is supported by the instrument mounting pads on the camera, is contained within a thermal enclosure in order to maintain it at a constant temperature.