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There is a choice of two collimators, one coated for the short wavelength region (Å ) and one for longer wavelengths (Å ). They are off-axis paraboloids, with a measured focal length for both collimators of 23590.5 mm and an off-axis distance of 2950.5 mm, implying an off-axis angle of 7.18 degrees. Space constraints mean that the instrument has in fact been constructed assuming an off-axis angle of 7.39 degrees. The diameter of the collimated beam is 215 mm in the absence of focal modifiers.