Both collimators are off-axis parabaloids with a focal length of 1650mm, and provide a collimated beam of 150mm diameter. The coating material on the collimators is optimised for the wavelength range of the particular camera, and is silver with a reflective stack overcoat for the red collimator and aluminium for the blue camera.
As with most astronomical grating spectrographs, an image of the pupil is formed on the grating in order to minimise the grating size required.
The collimators are remotely driven by stepper motors and their position is encoded. The spectrograph arms are normally focussed by driving the collimators; and the collimator position is repeatable to better than 10 m. With no extra refractive components (dichroics, prisms, Savart plate, filters) between the slit and the collimators the nominal focus positions for the ISIS collimators are 6000m for the blue arm, and 9000m for the red arm. The spectrograph should be focussed with the collimators within 3000m of these nominal values, otherwise the spectrograph will be astigmatic, and the best focus on a spectral line will result in a degradation of the spatial resolution along the slit.
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Figure: The ISIS Collimator Assembly