The collimator is an off-axis parabola of focal length 750,mm (nominal) aperture of 50,mm. The collimator can be removed from outside of the casing after removing the protective screw cap. As with other optics in the main optical train, two collimators are available, identical except for the coatings optimised for ``red'' or ``blue'' spectral regions. Information on the relative efficiencies of the two sets of optics is given in a later section. The ``red'' collimator is identified by a red painted disc on the collimator cell with a similar blue marking on the other unit.