Abstract
For correct alignment a Cassegrain reflecting telescope must satisfy three conditions: (1) The optic axis of the primary mirror must pass through the center of the secondary. (2) The optic axis of the secondary mirror must pass through the center of the primary. (3) The resulting common optic axis must pass through the desired central point in the focal plane. In a conventional Cassegrain, whose focus lies behind a central hole in the primary mirror, the center of the field automatically coincides with the axis of the primary, and the other adjustments can be made quickly and easily. Some reflectors, however, lack a central hole in the primary and use instead a tertiary diagonal flat to divert the Cassegrain focus outside the tube just above the primary. Collimating such a system is much more difficult, since no fixed points are available and the alignment error of each of three mirrors must be determined independently. I have not found any ready description of a method for performing this alignment and was therefore compelled to work it out for myself, when faced with the problem of adjusting a large Cassegrain reflector. Without any claim to originality, the method is described here.
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