Collimating a Three-Mirror Cassegrain Reflector

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