the mirror (see Figure 3b) should be accurately in the principal plane of the system. The two "pushers" lie approximately one mirror radius above and below this plane.
Satisfactory collimation can be achieved with the aid of a pair of fixtures, one made for each end of the telescope. Each fixture consists of a slender rod which can be attached to - or mounted directly in front of - a mirror. Each rod is long enough to extend across the mirror and out past the center of the light-path passing that mirror. Each rod carries two movable markers.
The following procedure is recommended:
1. Attach each fixture to its mirror, and make adjustments to have each rod pass directly in front of the center of its mirror and slide the mirror-marker along each rod until it is squarely in front of the center of its mirror.
2. Sight along the center of the eyepiece tube to the secondary mirror-marker and then rotate primary fixture until its rod falls in the line of sight.
3. Slide light-path marker along the rod until it is on the line of sight to the center of the secondary. This, then, places the light-path marker on the line running from the focal point to the center of the secondary. The primary mirror-marker is now on the line running from the center of the primary to the center of the secondary.
4. Measure the distance between these two markers; and, with the measured distance, S, calculate the angle, B.
5. With this value of B and Eq. (5), calculate angle, A. With A and S, calculate the distance from the center of the secondary mirror to the center of the light-path to a star.