22" Telescope Design Considerations
Choosing the design of a telescope is always a trade off between conflicting requirements. Some designs are inherently costly and are not versatile. Above a certain size portability becomes impossible. The availability of components is a major consideration for the amateur. Should new unproven technology be used? New technology drives up cost and increases uncertainty. Existing technology may not meet specific goals. The most important aspect of the design is to keep the telescope versatile. Telescopes last a long time so it is important to keep the design simple and capable of accepting new instrumentation. Usually it is not possible to meet every requirement for practical or financial reasons. All mountings are a comprimise at some level.
The Newtonian optical design was chosen for the 22" telescope. The Newtonian design allows the same primary to be used for a Classical Cassegrain. The Newtonian focal length was decided to be 99" yielding a focal ration of f4.5. The Cassegrain focal length was decided to be 264" for a focal ratio of f12. The English fork mount was decided on for compactness, access to the telescope focus, and stability. The conventional fork mount was modified by enlarging the polar axis and replacing the north polar bearing with a series of rollers and a disk, Figure 1
Figure 1. The progression of the load of a telescope, shown by the arrow, is moved toward the north polar bearing as the north bearing is enlarged. The enlargement of the north bearing provides better support for the telescope load.
The enlargement of the polar axis brings the load closer to the north bearing and makes an expensive large north bearing unnecessary. The north bearing was replaced with a series of small inexpensive rollers. The result was a modified fork design shown in, Figure 2.
Figure 2. The modification the
polar
axis to a cone and the use of rollers instead of a large north bearing
in a fork mount. This is a very stable arrangement for a fork mounting.