The amount of parabolization, Y, which when applied to the primary mirror, will correct for both mirrors, is given by the following expression:
When R1 is equal to R2 and T is 1.2 times S, the following is true:
This means that the primary mirror should be parabolized by 416 percent. Although this might sound like a lot of parabolization, in actual amount it is quite small and will be difficult to measure. To test for this amount of parabolization, we recommend either a caustic tester or a null test such as that proposed by Dall(2). Another alternative is to have the secondary mirror aluminized and then mount both mirrors in the telescope tube and test on a bright star with a high-power eyepiece. This method will require good seeing and may be much less convenient than a shop test, but it is very effective in obtaining the right amount of overall correction for the system.
Although the design procedure to be described here may seem somewhat unorthodox, it has the effect of forgiving the telescope maker for some of his mistakes in meeting tolerances or allowing him to just about ignore them. The first step, after the aperture has been decided upon, is to select a good value for the effective focal length, F. The few Yolo reflectors which have been