built (or started) to date have been designed as f/15 and f/17 instruments. Throughout this paper reference will be made to two of these, an 8-inch f/15 design described very briefly in Sky and Telescope(3), and a 12-inch f/17 design currently under construction for the Sacramento Valley Astronomical Society Observatory at Colfax, California. Although there seems to be no inherent reason why fairly short focal lengths should not be satisfactory, the prospective telescope maker is cautioned to not try anything much shorter than about f/12 or f/10 until some Yolo designs of these focal ratios have been built and thoroughly tested.
With values for D, and F decided upon, values for R, D2, S, and T are calculated, using Eqs. (7), (8), (9), and (10). Next, a scale drawing is made showing the two mirrors and the light-path, much like Figure 1. The secondary mirror blank is located so that it clears the parallel bundle of rays going to the primary by about one-twentieth of its diameter. This, then, permits angle, A, to be measured or calculated. With this known value for A, a value for angle, B, is calculated, using Eq. (11). This permits the focal point to be located and the outline of the light-path to be drawn to scale. Following this, the system of light-baffles is designed (see "Light-Baffle System," ahead) and drawn to scale.
At this point we are ready to start work on the construction of the telescope itself. The first step here is to rough-grind both blanks to the desired radius of curvature, thin-down the secondary blank (if this has been decided upon) and smooth up the backs of both blanks until they are reasonably flat and parallel to the front surfaces. Next, the mirror cells and warping