by the warping harness. This is given by the following equation:
where U is the calculated separation between the artificial star and eyepiece to be used in the test set-up for checking the performance of the warping harness in the optical shop, and is given by Eq. (19).
The weight of the warping harness shown in Figures 3a and 3b is carried by a thin sheet-metal spider and a set of counterweights. With this design the actual weight of the warping harness is unimportant so far as flexure is concerned. With the design shown in Figure 2, however, the total weight of the harness is carried by the mirror. This will produce unwanted deflections, or errors, in the surface curve of the mirror. In order to keep these errors to a minimum, the warping harness should be designed as light as possible. Lightness could be achieved by constructing the harness out of high-strength aluminum alloy; but it is felt that the high coefficient of thermal expansion of aluminum alloys might make this material unsatisfactory for this application. For this reason steel is preferred as the material for the warping harness. In order to avoid excessive weight with steel, some guide lines are needed.
The warping harness shown in Figure 3a was made of annealed mild steel. Assuming a yield strength of 40,000 lb per square inch and dimensions as measured, it should have been capable of sustaining a load of about 250 lb at the ends of each of its four arms. Substituting this value for P and putting the known values for t, D2, and wa for this unit into Eq. (14), a value of K = 14.5x106 lb/in3 is obtained. This warping harness was tested