The most that should be put between the metal and the glass is a single sheet of onion-skin paper. A material such as sheet cork should not be used between the metal and glass because, under the existing pressures, it will yield slowly and thus make the harness lose its adjustment. All the articulated joints, or pivot points in the warping harnesses shown in Figures 2 and 3a consist of one-eighth-inch steel balls pressed into drilled recesses in mild steel members. In order to facilitate assembly, the balls were soldered to one of the members at each joint, care being taken not to overheat the balls to the point of discoloring them in the soldering operation. It is felt that this construction is preferable to sharpened points, because with small points, pressures may easily reach the yield strength of one of the materials of the joint thus permitting plastic flow to take place.
Even though the areas of plastic flow may be quite small, they are still subject to a small amount of creep in the metal, and in time, may allow the warping harness to get out of adjustment.
One more adjustment - rotation - must be provided. In order to obtain satisfactory cancellation of the astigmatism, not only must the tension in the warping harness be adjusted to within about one-half of one percent, or less, of the required magnitude, but the phase, or rotation of the warping harness must be corrected to within about one-tenth of a degree. Provision for this adjustment can be made by attaching the secondary mirror-mount to a circular plate (parallel to the mirror blank) and then attaching this plate to the telescope tube so that it can be rotated a few degrees in either direction within its plane.
So far, we have spelled out requirements to be met by the warping harness. Since the mirror blank is a partner in this warping operation, it too must be