harness is shown in Figure 2. As far as could be determined by sensitive tests in the optical shop, this warping harness was completely satisfactory in every respect. Although the warping harness for the SVAS telescope shown in Figures 3a and 3b is more sophisticated in its entirety, it employs the same basic pattern of load distribution.
A second stratagem, which can be used to reduce the excess deflection of the mirror surface in the immediate vicinity of the pressure points, is to reduce the thickness of the mirror blank. The total deflection of the mirror surface at any given point is largely due to bending of the mirror blank; but in addition, and to a lesser extent, is due to shear-action. The deflection due to shear-action varies directly as the applied load, inversely as the thickness of the mirror, and builds up rather rapidly as we approach the pressure point. The deflection due to bending, on the other hand, varies inversely as the square of the thickness. A moderate reduction in thickness will, therefore, result in a rather large reduction in the force required to bend the mirror; and this will result in reduction of the shear-action component of the total deflection at each point on the mirror surface.
Even if we were to make the mirror blank wafer-thin, the deflection of the mirror surface would not fit the desired curve perfectly, so we should not try to carry this idea to an extreme. If we should make the mirror blank too thin, the optician would have difficulty in controlling the surface curve of the mirror to within the desired tolerances. In order to give some idea as to what is recommended in this respect - the 12.5-inch-diameter pyrex blank for the secondary of the SVAS 12-inch Yolo was thinned down from the standard 2.125-inch thickness to 1.75 inches.