The third stratagem which can be employed to eliminate localized excess deflections produced by the warping harness is to use what might otherwise considered an oversize glass blank and thus move the pressure-points on the glass well outside of the area of the mirror which is used to reflect the cone of rays of light. This stratagem has the further advantage of relieving the optician of some of his working tolerances and making an optical surface with a fairly bad turned edge perfectly acceptable - as long as the turned part of the edge does not extend too far in from the edge of the blank.
This device has the disadvantages that it increases the cost of the glass blank and it makes necessary larger angles of tilt in the design. Although larger tilt-angles result in larger tilt-aberrations (which are compensated for in the Yolo design) we should charge only a small fraction of this disadvantage against this idea since the larger tilt-angles reduce very appreciably the problem of baffling out the unwanted stray light which might get into the eyepiece. Although the warping harness might be applied successfully to either the primary or the secondary mirror, the increase in the cost of the blank resulting from the employment of this stratagem will be much less if secondary is warped rather than the primary. Furthermore, if the amount of this increase in blank diameter is chosen properly, the diameter of the blank for the secondary will be equal to that of the primary. When this is done, one grinding tool and one polishing lap will suffice for both mirrors; and the economics from this may more than offset the added cost of the increased diameter of the secondary blank. It is for this reason that we recommend that the warping harness be applied to the secondary rather than to the primary mirror.