thus only concerning the five Clebsch-Seidel modes a20, a22, a3i, a33, a42, the three first are negative for a paraboloid. The flexure to generate during the stress figuring is represented by the opposite coefficients.

• Determination of the figuring curvature 1 /Rs - Minimal stress: For simplification of the execution process we may assume that the spherical curvature of the figuring tool, 1/^S, is the same for all the segments. Next, the determination of RS must minimize the stress level arising at the surface of the meniscus. Because all the four modes a22, a31, a33, a42 have a null mean curvature, the stresses introduced for generating them cannot be minimized. Hence the practical way for generating the remaining mode a20 is only concerned with a stress minimization.

Denoting um = dm/R the slope of the marginal segment, the minimization of the stress is achieved when the sag is half the total sag variation. From (7.38), this entails l - —

which, with (7.38), fully determines the execution conditions. Therefore, the stressing is as follows.

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