E

Ouit/3,

The choice of n = 1 is preferable for making small corrective plates. For medium to large plates, a uniform loading at q2 = 0.85bar with n = 4 i.e. q1 = 0.2125bar allows reducing the amount of glass outside the clear aperture. The Sphe 3 adjustment is achieved by slight under or over pressure during the end of the polishing while maintaining the constancy of the n -ratio. Vacuum-pressure controllers allows maintaining the right pressures q1, q2 and the constancy of their ratio.

For aspherizing plates up to 1 m in diameter, a figuring machine has been built together with a melted fine grain steel table rotating onto a hydrostatic polyethylene-oil-steel pad. The Schmidt plates are directly supported on a ring emerging from a rotating table and accurate load controllers operate during the stress figuring process (Fig. 5.5).

For instance, with fused silica, a working stress cult/2 = c103s/2 (see Table 5.2) allows obtaining plates at the fast f-ratio of Q = 1.2 if only one face is aspherized (p = 1) or at f/1.2x2-1/3 = f/0.95 if both faces are figured with half asphericity on

Fig. 5.5 Aspherization of large plates with two partial vacuum zones and full-aperture flat tools. [SP] Schmidt plate, [FT] figuring rigid tool, [SR] merging support ring, [OR] O-ring and radial reaction blocks, [RT] rotating table, [DS] double rotating seal, [HP] hydrostatic polyethylene-oil-steel pad, [OC] oil compressor and regulator, [PP] primary vacuum pump, [VPC] vacuum-pressure controllers, [H] mercury absolute gauges, [V] valves, [PS] air pressure stabilizers, [AR] air flow regulators

Fig. 5.5 Aspherization of large plates with two partial vacuum zones and full-aperture flat tools. [SP] Schmidt plate, [FT] figuring rigid tool, [SR] merging support ring, [OR] O-ring and radial reaction blocks, [RT] rotating table, [DS] double rotating seal, [HP] hydrostatic polyethylene-oil-steel pad, [OC] oil compressor and regulator, [PP] primary vacuum pump, [VPC] vacuum-pressure controllers, [H] mercury absolute gauges, [V] valves, [PS] air pressure stabilizers, [AR] air flow regulators

Table 5.2 Tensile rupture stresses - or tensile ultimate strengths - a of some brittle materials versus loading time [MPa]

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