One of the problems of making aspherical mirrors by thin-film deposition of aluminium is the low reflectance of the films. Such a low reflectance of the films is due to the fact that the aluminum film is too thick and makes the surface rough, resulting in scattering of light 1-5. Another problem is that the reflectance of an aspherical mirror is not the same at different zones, because the thickness distribution of an aluminium film is not uniform. For example, for a desired paraboloid with a vertex radius, R, and a starting spherical substrate with a curvature radius, R', and a maximum radius, S-0, the required film thickness distribution, D(S), is S-2(S-0(2)-S-2)/8R'(3), where S is the radius 5. The relationship between R and R' is R = (4R'(3))/(4R'(2) + S-0(2)). Differentiating D with respect to S, we then find that the largest film thickness is S-0(4)/32R'(3) at S = (1/2)S-1/2(0). Assuming that R = 500 mm and S-0 = 50 mm, the thickness of the film is 1.661 mu m, which is too thick to have such high reflectance as a thin aluminium film does. To deposit thick films and still maintain good optical quality is state of the art. Dobrowolski and Weinstein used zinc sulfide to make aspherical lenses that exhibited no peeling or cracking 3, 4. A zinc sulfide-thorium fluoride mixture evaporation source was suggested to make aspherical mirrors by Kurdock and Austin 6. In this paper we show the results obtained when an Al2O3 layer is deposited to form an aspherical surface, followed by deposition of a thin layer of aluminium. References: 6
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