GB569046A - Method and apparatus for improving optical elements - Google Patents

Abstract

569,046. Optical elements. TWYMAN, F., and STERNBERG, R. S. Dec. 16, 1942, No. 17905. [Class 97 (i)] Improvement of optical elements to correct for defects in the emergent wave front, as determined by interferometric means, are effected by adding transparent material, such as silica, cryolite, or fluorite, to parts of a surface of the element while other parts are masked in accordance with the results of the interferometer test ; the transparent material, may be deposited by sublimation in an apparatus. incorporating interferometric observation apparatus. The element may be a prism, lens, or mirror, and examination for determination of defects may be made by a Twyman and Green interferometer as described in Specifications 103,832, 130,224 and 213,674, [all in Class 97 (i)]. Deposition may be effected by sublimation in vacuo, as described in Specifications 538,272, 538,273, &c., under the control of masks prepared in accordance with the patterns obtained by the interferometric test. If the element is not a solid of revolution, the masks are cut to the shapes of the different " contour lines " of the pattern, and are used successively to enable a greater deposition to be made in the " lower " zones. Fig. 4 shows apparatus, for a lens or other solid of revolution, comprising an evacuated casing 12, a heating element 14 for subliming the material, and a holder 18 for a lens 15 above an aperture 16 in a partition 13. The holder 18 is rotated over a mask 17 by a rotatable magnet outside the casing, and is connected by a pulley 33 and belt 32 to a rotatable holder 30 for a mask 29, similar to the mask 17, located in front of a stationary concave mirror 21. The mirror 21 is an element in an interferometer system, comprising also a similar mirror 26, a window 22 and its compensating plate 25, and a transmitting reflector 23 and its compensating plate 24. Each mask has an aperture shaped in accordance with the initial interferometric test, and the mirror 21 and lens 15 are so placed that deposition occurs on them at equal rates, interferometric observation being made of the deposit on the mirror 21, and deposition being stopped when the observed pattern resembles that obtained by the initial test on the lens. Simplified apparatus is described, in which the deposit on a wedge-shaped glass plate, replacing the mirror 21 and mask 29, is observed by an " interferoscope." For the correction of a concave mirror, the mirror replaces the mirror 21 and observation is made directly thereon.