Patent GB-1185322-A (GB)
Title: Structures Including Position Determining Apparatus.
Inventor: NORTH JAMES CLIFFORD
IPC: G02B27/10
Filed: 1966-05-25
Abstract: 1,185,322. Optical detection of aerial dish distortion. POST OFFICE. 30 June, 1967 [25 May, 1966], No. 23498/66. Heading H4D. The Specification describes an 85 feet diameter parabolic dish aerial fitted with apparatus comprising a laser multibeam light source and a plurality of photo-cells mounted on the dish, for determining departure of the shape of the dish from the ideal. The dish comprises a central portion 1, Fig. 1 and 24 outer sections 2 hinged on the central portion. Pairs of photo-cells 28, 29, Fig. 4, connected in opposition, are arranged in concentric rings 3, 4, 5, 6, 7 and 8. Ring 4 will provide a reference signal/ring 3 will indicate deviation of the aerial feed from its correct position and rings 5, 6, 7 and 8 indicate distortion of adjacent parts of the respective sector. An axially mounted prism 11 directs light from laser 9 to a stacked arrangement of pentaprisms 12 (see Fig. 2) which produce five divergent beams 23, 24, 25, 26, 27, which, when the prism arrangement is rotated, exactly scan the middle of the photo-cell pairs in the respective rings 4, 5, 6, 7, 8, when no distortion of the dish is present. The plurality of beams may alternately be produced as different diffraction orders of a diffraction grating. Each photocell pair is mounted to be normal to the incident light beam, such that distortion of a surrounding sector part causes the photo-cell pair to move whereby the light beam 23 illuminates one cell more than the other and the differential output of the pair gives an error signal, giving by magnitude and sign, the magnitude and sense of the distortion. The outputs of the photocells may be displayed on a multibeam oscilloscope as shown in Fig. 4, one line to each ring of detectors. Reference ring 4 on the non- distorting central aerial portion 1, produces display 41a. The output of the photo-cells may control servo means repositioning the aerial segments. Prism 15 is mounted on the aerial feed and then reflects by angle # the laser light -axially offset by prism 13-to scan photo-cell ring 3. If the feed mount is displaced along the axis by #X and perpendicular to the axis by #r, then The direction of #r is given by the direction of the photo-cell pair giving the maximum pulse height. (Alternatively, Fig. 5, three photo-cell pairs 42a/42, 43a/43, 44a/44, are connected to the aerial feed, and are scanned by beams 45 and 46 marking angles #1, and #2 respectively with the aerial axis. The pulse outputs of cell pairs 42a/42 and 43a/43 are added together to give a pulse height proportional to #X (tan # 1  + tan # 2 ). The individual pulse height from cell pairs 42a/42 and 44a/44 are approximately proportional to #y and #z respectively, where #y and #z are orthogonal components of #r. Beams 47 and 48 may be stationary, Fig. 6, and originate from the prisms and fall on photocell groups 49/50/51/52 and 53/54 respectively. If the pulse height from a photo-cell is indicated by the number of that cell, then (50 + 51) - (49 + 52)α#y (51 + 52) - (49 + 50)α#z (54 - 53)α#y + #X tan # (54 - 53) - (50 + 51) + (49 + 52)α#x tan #.