GB852302A - Improvements in optical objective systems - Google Patents

Abstract

852,302. Vari-focal optical systems. COX, HARGREAVES & THOMSON Ltd., and HARGREAVES, F. J. May 4, 1959 [May 12, 1958], No. 15217/58. Class 97(1) An optical objective comprises a concave primary mirror having a central hole, a secondary mirror co-operating with the primary mirror, a collective lens interposed between the secondary mirror and the focal plane of the system and means for simultaneously moving at least two of the components longitudinally to vary the equivalent focal length of the system while maintaining the focus thereof in a constant position relative to the tube or casing of the system. In the form shown in Fig. 6, a concave primary mirror 10 is held in a cell 44, which is integral with a sleeve 42 movable for focusing on an extension from the back plate 34 of a casing 32, the focusing control being effected on rotation of a hand wheel coupled by rack and pinion with the sleeve 42. Within the extension 38 is a tube 52 carrying a collective lens 30 and a negative lens 28 of lower power. These lenses are movable together with a convex secondary mirror 12 for varying the focal length of the system. For this purpose, the tube 52 is connected to a flexible steel strip 56 wound on a drum 58 rotatable, through gearing, by a hand wheel. This drum is also geared to a wheel secured to a cylindrical edge cam 68 against which is pressed a roller 70 carried by a ring 74 and a rod 76, this rod being slidable in guides 78 and fixed to a ring 80 which is joined by a tube 82 to the ring 74. The ring 80 carries the convex mirror 12 which thus moves with the tube 52 when the cam 68 is rotated. The extension 38 from the back plate 34 and a lens hood 88 cut out extraneous light and a flange 40 at the rear of the extension 38 serves for attaching the vari-focal system to the lens turret of a television camera and between this flange and the back plate 34 a sliding grey wedge may be disposed to serve the purposes of an iris diaphragm. Optical data are given for one example of the lens combination 28 30 and of the mirrors 10 12; these mirrors may be figured. Another form is described in which a collective lens is disposed with its rear vertex at the vertex of the concave primary mirror and in which all three components are moved to vary the focal length and maintain the image in focus on a fixed plane. The system may also include an aspherical correcting plate or its equivalent.