Patent GB-590496-A (GB)
Title: Improvements in and relating to sighting instruments
IPC: G01C21/18
Filed: 1944-11-29
Abstract: 590,496. Sextants; gyroscopic apparatus. SPERRY GYROSCOPE CO., Inc. Nov. 29, 1944, No. 23849. Convention date, Sept. 26,1941. [Classes 97 (i), 97 (ii) and 97 (iii)] In a sighting instrument such as a sextant, an image of a gyro-stabilized reticule is formed on a translucent screen which tilts with the instrument and the screen is seen superimposed on the sighted object. The sextant shown in Fig. 1 is enclosed in a casing 37 and tube 29, the latter being rotatably supported at 31 in supporting case 9. The reticule 101 comprises a blocking screen permitting light to pass only through crossed slits. The reticule image is obtained by projecting a light source 93, Fig. 6, through a condenser 95 on to an opal 97 which acts as a secondary source. Light from the opal 97 is collimated at 99 and projected in parallel rays through reticule 101 on to the translucent screen 104. The reticule 101, collimator 99 and opal 97 are mounted in a tube 117 which is pivoted in a fork 116 fixed to the transverse trunnion 114 of a gyro vertical 112 and is maintained parallel to the rotor spin axis by parallelogram linkage 119. In a modification, the reticule 101 is moved to the middle of the tube 117 and an image thereof is focused on the screen 104 by additional lenses. The secondary light source may be dispensed with and the primary light scurce 93 placed at the point 97. An image of the screen 104, Fig. 1, is projected by the optical system 106, 107, 108, 109, 110 and semitransparent reflector 83 into the eyepiece 91. An image of the sighted object reaches the eyepiece 91. via the entrance reflecting prism 76, lenses 77, 79, 81 and through the semi-transparent reflector 83. Azimuth control of the instrument is effected from a handle located near the eyepiece and driving through a train of gearing a pinion 57 which runs round a ring gear 59 on the casing 9. A scale 63 is provided on the casing 9 to co-operate with an index on a part 35 of the instrument casing 37. A similar handle operates the elevation control through a train of gearing (not shown in Fig. 1) which terminates in a worm co-operating with a toothed quadrant to move the prism 76 about a horizontal axis in its supporting bracket 151. A nut is arranged to travel on a threaded shaft in this train of gearing and stops are provided to limit the movement of the nut so that injurious contact between the prism 76 and its mount is prevented. The angle of elevation is indicated by a counter 138. Both trains of gearing drive selsyn transmitters for repeating the indications at a remote point. The prism 76 is enclosed in a hood 33 having a window 73 which either carries electric heating wires to prevent frosting or is made of a material which converts light radiation into heat. Three filters 161, 162, 163 may be swung into the optical path at will by gearing operated from remote handles. The lowermost filter may be formed of dark glass for viewing the sun. The upper pair of filters may be formed of lightpolarizing material so that the light intensity passing into the optical system may be varied by relative rotation of the filters. In order to indicate the attitude of the supporting craft, an outline of an aircraft is drawn with fluorescent material on the screen 104 and the intensity of this outline may be varied by controlling the amount of ultra-violet light irradiating the fluorescent material. The screen 104 is mounted in a rotatable ring which is geared to the azimuth control so as to maintain. its position relative to the aircraft when the sectant is turned in azimuth. Tilting of the aircraft is then indicated by relative movement between the reticule image and the outline of the aircraft. In order that these two references may be made to coincide for the trim which is, for the time being, normal to the aircraft, the casing 9 supporting the sectant is pivoted about an athwartship axis 11 in a spider 3 and may be clamped in its adjusted position about this axis by the handle 25. The spider 3 is hung from the wall 1 of the aircraft cabin by resilient mountings 5. When used in pressure type cabins, the casing 9 is connected to the cabin wall 1 by flexible membranes 27 and the rotatable mounting of the tube 29 in the casing 9 is made air-tight by tight fitting sleeves 12, 14 and an annular groove 10 filled with lubricant. U.S.A. Specification 2,177,094 is referred to.