GB848692A - Improvements in or relating to stellar orientation detector - Google Patents

Abstract

848,692. Photo-electric determination of direction. NORTH AMERICAN AVIATION Inc. Sept. 7, 1956, No. 27534/56. Class 40(3). In order to generate voltages indicative of the deviation of the axis of an optical system from a line of sight to a star, light from the star is split into two beams each directed on to a respective photo-electric cell and means are provided to modulate each beam with a phase relation dependent on both the magnitude and sense of said deviation. The optical system comprises an objective lens 1, Fig. 1, and beam-splitting prisms 2, 3, the light from which falls on photocells 10, 11 respectively after being modulated by rotating reticles 4, 5 respectively. The reticles 4, 5 Fig. 2 are formed with two and three apertures, respectively, bounded by Archimedes spirals having curvatures in the ratio 2:3 and are driven by gears 7, 6, respectively, from gear 9 and motor 8 at speeds in the ratio 3:2. The generator 12, driven by the motor 8, produces a signal S 3 of standard phase at the same frequency as that of the signals S 2 , S 1 respectively arising in the photocells 10, 11, due to the modulation of the light beams by reticles 4, 5. Signals S 1 ,S 2 are fed to a differential synchro 19, Fig. 3, the shaft output of which corresponds to the phase difference # between the signals and hence to the star image angle measured clockwise from the positive Y-axis Fig. 2. This shaft output is doubled by gearbox 20 and causes the phase-shifter 14 to shift the phase of signal S 3 , applied thereto, by an angle 2#. The output S 4 from phase-shifter 14 is fed, together with signal S 2 ,to a differential synchro 16 the shaft output of which is thus proportional to #, the phase angle due to the spiral curvature of the reticle apertures. A mechanical computer 15 receives the quantities # and # as shaft rotations and produces therefrom the rectangular coordinates, y = K# cos # and x = K# sin # ,of the star image in the telescope field, which may be 'used to change the telescope orientation so that its optical axis coincides with the line of sight to the star. Details of a suitable mechanical computer are described, Figs. 5, 6, 7 (not shown). In a modified form of reticle, the spiral portion of the apertures occupies only the central portion, the remainder of the aperture being sector-shaped Fig. 4 (not shown). The modulation effect of a uniform sky background light or a linearly varying background light is avoided by the fact that the centroid of the apertured area of a reticle coincides with the axis of rotation. Specification 848,691 is referred to.