GB486851A - Improvements in or relating to sighting telescopes - Google Patents

Abstract

486,851. Telescopes. REASON, R. E., and KAPELLA, Ltd. Dec. 11, 1936, Nos. 30740/36 and 663/38. [Class 97 (i)] In a sighting telescope, particularly for measuring the angular or lateral displacement of a member by observing the displacement of the image of a mark &c. formed by an objective in the telescope, the observing system is provided with means for reversing the displacement of one portion of the image relatively to that of the other portion and means are also provided in the telescope whereby the two portions are brought into superposition or into alignment with one another. Fig. 1 shows a form in which the mark is within the telescope and consists of a slit D illuminated by a lamp C and condenser C<1>. The rays are directed along the axis of the telescope by a thin film E constituting a semi-transparent mirror. The optical distance from the slit to the telescope objective A is equal to the focal length of the objective so that the rays on emerging from the objective are parallel. These rays are received by a mirror F on the member whose angular position is to be measured and are returned through the objective to form an image in the focal plane. In this plane the rays are divided, for example by a pair of segmental lenses G<1>, G<2>, and the two beams pass to an objective H having its principal focus in the plane of the image, and thence through apertures J<1>, J<2> in a screen J to an objective H<1> forming a second image, the beams being recombined at such image by a biprism K and transmitted to an eyepiece K<1>. Between the objectives H, H<1>, one of the beams passes through a reversing prism L<1> and the other beam passes through a glass block L to equalize the ray paths. To bring the two half images into alignment, a prism M displaceable along the axis of the telescope is provided, this prism being provided with a pointer M<1> which indicates on a scale M<2> the displacement of the mirror F. Instead of an auto-collimating telescope as above described, a focusing telescope as in Fig. 3 may be used in conjunction with a sighting mark on the object whose displacement is to be measured. In Fig. 3, the whole of the ray-dividing and observing system is mounted in a tube S which can be moved axially for focusing by a rack S<1> and pinion S<2>. To prevent lateral errors in the focusing movement the tube S is mounted in the tube B in two bearing rings T having sharp-edged recesses T<1>, Fig. 4, constituting a four point support, against which the tube is pressed by springs U acting through balls U<1>. To enable this instrument to be used alternatively as an auto-collimating telescope, a slit D and lamp C may be provided. Specification 430,761 is referred to.