GB2178865A - A device for holding porro prism inversion systems - Google Patents

Abstract

A device for holding a Porro prism inversion system of the type in which the longitudinal axes of two 90 DEG single prisms 1,2 form a right angle and the two 90 DEG single prisms 1,2 are cemented together at the partially overlapping hypotenuse faces 3. The two side faces of the prism 1 defined in each case by the hypotenuse and the two shorter sides of the prism, are formed with respective sloping portions 4,5. The free partial hypotenuse face of the prism 1 rests on a support surface 6 in a housing 7. An inclined locating surface 12 is provided in the housing 7, against which the sloping portion 4 abuts. A fixing wedge 10 is disposed between the housing 7 and the sloping portion 5 to secure the prism 1,2 rigidly in the housing. <IMAGE>

Description

SPECIFICATION A device for holding porro prism inversion systems of the "first type" The present invention is concerned with devices for holding Porro prism inversion systems of the "first type", as hereinafter defined, and preferably, but not exclusively, with devices for holding Porro prism inversion systems of the first type in observation instruments such as telescopes,binoculars, and the like.

A Porro prism inversion system ofthe "first type" is one in which the longitudinal axes of two 900 single prisms form a right angle and the hypotenuse faces are directed toward each other such that they overlap by approximately a half.

Various means of holding the two prisms are already known in that art and can essentially be divided into three main variants.

In the first variant, the hypotenuse faces of the two prisms are joined to a plate located between the prisms on which corresponding prism support areas have been provided and are each pressed against them bya resilientclampclip. Following adjustment ofthe two prisms in the optical ray path, the prisms are generally prevented from slipping sidesways by screwing, sticking or similar measures.

Disadvantages of the first variant are that: - light losses result from two glass-airtransitions in the optical ray path through the plate located between the prisms; - complex adjustment is necessary for holding and fixing the prisms in the housing; and -the plate is provided on both sides with prism locating surfaces, sothattwo-sided opening ofthe housing is necessary, with the plate being fixed.

In the second variant,the overlapping partial hypotenuse faces of the two prisms are cemented together, wherein on the one hand light losses are reduced, but, on the other hand, the size ofthe remaining support areas for aligning the prism system in the housing is greatly reduced,thus rendering accurate and secure holding much more difficult.

Usually the two residual hypotenuse faces are used to support the prism system. Following alignment in the optical ray path, each prism ofthe prism system is held by clamp clips, adhesive orscrewconnections.

Disadvantages of the second variant are that: redundancy occurs in determination of the prism position in the housing as a result of alignment being effected in accordance with several locating surfaces; -mechanical stress occurs on thecementconnec- tion between the prisms due to the securing ofthe two prisms, in particular in the case oftemperature changes resulting from the differing expansion ofthe prism glass and the housing material, complex and expensive housing manufacture, as well as elements for the prism holder.

In the third variant,thetwo prisms are stuck either singlyorasacemented prism system onto thesup- port area ofthe prism plate.

Disadvantages ofthe third variant are that: -careful preparation and clean execution ofthe adhesive connections are necessary; - use of holding devices is necessary; - assembly is interrupted because of the setting time ofthe adhesive; and -the system cannot be diassembled for repairs or maintenance.

It isthe overall aimofthepresentinventionto provide an extremely simple, cost-effective device for holding Porro prism inversion systems which functions reliably and in which the disadvantages ofthe priorartare eliminated.

It is a particular object of the present invention to implementadeviceforholding Porro prism inversion systems using particularlysimplestmeanswith which light losses from glass-airtransitions as well as complex working, assembly and adjustment of the prism, housing and prism holder are avoided.

In accordance with the present invention, there is provided a device for holding a Porro prism inversion system of the first type (as hereinbefore defined), wherein the two 900 single prisms are cemented together at the partially overlapping hypotenuse faces, the two side faces of one of the two prisms, defined in each case by the hypotenuse and the two shorter sides ofthe prism, are formed with respective sloping portions, the free partial hypotenuse face of said one ofthe prisms rests on a support surface in a housing, an inclined locating surface is provided in the housing againstwhich oneofsaid sloping portions of said one prism abuts, and a fixing wedge is disposed between the housing and the other one of said sloping portions of said one prism.

Preferably, the fixing wedge, at the side thereof adjacent the housing, abuts against a point fixed to íhe housing, the element acting as thefixing part being a prismatic element whose cross section is an irregular polygon, one edge ofthis prismatic element abutting againstthe housing and the largestface of the prismatic element opposite this edge abutting against said one prism provided with the sloping portions.

In orderto ensure that the position ofthe Porro prism inversion system in the housing is secure and stable, the point fixed to the housing orthe edge of the prismatic element abutting against the housing and acting as the fixing wedge should advantageously lie between the support area and the edge ofthe inclined locating surface which isfurther awayfrom the support area.

The invention is described further below, byway of example only, with reference to the accompanying drawings, in which: Figure lisa side view of a Porro prism inversion system of the first type disposed in a housing; and Figure 2 is a plan view of the Porro prism inversion system of Figure 1.

In Figures 1 and 2, prism 1 and prism 2 are cemented together by way of the overlapping hypotenuse subfaces 3. Sloping portions 4 and 5 are provided on both sides ofthe prism 1.

The free partial hypotenuse face of prism 1 ofthe prism system is placed on a support area 6 of the housing 7. The sloping surface 4 abuts against an inclined locating face of a lateral locating stop 8. In addition, the lower prism 2 is placed against a longitudinal locating stop 9.

Afixing wedge 10 is inserted opposite the lateral stop 8 between the sloping surface 5 and the housing 7. On the housing side, the fixing wedge 10 abuts against a fixed point 11 on the housing, so thatthe direction of pressure remains unaltered even if the wedge is inserted to various depths as a resu It of manufacturing tolerences. Atthe same time, the point 11 fixed to the housing is closerto the bearing area 6 by an amount A compared to the stop edge 12, thus achieving a secure and stable position for the prism system having the suspended prism 2.

Claims (4)

1. A device for holding a Porro prism inversion system of the first type (as hereinbefore defined), wherein the two 90" single prism are cemented together at the partially overlapping hypotenuse faces, the two side faces of one ofthe two prisms, defined in each case by the hypotenuse and the two shorter sides ofthe prism, are formed with respective sloping portions, the free partial hypotenuse face of said one ofthe prisms rests on a support surface in a housing, an inclined locating surface is provided in the housing against which one of said sloping portions of said one prism abuts, and a fixing wedge is disposed between the housing and the other one of said sloping portions of said one prism.

2. Adevice as claimed in claim 1, wherein, the fixing wedge, atthe side thereof adjacent the housing, abuts against a point fixed to the housing.

3. A device as claimed in claim 1, wherein the element acting as the fixing wedge is a prismatic element whose cross section is an irregular polygon, and wherein one edge of this prismatic element abuts against the housing and the largest area ofthepris- matic element opposite said edge abuts against the prism provided with the sloping portions.

4. A device for holding Porro prism inversion systems ofthefirsttype, substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.