right angle of the telescope and the erection of the image is secured by reflection on two of its surfaces. It can be mounted securely and thus is suitable for telescopes which are mounted on gun carriages. Combined with a rotating prism and a right-angle prism, it is used in the erecting system of panoramic telescopes.

(4) DOVE OR ROTATING PRISM. The Dove or rotating prism (fig. 90) is the means by which the image in a panoramic telescope is caused to remain erect as the head of the instrument is rotated. The entering rays are diverted downward but are reverted by a right-angle prism in the upper head of the instrument. This prism is called the objective prism. The reverting effect of the objective prism is neutralized and the light rays are kept in alinement by a dove or rotating prism. If the dove or rotating prism were to be fixed in the instrument and the objective prism were to be rotated in azimuth to the side or rear, the image viewed through the instrument would rotate until it became inverted. This difficulty is overcome by connecting the rotating prism to the objective prism with gearing which causes it to revolve with one-half the speed of the objective prism. This serves to keep the image in a vertical position regardless of the direction in which the upper head of the instrument is turned (fig. 98).

(5) ERECTING PRISM ASSEMBLY. The erecting prism assembly used in Panoramic Telescope M8 consists of two prisms separated at an angle of 45 degrees. Light rays entering this prism assembly are reflected five times before emerging (fig. 100); twice in one element and three times in the other. The incident rays and the emergent rays are in alinement and travel in the same direction. This assembly brings about an odd number of reflections and neutralizes the inversion caused by the objective prism.

Optical Components, Coated Optics, and Construction Features

70. MIRRORS.

a.

The mirrors used to reflect the paths of light in fire control instruments are not the common household type made of plate glass. The back reflecting surface on household mirrors produces a faint reflection from the glass surface in addition to that produced by the silvered back. To eliminate this "ghost image," the reflecting surfaces of optical mirrors are placed on their front faces. Mirrors of this type are termed first surface mirrors. The reflecting surface is a thin film of metal chosen for its resistance to tarnish. Such mirrors must be handled carefully as they can easily be scratched and become useless until they can be resilvered.

a. Reticles are used in fire control instruments for superimposing a predetermined pattern of range and deflection graduations on a target. A simple reticle may consist of two intersecting lines. The line of sight through their intersection is in the center of the field of view, and represents the axis of the bore of the weapon.

b. The majority of reticles are glass disks with plane parallel surfaces. Appropriate markings are engraved or etched on one of the surfaces. In some cases, a plano-convex lens is required at the point where the reticle would be mounted. In such cases, the markings are engraved on the plane surface of the lens. The cross lines of some instruments are strands of fine wire.

C. The reticle is placed in the instrument in the focal plane of the objective. With a lens erecting system, the reticle may be placed at the front or rear of the erecting system, or reticles may be placed at both points. The preferable position with this type of erecting system is in front of the system, as the objective and reticle then form a unit, and any shift of the erecting system does not disturb the alinement of these elements. With a prism erecting system, the reticle is usually placed in back of the erecting system.

d. There are many different types of reticle patterns (fig. 101) designed for different purposes. The reticle of an instrument designed for use on a particular weapon, when used with specific ammunition, is identified by a group of small figures and letters which appear near the bottom or top of the field of view (C and E, fig. 101). These figures and letters may be Firing Table numbers or F. T. numbers, to indicate the Firing Table used in designing the reticle, or may be other characteristic identification information.

72. DIAPHRAGMS AND FIELD STOPS.

a.

Diaphragms are rings of opaque material, placed in an optical system so that light passes through their centers. As their apertures limit the field of view, they are sometimes called field stops. When placed between the objective and the erecting system or between the

Figure 101 - Representative Types of Reticle Patterns erecting system and the eyepiece, or in both of these positions, they eliminate marginal rays which would otherwise cause glare by reflecting from the inside walls. When placed around the edges of lenses or within lens systems, they prevent rays from passing through the margins of the lenses and causing aberrations.

73. FILTERS.

a. General. Filters or ray filters are colored glass disks with plane, parallel surfaces. They are placed in the path of light through the optical system of a fire control instrument to reduce glare and light intensities. They are provided as separate elements which may

be attached and detached (A, fig. 102) or are mounted so they may be placed in or out of position as desired (B and C, fig. 102).

b. Colored Filters. Filters of various colors are used to improve visibility under different atmospheric and light conditions. Among the colors of filters used are: smoke, yellow, amber, blue, red, and greenish-yellow.

(1) The smoke (neutral) filter reduces the intensity of light and is effective when observing against or in close vicinity of the sun or a searchlight; usually it is too dark for other purposes.

(2) The yellow and amber are used to protect the eyes from the reflection of sunlight on water and other general conditions of glare.

(3)

RA PD 30145

Figure 105- Instrument Light M1 for BC
Telescope M1915A1

The amber and red filters are usually employed under various conditions of fog and ground haze. Red filters are also used in observing tracer fire.

(4) The blue filter is helpful in detecting the outlines of camouflaged objects.

(5) The greenish-yellow filter is intended to serve the purpose of both smoke and amber.

C. Polarizing Filters.

(1) Polarizing filters do not change the color of objects but merely decrease light intensity. When mounted in pairs, they can be used to provide continuous control of light intensity.

(2) Light polarizing substances can be considered as being made up of parallel bars or grain (A and C, fig. 103). Light vibrates in waves with the crests of the waves in all directions. A polarizing substance, placed in the path of the light, permits waves vibrating parallel to the direction of the waves or grain to pass through; waves vibrating at right angles to the bars or grain are stopped (B, fig. 103). By eliminating a portion of the light, the intensity of the light is decreased. When two polarizing substances are held with the grain of one at right angles to that of the other, no light passes through. By shifting the substances to intermediate angles, light at different degrees of intensity is permitted to pass through.