Representative Fire Control Instruments tion of 1.44 power, a field of view of 9 degrees, and considerable eye relief. The magnification is relatively low so that the movement of the tank will keep movement of the field to a minimum, yet provide enough magnification to permit the reticle to be seen readily. The objective is a compound achromatic lens. An Abbe prism erecting system (fig. 98) is used. The Kellner type eyepiece contains an equiconvex eyelens and a compound field lens. The reticle is inverted in order to appear in its normal position after reflection from the lower mirror. d. Adjustment knobs with graduated scales for lateral deflection and elevation are located on the rear of the body. These permit bore sighting of individual heads with the gun. Adjustment data is recorded, prior to action, on notation labels attached to the rear of spare heads. When replacing a shattered head, this adjustment data is used to make the required adjustment in the field without bore sighting. Section V 104. GENERAL. a. A military binocular or field glass consists of two prism offset type telescopes pivoted about a hinge which provides adjustment for interpupillary distance. The left telescope of many models contains a reticle with a range scale pattern (fig. 118). An erecting system is required in each of the halves of the instrument; prisms are used to increase stereoscopic vision and to provide a compact instrument. b. The power of magnification of the binocular, like that of the monocular telescope, depends upon the focal lengths of the objective and eyepiece groups. The true field of view depends upon the design of the lenses and the power. The brightness of image depends upon the size of the objective. The amount of the exit pupil that can be used depends upon the size of the pupil of the eye of the observer, which varies from about 0.1 inch for very brilliant illumination to about 0.3 inch for very faint illumination. The design of an instrument of this type determines its suitability for a specific purpose. A binocular with large objectives and exit pupils is generally better suited for observation at night and other conditions of poor visibility. A binocular of this type is often referred to as a "night glass." C. Binoculars are usually designated by the power of magnification and the diameter of the objectives. Thus, a 6 by 30 binocular magnifies 6 diameters and has objectives which are 30 millimeters in diameter. This designation is usually stamped on the instrument. d. Binoculars permit the use of and increase the radius of stereoscopic vision. The observer views the object from the two objec Figure 129 - Binocular M13A1-Assembled View, Optical System, and Optical Diagram Representative Fire Control Instruments RA PD 102544 tives which are more widely separated than his eyes while the magnification provided by the instrument increases his range of vision. For example, if the distance between the lines of sight of his eyes is doubled by the use of prism binoculars, and a 6-power instrument is used, the radius of stereo-vision is increased from a normal of approximately 500 yards to approximately 6,000 yards (500 yards times 2 times 6). 105. BINOCULAR M13A1 (GENERAL USE). a. The Binocular M13A1 (fig. 129) is a general purpose instrument for use by all services. It has a magnification of 6 power, the objective diameter is 30 millimeters, and the field of view is 8 degrees 30 minutes. Each of the prism offset type telescopes has an achromatic objective and a Kellner type eyepiece with a compound eyelens and a plano-convex field lens. A Porro prism erecting system is employed in each telescope. The eyepieces are individually adjustable from plus 4 to minus 4 diopters to meet eyesight variations. Graduated diopter scales permit prefocusing of both eyepieces. The interpupillary distance adjustment also is provided with graduations to permit presetting. b. A reticle is included in the optical system for the left eye. The horizontal scale of this reticle pattern (fig. 101) is graduated in mils. Above the center and on the left-hand portion are reference marks Representative Fire Control Instruments spaced 5 mils apart for convenience in observing fire. Crossing this horizontal scale, to the right, is a vertical scale graduated in hundreds of yards. The binocular is equipped with a filter and a carrying strap. 106. a. BATTERY COMMANDER'S TELESCOPE M1915A1 (FIELD ARTILLERY). The Battery Commander's Telescope M1915A1 is a binocular instrument used for observation and for measuring angles. In conjunction with the tripod head and mount, the instrument measures azimuth (horizontal) angles and angles of site (vertical). The telescope tubes may be positioned horizontally (fig. 130) to provide an accentuated stereoscopic effect, or they may be positioned vertically, with the eyes below the line of sighting, permitting the instrument to be used as a periscope. b. This instrument has a magnification of 10 power and a field of view of 4 degrees 15 minutes. The two telescopes have identical periscopic type optical systems. The right telescope contains a reticle. The reticle is provided with a rotating ring for erecting it when the telescopes are in horizontal or vertical position. The hinge arrangement which connects the two telescopes provides for the proper setting of the interpupillary distances of individual observers; a scale permits presetting. C. The head of each telescope contains a right-angle objective prism protected by a window (fig. 131). Porro prism systems erect the images. The achromatic objectives are mounted between the objective prisms and the erecting systems. The eyepieces of Kellner type each contain a compound eyelens and a plano-convex field lens. Detachable sunshades may be fitted on the objective ends of the telescopes, and detachable amber filters may be placed on the eyepieces when required. Leather objective caps are attached to the telescopes for installation when the instrument is not in use. Both eyepieces have diopter adjustment to accommodate the optical systems for eyesight variations. Section VI RANGE FINDERS AND HEIGHT FINDERS 107. GENERAL. a. The purpose of a range finder is to find the range of an object or target; the height finder gages either its range or altitude. These instruments measure distance by triangulation. b. The range finder contains two optical systems which permit a single observer to view the target from points some distance apart. This distance serves as the known leg of the triangle. It is termed the base line (A, fig. 132) and is one form of designation of a range |