Representative Fire Control Instruments finder, that is, a 1-meter base instrument. One of the two angles from which the target is observed is fixed at 90 degrees; the other angle is variable, depending upon the distance of the target from the instrument. The angle to which the 90-degree and the variable angle converge is termed the parallactic angle. It becomes larger as the distance to the observed object becomes less. Because of the extreme shortness of the base line in comparison with the range to be computed, the utmost accuracy and precision are required in the determination of the angles at the two ends of the base line. c. A height finder measures altitude by solving a vertical triangle. In this case, the slant range to the target (B, fig. 132) as determined by the observer is multiplied by the sine of the angular height. The multiplication is performed mechanically or optically. d. The optical system of the coincidence range finder causes the images entering the two ends of the instrument to coincide or combine in a centrally located image plane, and form two half images which are viewed through a single eyepiece. (1) Certain instruments of this type form two half images which, except for a dividing or halving line, appear formed by a single objective. This is known as the erect type image. On other instruments for two half images, the upper half is inverted and appears to be the mirrored image of the lower half. This is known as the invert type image. When perfect halving and coincidence have been secured in either type, the range is indicated on the range scale. The erect type of field is usually preferred when the target is of a regular geometrical shape with well defined boundaries. The inverted type is considered to have advantages when the target is poorly defined or very irregular in shape, as where the targets sighted upon may be trees or trench parapets. (2) There are two types of coincidence range finders which operate on different principles. In instruments of one design, a measuring wedge (par. 97) is employed to deviate the rays of light. This wedge is moved backward or forward in the line of sight until coincidence is secured. In the other design (known as the azimuth type) the right and left objectives have slightly different powers of magnification. Swinging the instrument in azimuth about its vertical axis causes the erect and inverted half images to traverse the field at slightly different rates so that coincidence for an object or target at any given range is obtained at only one location in the field. e. The stereoscopic range and height finder may be considered as a binocular telescope, fitted with reticles for measuring distance and with the objectives located at a considerable distance from one another. This increased interobjective distance combined with high magnification greatly accentuates stereo-vision. Each of the two optical systems form a complete image. The two images are viewed with both eyes of the observer through a pair of eyepieces and fused into a single image by his brain in the same manner as in normal binocular vision. The line of sight through the variable angle end of the instrument is deviated by a system of rotating wedges (par. 68f). A reticle in each optical system of the instrument places a reference pattern in the focal plane of each eye. These patterns appear to fuse into a single pattern in relief when viewed at once by both eyes. The center of the pattern can be brought exactly onto the fused target image by moving the rotating wedges. This is termed stereoscopic contact. The motion of the rotating wedges is mechanically transmitted to a calibrated scale or range drum, and the range can be read directly on the scale or drum when stereoscopic contact has been established. 108. RANGE FINDER, 1-METER BASE, M1916 (FIELD ARTILLERY). a. The 1-meter Base Range Finder M1916 (fig. 133) is of the measuring wedge coincidence type producing an invert image. Coincidence is obtained by turning a range drum knob until the half images of the point selected appear in coincidence. The range in yards is read on the range drum against the sliding range pointer. This instrument has a magnifying power of 15 and a field of view of 3 degrees 10 minutes. b. Light enters the front side of the two ends of the instrument through the end box windows and is bent at right angles by the Representative Fire Control Instruments penta prisms toward the centrally located eyepiece (fig. 134). The end box windows are optical wedges which are carefully adjusted in manufacture to compensate for combined deviations of the optical system. End box sleeves can be rotated to cover the window openings for protection against breakage and dirt. Padded leather buffers on the ends of the instrument afford protection against minor shocks. C. The two objectives, measuring wedge, correction lens, and ocular prism assembly are mounted in a separate optical tube within the outside tube. The optical tube is supported at the ends. One of the supports provides for adjustment in height for the proper position of the half images at the halving line. The objectives are mounted in the ends of the optical tube. d. The measuring wedge is mounted between the left objective and the ocular prism assembly. This wedge can be moved toward and away from the left objective. Light entering the left end box window is deflected by the measuring wedge through an angle sufficient to provide coincidence. This wedge is operated simultaneously with the range drum by the range drum knob. e. The right-hand portion of the instrument contains the adjustment elements. These are the correction lens, located in the optical tube between the right objective and the ocular prism assembly, and the correction wedge, mounted in a separate cell, sleeve, and adapter between the right penta prism and the right objective. The correction lens is provided to insure that both objectives will have the same magnifying power. The correction wedge is used to correct optical errors in the system resulting from temperature changes, and personal errors peculiar to the individual operator in his adjustment of the instrument. The correction wedge is fitted with a shaft for adjustment and a scale to record movement. The scale is viewed through a window located to the right of the eyepiece. f. The ocular prism assembly or coincidence prism is a system of small prisms cemented together. This assembly receives the light from both ends of the range finder, suppresses the upper half of both images, erects the lower half of one image and reverts the other, and produces these images, one inverted above the halving line, and the other erect and below it. These half images and the halving line are focused and magnified by the eyepiece when viewed by the observer. g. The eyepiece is of symmetrical type with a compound eyelens and a compound field lens. It is focused by rotation of a diopter scale. The eyepiece is fitted with amber and smoked ray filters controlled by a ray filter lever. The filters are kept in a filter holder located between the eyepiece and the ocular prism assembly. An open sight near the eyepiece facilitates the training of the range finder on the desired target. |