Product Overview: M92D Elbow Telescope
NSN: 1240-00-564-6339
P/N: 8299680

Description
Elbow telescope M92D is a fixed-focus fixed-reticle-type elbow telescope having 3-power magnification and a field of view of 12 degrees 12 minutes. When the elbow telescope M92D is installed in the telescope mount M90 (T183) which, in turn, is mounted on the rear bracket of the 106-M recoilless rifle M40 (T170E1), it is the direct sight for this weapon.

Functional Description
a. Mechanical Components
The objective end of the body of the elbow is tapered and slotted to fit the bore and key of telescope mount M90 (T183), and the extreme end is threaded so that the elbow telescope can be locked in place in the telescope mount by means of a round nut which is an integral part of the telescope mount. A level vial at the elbow of the body is used in conjunction with the cant correction mechanism of the telescope mount M90 (T183) to determine when the elbow telescope M92D is level. The band assembly serves as a means for attachment of the lamp bracket of the instrument light M42, to illuminate the reticle through a window in the body. The eyeshield enables the observer to sight on the target without interference from light or wind and to position his eye at the proper distance from the eyepiece.

b. Optical System
(1) General. The image of the target enters the elbow telescope M92D and the objective lens in its correct upright condition. The objective lens focuses or converges the light rays to a point, and when this lens is properly positioned, it will cause this focal point to fall exactly on the etched side of the reticle. Because the image would then appear inverted and reversed due to the crossing of the light rays at the focal point, the amici prism is positioned between the reticle and objective lens. In this position it serves two purposes: (a) It changes the direction of the light rays 90 degrees so they will leave through the telescope eyepiece. (b) It inverts and reverses the image before it appears on the reticle, so that when it is reversed and inverted a second time at the focal point it will then appear in its correct upright position. The image appearing on the reticle, however, would now be to small to serve any practical purpose. The eye and field lens correct this difficulty by magnifying the image and brightening it to make all details stand out sharply throughout the entire field.

(2) Objective. The objective is simply a converging lens whose function it is to focus the light rays on the reticle forming an image of the field of view on that lens.

(3) Amici Prism. The amici prism bends the light rays 90 degrees so they are projected towards the reticle. This prism also inverts and reverses the image so that the image will appear in its normal upright position when seen by the observer.

(4) Reticle. The reticle provides reference lines etched or engraved into the glass for keeping the target centered in the field of view and for making angular measurements. The reticle pattern includes curved stadia lines for estimating target range on targets of known dimensions (10 or 20 feet), and a series of broken horizontal and vertical lines. At the top of the pattern is a cross hair representing zero deflection and zero range and is used as the bore sight cross. Each vertical line and each space represent 100 yards of range. Each horizontal line and each space represent an angular lead of 5 mils. The center point of the small circles within the reticle pattern represents the ends of lead lines that have been eliminated to insure a clearer field of view through the reticle. At the bottom of the reticle is a horizontal and vertical mil scale to permit emergency use of the direct sighting instruments for indirect fire. The horizontal scale is marked in 1-mil increments from 0 to 30 mils right and left of the vertical centerline. Note that the first mark right and left of the centerline represents 2 mils. The mark for 1 mil has been eliminated for greater clarity. At the center of the scale is a vertical mil scale graduated in 1-mil increments, 5 mils above and 5 mils below the horizontal scale. At approximately the 800-yard range point, a horizontal dash line indicates the centerline of the reticle.

(5) Field Lens. The field lens is one of the lenses that forms the eyepiece of the elbow telescope M92D. Its main function is to gather light from the objective and converge or direct it into the eyelens. Without the light gathering qualities of the field lens, the image as viewed through the eyelens would not be clearly and uniformly illuminated, but would be dark and shadowed along the edges.

(6) Eyelens. The eyelens is that lens of the elbow telescope M92D eyepiece which is closest to the observer's eye. This lens acts as a magnifying lens and enlarges the image and the reticle pattern so that it can be more readily seen.

Tabulated Data