Optical Components, Coated Optics, and Construction Features

Section III

GENERAL CONSTRUCTION FEATURES

79. LENS CELLS, SEPARATORS, AND RINGS.

a. A lens cell (fig. 109) is a tubular mounting or frame made of metal, plastic, or rubber which holds a lens or a number of lenses in the proper position within an optical instrument. The lens is usually secured into the cell by a lens retainer ring or by turning over a thinly feathered edge of the cell (burnishing). When two or more lenses are mounted in a cell, the different elements are maintained at the proper distance from one another by spacers or separators. The cell mounting permits the entire assembly to be handled and mounted as a unit.

b. Separators or spacers are smooth or threaded tubular sections which separate or space the elements of a lens system in the proper relation with one another (fig. 109). Adjoining faces are usually beveled to fit the faces of lenses snugly. Lens retaining rings are threaded about their outer diameters and are screwed in behind lenses to secure the lenses to their cells. An adapter may be used to mount an element or part of smaller diameter into a part of the instrument body which is of larger diameter.

80. CENTERING DEVICES.

a. Light always bends toward the thickest part of a lens or prism. When the center of such a lens is moved, it can be made to cause the light to be bent in the desired direction. Movement of the center of the objective is accomplished by mounting it in a pair of eccentric rings (fig. 110). By rotating the objective about in the inner ring or by rotating the inner ring about in the outer ring, the axis of the objective may be moved to any point in a relatively large area. When properly positioned, the rings may be locked in position. Skill is required to set the rings in the proper manner.

b. Prisms are often placed in mountings designed to hold them firmly and yet afford movement for exact adjustment. Screws bear upon two faces of the prism to lock it in place. By letting out on one screw and taking up on the other, the prism may be positioned exactly. Extremely fine adjustments can be made.

81. EYESHIELDS.

a.

Eyeshields or eyeguards are fitted to fire control instruments to maintain proper eye distance and to protect the eyes of the observer from stray light, wind, and injury due to the shock of gun fire or similar disturbances. Eyeshields are made of rubber, plastic,

RA PD 30149

Figure 109 Lens Cell, Separators, Lenses, and Retainer Ring

and metal, but only those made of soft rubber can most effectively meet all of these requirements.

82. SUNSHADES AND OBJECTIVE CAPS.

a.

Sunshades are tubular sections of metal, usually with the lower portion cut away, which are fitted into slots around the objectives of many instruments to protect against rain and the direct rays of the sun (fig. 111). They not only reduce glare which would be caused by sunlight striking the outer face of the objective, but they protect from extreme heat the canada balsam used to cement the elements of the objective.

b.

Objective caps (fig. 111) are leather covers which are fitted over either the sunshade or the objective end of the instrument to protect the objective when the instrument is not in use. The caps are usually attached to the instrument by strips of leather to prevent loss.

83. FOCUSING DEVICES.

a. The distance between the reticle and the eyepiece must be adjusted to the observer's eye so that the reticle and image of the object will be sharply defined, and to eliminate eye fatigue. To provide this adjustment, the two or more lenses of the eyepiece are mounted in a single tube or lens cell and its distance from the reticle (and the focal plane of the objective) can be adjusted by a rack and pinion by a simple draw tube, or by rotating the entire eyepiece by means of the diopter scale (fig. 112), causing it to screw in or out. This is referred to as the diopter movement. The knurled ring of the "screw out" adjustment type is provided with a scale reading in diopters by which this adjustment can be made directly, if the correction required by the eye is known.

84. OPTICAL BARS.

a. In a range or height finder the angles of the lines of sight from both ends of the instrument are employed to determine the range. Heat and cold cause unequal expansion and contraction of

metal parts, disturbing the angular alinement of elements mounted on or in them. The extreme lengths of the large range and height finders would tend to emphasize the deviation caused by any misalinement if special measures were not taken to keep the optical systems stable and in perfect alinement.

b. One of these measures consists of mounting the most sensitive parts of the instrument in a metal tube known as the optical bar. The optical bar is made of specially treated steel with a low coefficient of expansion. It is perfectly balanced and is supported by the inner or main tube of the instrument in such a manner that the outer tube may expand or contract without affecting the alinement of the inner tube and optical bar. In addition, the instrument is insulated to reduce temperature changes to a minimum.

Optical Components, Coated Optics, and Construction Features 85. COLLIMATORS.

a. The collimator (fig. 113) is an ingenious and relatively inexpensive type of sighting device. The collimator is inferior to the telescope in effectiveness or convenience of operation, but its simplicity, ruggedness, and low cost make it particularly desirable as a sighting device for mortars.

b. The principle of the collimator is that a reticle, fairly close to the observer, can be optically transferred to a position infinitely distant from the observer. Parallax between the reticle and the target is thereby eliminated. Since there are only two optical elements, a reticle and an eyelens, the entire structure can be housed in a compact tube. Generally, the pattern of this reticle is a transparent or illuminated cross on an opaque or dark background. When employed as the sighting device for a weapon, the mount of the collimator is provided with leveling mechanisms and scales which permit the weapon to be placed at a prescribed elevation with relation to the line of sight established by the collimator. Sighting is accomplished by looking into the collimator and at the target simultaneously or successively to superimpose the cross upon the target.