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1. Range finders are optical instruments used to determine the distance to an object by triangulation. In the United States Navy there are two types in general use:

On both types the optical parts may be divided into a main and several auxiliary optical systems. The main optical systems differ in principle for the two types. The auxiliary optical systems are similar in principal for both types. The altimeter is a special type of coincidence range finder.

The Coincidence Type Rangefinder


2. The trianglewhich the coincidence rangefinder solves in range-taking, consists of the base length of the rangefinder, i.e., the distance between the two lines of sight, one at each end window, and of the lines of sight from the left and from the right end window to the target. The angle between the base and the left line of sight is generally maintained as a right angle. The base is constant for any particular range finder. The small angle at the target subtended by the base is known as the parallactic angle (Ө). It is by measurement of this parallactic angle that the range is obtained.

3. The base, being self contained in the instrument, is comparatively very short but the "virtual" base is much greater, being equal to the length of the actual base (B) multiplied by the magnifying power (M) of the instrument.

4. Suppose a range finder of base length B is sighted at a target T at true distance R. The rays from T pass through the two end windows, are reflected by the end reflectors, and are then refracted by the main objectives to form the target image in the coincidence prism, which suppresses opposite halves of each image of T and projects the desired halves of the image to the eye of the observer. (See Plate 2).

5. These two half images will not coincide if the distance R is less than infinity, for the virtual base length BM will subtend an appreciable parallactic angle Ө; therefor the rays will still be at an angle 9 unless means are taken to deflect the rays into parallelism within the instrument. This deflection in to parallelism is accomplished by means of a measuring wedge or



compensator wedges which deviate the right end ray so that the half images of T exactly coincide in the observer's field.

6. As the distance R increases, the angle 0 decreases, thus requiring less deviation of the rays until at an infinite distance the rays from T are parallel and no deviation is required. Conversely as the distance R decreases, the angle 6 increases, and more and more deviation is required to make coincidence until the limit of practical movement of the measuring wedge or rotation of the compensator wedges, is reached.

7. It is evident, then, that at every range, formerly particular range finder, there will be a certain angle subtended and a certain corresponding position of the measuring wedge, or compensator wedges. With, measuring wedge or compensators connected to a suitably graduated scale, the range may be read directly in yards.

8. Mathematically from the range-finding traingle

tan Ө = B/R
or since Ө is very small and for small angles Ө - tan Ө
Ө = B/R radians           (1)
This is known as the range-finder formula.

9. Since 1 radian = 360/ x 60 x 60 seconds = 206,265 seconds

Ө = 206,265 B/R seconds, where B and R are expressed in yards.

10. Letting e be a small error in range corresponding to a small error Δ (radians) in the measurement of the parallactic angle, it may be shown that

e = ±R2 Δ/B M           (2)

This is the basic formula of all rangefinders. It holds only when e is very small as compared to R. By experiment it has been determined that the mean error of alignment of a normal well-trained operator under good conditions of observation for a small series of observations with magnification of unity, should not exceed 12 seconds of arc. Substituting this value of A and expressing R in thousands of yards, equation (2) becomes

e = 58.2 R2/B M

This value of e is known as the unit of error, and may be considered to be the probable error of a single observation by an excellent observer under ideal conditions.


Main Optical System - Coincidence Type.

11. The main optical system of a coincidence type rangefinder consists of (Plate 2):

12. The end windows serve as a seal against dirt and moisture. They are ground as very slight wedges, so that the entering rays may be slightly deviated to compensate for any slight inaccuracy in the reflecting angles of the end penta prisms or elsewhere, in the instrument. Setting of the end windows is accomplished by rotation of the wedge to the desired position. The setting is a factory adjustment at final inspection after manufacture or repair. When once set, it should not be necessary to reset end windows in service.

13. The end penta prisms possess the property of deflecting an incident ray through a constant angle by two successive reflections in the plane of triangulation, so that the rays from a target are projected 90°, or nearly 90° toward the center of the instrument from opposite ends. End reflectors are "built up" prisms used on certain long base range finders and function like the solid penta prisms.

14. The measuring wedge or compensator wedges deviate the rays (usually from the right end penta prism) in the plane of triangulation. They are of two general types:

15. The compensator wedges are usually positioned in the rangefinder outboard of the right main objective. The entering rays are deviated from the right end so as to make them coincide at the halving line with the undeviated rays from the left end. Movement of the wedge in making coincidence automatically turns the connected internal and external range scales to register the


range of the target. When a measuring wedge is used, it is, located between the main objective lens and coincidence prism and its shifting along the longitudinal axis increases or decreases the amount of deviation of the light rays at the image plane, as its-distance from this plane is increased and decreased.

16. The correction wedge is included in the main optical system as it lies in the path of light through this system, usually on the side opposite the measuring wedge or compensators. It has rotational movement about the -longitudinal axis. The function is to correct the range-finder for errors in the main optical system due to temperature variations, etc. It willbe further described under Internal Adjusting System.

17. The main objective lenses (right and left) bring the rays from each end of the instrument to a focus at the image plane. A lens group or pair is used at each end, in late type instruments, each being equivalent to one lens corrected for the various aberrations. The use of a pair facilitates adjustment for obtaining equal magnification in both fields.

18. The coincidence prism is mounted near the center of the instrument and consists of a system of small prisms usually cemented together. There are several types, but in all the arrangement is such that of the two complete images from the main objectives, the upper half from the right objective and the lower half from the left are suppressed; the remaining opposite halves together forming one erect whole image at the halving line. To effect coincidence the lower half of the image moves horizon tally when the measuring knob is turned.

19. The eyepiece serves to magnify the image of the object viewed at the halving line of the coincidence prism. Provision is made for focusing the eyepiece, usually from plus 2 to minus 4 diopters. The most usual type of eyepiece is the 2-doublet (achromatic) eyepiece.

20. The astigmatlzersconsist of two cylindrically ground lenses, such lenses forming line images parallel to their own axes. In the range-finder this line is imaged at and perpencidular to the halving line. The astigmatizers are mounted one on each side of the coincidence prism and may be thrown simultan eously and retained in the "in" position (to intercept the rays just before they enter the coincidence prism) or to the '"out" position (clear of the path of light) by means of the astigmatizer knob. Coincidence, using the astigmatizers, is made by bringing the line images of the upper and lower halves of the field into alignment. Astigmatizers are used for taking ranges on lights at night.


21. Ray filters are usually installed in the optical system near the eye-piece. "Various colors are provided, those on most recent instruments being designated as dark smoke, light smoke, yellow and red. They permit ranging on brilliant targets, or against the sun. Their use may improve the visibility under certain adverse atmospheric conditions due to heat waves, haze or mist.

The Altimeter (Rangefinder Mk XXXII).

22. This altimeter is a vertical base coincidence type rangefinder capable of being-used either as an altimeter or as a range-finder. It contains as a special feature a rotatable tube which carries a compensator wedge group secured at each end and its function is to rotate them bodily as a group to change the angle between the plane of deviation of both compensator groups and the plane of triangulation. The rotatable tube is controlled by the leveling shaft mechanism through appropriate gearing, when the instrument is used as an altimeter, so that the angle between the plane of deviation and the plane of triangulation will always be 90° - α, where a is the angular elevation of the target.

In taking altitudes the measuring knob is moved to set the compensator wedges to measure the angle θ.

Let H be the target height and 9' be the component of the deviation 6 in the plane of triangulation:

Then Θ' = &Theta cos (90° - α) = &Theta sin α
From par. 8 Θ' = B/R and since H = R sin α
Θ' = B sin α/H = &Theta sin α
(thus) Θ = B/H

Thus in moving the compensator wedges to measure the deviation 0, the altitude of the target has been measured and is read directly on the scale in feet.

23. To use the instrument as a rangefinder the rotatable tube is clamped in position where the plane of deviation of the compensators is at an angle of 70°31T45lf = 1/3, this introduces the factor 1/3 and the range in yards is read on the same scale as was used for altitude readings in feet.

The Stereoscopic Type Range-finder



24. Stereoscopic vision is the ability to perceive spacial depth. Due to the distance between the eyes-interocular distance, a dissimilar image is formed on the retina of each eye; this is the explanation of depth perception. To see an object sharply with two eyes, both lines of sight must converge directly on it. The angle of this convergence varies inversely as the distance. The smallest angle at which differences in the amount of convergence are detectable is called the limiting angle of resolution (θ0).

25. The radius of stereoscopic vision (γ0) for the unaided eye of the average person whose limiting angle of resolution is 30 seconds or 0.000145 radians and whose interocular distance is 63 mm. is:

γ0 = B0/Θ0 = 63/.000145 = 434000 mm. = 434 meters.

The radius of stereoscopic vision may be increased by (1) base magnification and (2) telescopic magnification. Let M be the telescopic magnification and n (= B/B0) be the base magnification. The stereoscopic radius, S, will be:

S = γ0 Mn.

Since Θ0 = B0/γ0 equation (92) for stereoscopic rangefinders becomes:

e = ± R2 Θ0/Mn B0 × 1/206,265
e =   R2/Mn γ0 × 1/206,265
e =   R2/S × 1/206,265           (2s)

Since the angular error (A) depends on the limiting angle of resolution (γ0), which should not proceed plus or minus 12 seconds of arc for the unaided eye for a small series of observations, the units of error (e) for stereoscopic and coincidence type instruments of the same telescopic and base magnification are equal:

where R = range in thousands of yards.


Main-Optical System - Stereoscopic Type.

26. The main optical system of the stereoscopic; type rangefinder consists of (Plate 8):

Through (e) the system is the same as described for the coincidence type range-finder.

27. The ocular prisms are mounted near the center of the instrument. Each receives the rays from its end, reflects them upward 45° in a vertical plane through the axis of the rangefinder, and then through a second 45° upward in a vertical plane perpendicular to the axis of the range-finder. Thus if the lines of sight to the target are horizontal, the operator looks down on the target images at an angle of 45°.

28. The reticules, piano-parallel glass discs on which are etched the reference marks capable of blending stereoscopically (Plate 7) are placed in the focal planes of the main objectives and there viewed from the main eyepieces. On each reticule of the wandermark type the smaller reference marks are arranged about the center reference diamond, called the measuring mark. Ranges are taken by adjustment of the compensator wedge until the target appears to lie stereoscopically at the same depth as this measuring mark.

Another type of reticule has the reference marks lying in zig-zag lines. The marks are numbered according to the range and blend stereoscopically. Instruments having these zig-zag type reticules do not have compensator wedges and ranges are taken directly by noting the range at which depth the target is perceived to lie. The Mark V- spotting glass uses such reticules, except that the reference marks are not numbered but show 500 yard increments of range.

29. The rhomboid prisms are employed to obtain variation in interpupillary distance. They are so mounted that the distance between the centers of their forward faces is constant, while, the prisms rotate about these centers to suit different interpupillary distances.


Plate 7


30. Each collective lens gathers the rays from its rhomboid prism and bends them to a focal plane in front of its eyepiece lenses.

31. Each main eyepiece group consists of two achromatic doublets which may be moved in "or out by rotation of the focusing ring within a range of plus 2 to minus 4 diopters. At the proper distance the eyepieces sharply define the target image rays and project them to the observer's eyes with the required magnification.

32. The ray filters are usually placed between the end penta prisms and the objective lenses. Their designation and use are as explained for the coincidence type range-finders. The stereoscopic type instrument may be used at night as in daylight; astigmatizers are unnecessary and are not provided.

Auxiliary Optical Systems - Coincidence and Stereoscopic Types.

33. In addition to the main optical system there are in general the following auxiliary systems:

34. Types of range scales found in service are: (1) inside range scale, (2) outside range scale, and (5) a combination of inside and outside scales. The last is most common. Inside scales are directly connected with the measuring wedge or compensator. They are positioned so as to be read in various ways, as: by the range observer as a part of the field where a single eyepiece is used; by the range observer using his left eye in a 2-eyepiece range-finder where the right eye is used for viewing the target; or by a range talker. Outside range scales are usually engraved spirally on a dial located on a housing attached to the rear side of range-finder. The position of the measuring wedge or compensator wedges is transmitted through suitable gearing to the sliding index which indicates the range.

35. With most range-finders internal-adjustment is effected by the rotation of a correction wedge. Various types of index scales are used to determine the amount of rotation, or the position of the correction wedge. In some instruments the adjuster scale is engraved on a drum carried as a part of the correction wedge mount and is viewed through a window. In others the rotation of the wedge is transmitted by worm gearing to an outside dial with circular scale and index. Other types use a circular transparent scale attached to the correction wedge carrier.


In range-finders without correction wedges, adjuster corrections are effected by shifting the index on the outside range scale or by shifting the range scale with respect to the measuring wedge.

In older instruments adjuster scale graduations are arbitrary. In later types the graduations are in "units of error" (within a tolerance of 10$) and are therefore proportional to the square of the range in yards, just as the theoretical error varies with the square of the range; decreasing the reading on the scale will increase the range readings of the instrument and vice versa.

36. Trainer and finder telescopes are frequently used to assist in locating the target and keeping it centered in the main field of the range-finder. Such systems are of comparatively low power and large field, and contain reticules with a vertical line, cross lines or concentric circles for training and centering on the target. Plate. 8.shows one type of finder telescope system.and Plate 2 a more elaborate system. Other sys tems which differ slightly in the prism and erecting parts may be found in service. Frequently externally mounted, monocular, periscopic telescopes are used on long base instruments.

37. The internal adjuster system provides a practical, ever available, artificial infinity by means of which the range-finder may be adjusted to read correctly at infinity under the existing conditions. Internal adjuster systems vary with the different marks and modifications of range-finders but they may be generally classified as follows:

  1. Center light colliniating system.

    1. Adjuster penta prism type (Stereoscopic rangefinders)

    2. Triple mirror prism type (Coincidence range-finders)

    3. Lost ray type (Coincidence range-finders)

  2. Double-end collimator system.

    1. Direct type (Coincidence range-finders)

    2. Optical square type (Both types of range-finders)

The center light collimating system corrects only for errors in the main optical system. The double end collimator is theoretically designed to correct for changes in both the main optical system and in. the adjuster system. Only the adjuster penta prism and the lost ray type of center light collimating systems will be described here.

Adjuster Penta prism Type.

38. Plate 8 shows the adjuster penta prism type of internal adjuster. The adjuster penta prisms are swung into position for


making internal adjustment by rotating an appropriate knob. Light introduced through the right eye piece is reflected through the right main optical system to the end penta prism or reflector, is received by the right adjuster penta prism and is reflected into the left adjuster penta prism. It is then reflected as from an infinite distance into the left end penta prism or end reflector and through the left main optical system to be viewed in the left focal plane through the left eye-piece. The reference marks should coincide at infinity.

Lost Ray Type.

39. Plate 2 shows the lost ray type of internal adjuster. It operates on the principle of delivering to the range-finders main optical system two partial images of the adjuster target which are exactly parallel to each other and then adjusting the instrument so that when these partial images coincide to form a perfect picture of the target in the field of the main eyepiece, the instrument reads infinity on the inside range scale.

40. The adjuster target is a straight black line in the center of a rectangle engraved on the target reticule. The target illuminator is positioned on the front of the range-finder and in line with the reticule. Rays from the illuminator project the target image through the target objective lens to the coincidence prism, then upward through a small reflecting prism, bringing the image to a focus at the halving line and in the same position as an image from an external target, except that the light path is from a different direction. Dividing at the halving line in the coincidence pr-ism, each half of the target image rays traverses in the opposite direction the same optical path, as enter ing rays from an exterior target until the rays emerge from each end penta prism. There they are intercepted by the two adjuster penta prisms which have been swung into position by rotation of the outside adjuster knob. Each adjuster penta prism tarns the rays through 90° and projects them through the adjust er dove prism and the auxiliary wedge to the opposite adjuster penta prism which in turn presents them to the end penta prism as though they were rays from an outside target at infinite distance.

41. The two parallel partial images now return along the normal light path and will coincide to form a perfect image or unbroken line if the line of collimation through the optical tube is correct. The adjustment of the instrument is such that it will now read infinity on an external target at an infinite distance. If the two halves do not form a perfect line, the line of collimation is not correct, and the observer turns the correction wedge knob, which rotates the correction wedge and deviates the rays from the left end, sufficiently to effect perfect coincidence.

The range scale must be set at infinity before making this adjustment.


42. The dove prism erects the image ray entering from the right side and inverts the rays entering from the left side so that the half images inverted by the objective lenses during the circuit, arrive back at the coincidence prism so as to each fall upon its proper reflecting surface so as to form a complete image.

The adjuster auxiliary wedge is introduced to correct any residual errors in the adjustment system. Its position is not changed after the, instrument is finally adjusted at assembly.

Structural Features.

43. The principal structural elements of a range-finder are:

The outer and inner tubes are of circular cross section and are made of seamless drawn steel. For large instruments each is of two sections welded together. The air space between the outer and inner tubes assists, in insulating the optical elements' against temperature variations. The outer tube is lagged to provide further insulation.

The end boxes are of strong construction rigidly secured to the ends of the inner tube. These boxes contain the end windows, end reflectors and adjuster penta prisms.

The optical tube is really the double telescope of the main optical system. To avoid distortion it is machined from a solid nickel steel forging of special homogeneous quality. It was formerly the practice in large range-finders to suspend the optical tube on tension wires so any bending or sagging of the inner tube would not be transmitted to the optical tube. Tension wire reliefs installed at each end of the optical tube provide support to take the load when the range-finder is not in use thus preventing permanent set in the wires. In late design instruments the tension wires are omitted; the optical tube resting in two brackets secured directly to the inner tube. In order that halving adjustment may be made the left bracket contains a gimbal joint and bearing.

The outer tube serves only for support, protection and insulation.

Errors and Adjustments.

44. It has been shown (par. 10) that the unit of error under theoretically perfect condition is:


e = 58.2 R2/BM

Errors are of two general types: (1) inherent instrumental er-rors and (2) errors of observation. The causes and possible magnitudes of the inherent errors should be understood and should always be considered when analyzing or using range finder data. Errors of observation can be traced directly to personnel, and are generally of a variable character so that they cannot, as a rule, be allowed for in service use. Both instrumental and personal errors, however, should be within the specified limits of accuracy for the particular range-finder used. Unit of error tables should be computed for each particular type of range-finder, used.

45. Range-finders are very sensitive to variations in temperature; errors may be expected when radical temperature changes are encountered. The magnitude of these errors will vary widely among similar instruments so that the behavior of each instrument under these circumstances is an individual problem. The effect and advisability of constantly changing the internal adjuster setting should be determined. As a rule, in the later instruments frequent changes in internal adjuster settings will give a very good range line, but in earlier instruments this does not hold true.

46. Every available opportunity should be seized to collect and keep up to date range-finder data. Satisfactory results can be obtained only by frequent, careful checking. A curve, establishing internal adjuster setting as determined on known, ranges under various temperature and atmospheric conditions, should be checked frequently and kept up to date.

Instrumental Errors.

47. Instrumental errors are as follow:

  1. Optical errors.

    1. Field position
    2. Inclined image
    3. Optical misalignment
    4. Parallax
    5. Ageing of optical glass
    6. Ray filters
    7. Astigmatizers
    8. Rotation of end windows
    9. Internal adjuster system
  2. Mechanical errors.

    1. Range scales.
    2. Gear trains
    3. Optical tube bearings
    4. Correction scales
    5. Loose internal and halving adjusters,


48. For the best accuracy in range taking, the target should be centered in the field. If the target position is half way from the center to the edge of the field the error will be about four times that at the center. An inclined target serves to multiply this field position error. If a vertical target is not available, an inclined one may be used, but to produce any satisfactory range readings, the readings must be made at the center of the field.

49. Optical alignment may be tested by inking figures at the exact center of the end windows and with an auxiliary telescope bringing these figures to a focus in the eyepiece. The two figures will be superimposed if the optical alignment is correct. If widely separated, misalignment exists and the services of an optical expert should be requested.

50. With a coincidence range finder directed on a target at least 8000 yards distant, vertical or horizontal movement of the observer's eye should not cause any perceptible relative motion of the two half images of the target. With a stereoscopic range finder directed on a target at least 3000 yards distant, so that the target image is near the measuring mark, vertical or horizontal motion of the observer's eye should cause no perceptible change in the apparent, relative distances of target and mark as seen stereoscopically. These tests should be made under good observing conditions only. If excessive parallax is noted the services of an optical expert should be requested.

51. Ageing optical glass in some instruments introduces a progressive error for about the first two years of service. This can be detected by frequent checking on accurately known "ranges and can be compensated for, when present by offsetting the internal adjuster.

52. Ray filters cause varying errors which should be determined for each type of filter furnished for each instrument.

The errors will vary as the square of the range and compensation may be accomplished by offsetting the internal adjuster the proper amount.

53. Astigmatizers should not introduce an error greater than one unit of error. Comparative tests with astigmatizers "in" and "out" should be made for both day and night conditions, and curves of astigmatizer errors prepared for each instrument, In some earlier instruments these errors may be sufficiently large to require offsetting the internal adjuster. With the astigmatizer thrown "in", a slight change of ocular focus is to be expected.

54. If end windows are found loose, the services of an optical expert should be requested to reset them.


55. With constant temperature the average spread of three series of five internal adjuster readings should not exceed half of a unit of error and the spread for any one set should not exceed one unit. Between two well trained operators the mean of three groups of five internal adjuster settings should not differ by more than one unit. These figures are for ranging under "perfect" conditions.

56. In later instruments small errors of range scale may be expected. The error between inside and outside scale should be checked throughout the full range. These may Vary but should not exceed one half unit of error. Lost motion-in gear trains can usually be detected by feel. The outside scale should be frequently checked against the inside scale for lost motion. The extent of error due to lost motion is variable. If the lost motion is believed excessive, the services of an expert optical repair man should be requested.

57. Looseness of the optical tube in its bearings will affect halving, producing varying errors on inclined targets.

58. Correction scales are graduated arbitrarily in earlier instruments, in units of error in later instruments. The value of one at different parts of the scale should, be determined.

59. If internal or halving adjusters are loose, vibrations or gun blast may cause changes in settings of these adjusters.

Personal Errors.

60. The principal errors of observation, or personal errors are:

Consistency in making coincidence can be acquired by long practice only. Heat waves and vibrations are sources of large errors which can be materially reduced by good judgments.

Improper halving adjustment introduces variable errors on inclined targets.

The internal adjuster setting is the largest potential source of observational errors. Since the range-finder cannot be more accurate than the internal adjuster, these adjustments must be made with extreme care and frequently to avoid the introduction of large unnecessary errors.


Ideal range taking conditions are a clear bright atmosphere, no heat waves, stationary platform and steady target. These conditions are rarely obtained at sea. Heat waves and vibrations, or poor light conditions or hazy weather make accurate coincidence so difficult that range taking becomes more a matter of judgment than of alignment. Errors are so dependent on the individual operator's judgment that only a rough guess of the range accuracy can be made.

Adjustments - Coincidence Type.

61. The adjustments of a coincidence type range-finder should be made in the following order:

Focusing is accomplished on a target having a background of good contract, starting from the extreme plus position, and moving the eyepiece in until the halving line is sharply defined.

Halving adjustment is the act of moving two focal images in a direction perpendicular to the halving line until they form together one whole image while the range-finder is at a constant elevation on a motionless target. It is accomplished by the halving adjustment knob on a straight line target, as a roof, etc., at a range not less than the minimum on the instrument range scale. If the halving is correct the top of the target will appear in the upper half-field the instant it disappears in the lower half-field. If it appears too soon, there is an error of "duplication", if too late, of "deficiency".

The steps in making internal adjustment (lost ray type of adjuster) are:


Adjustments - Stereoscopic Type.

62. The adjustments of the stereoscopic type range-finder are:

In focusing the right eyepiece the left end-window should be covered and vice versa.

Each operator should have his interpupillary distance measured with a opeciil ocular instrument for that purpose. Knowwing his interpupillary distance the operator should set the range-finder interpupillary scale at that figure or half a millimeter more, never less.

Height adjustment consists in moving the reference marks vertically until they appear at the same height. It is accomplished by setting the range at infinity on a. near target the top of which makes a sharply defined horizontal line against the sky. The height adjustment knob or key is turned until the target image makes contact with the two measuring marks at the same time.

Internal adjustment (adjuster penta prism type) is accomplished as follows:


Adjustment of End Windows.

63. The adjustment of end windows is a factory adjustment, the weak wedges being used to correct slight errors due to lack of parallelism or slight inaccuracies in optical parts, etc. Once set and secured, in position at the factory there is ordinarily no reason why they should be changed or tampered with. However adjustment is considered necessary under the following conditions:

Looseness - If it will rotate with pressure of fingers against the glass. Instrumental error - If a large constant error is found as indicated by reliable checking data.

These windows should never be shifted in position nor removed for cleaning except by an optical expert.


Casting Loose.

64. In turrets, when possible end ports should be opened and blowers started at least two hours before range-finders are to be used, in order to obtain steady temperature conditions. The steps in casting' loose are:

65. The steps in securing a range-finder are:


66. Range taking procedure:

Making Coincidence.

67. Consistency in making coincidence can be acquired only by long practice. Standardized training methods greatly reduce personal errors. The two general methods of making coincidence are the direct method and alternate method. The direct method consists in turning the measuring knob in one direction only - from high to low range - until perfect coincidence is made. This method is preferred for establishing an accurate rate of change, and when ideal ranging conditions exist.

The alternate method consists in turning the range Knob until coincidence is made, continuing turning in the same direction until coincidence just breaks. Reverse the movement of the knob until coincidence just breaks on the opposite side; then carefully make coincidence. By this method a better sense of exact coincidence is obtained under conditions of poor visibility.

These methods are also applicable to range taking with stereoscopic type range-finders.


Care and Handling of Range-Finders

68. Range finders are delicate instruments, requiring extremely careful handling. Their use should be restricted to regular range-finder personnel only, Then in use, they should be inspected daily and all operating parts moved. Mechanical parts, as training gear, bearing rings, etc., should be well lubricated. Parts extending into the range-finder should be oiled only when necessary and then very sparingly.

69. When avoidable, instruments should never be exposed to inclement weather. Range-finders in exposed positions should be protected with water-proof covers from spray, rain, snow, rapid temperature changes, etc. In dry clement weather waterproof covering should be removed to thoroughly air instruments and prevent fogging of optical parts. Proper ventilation must be maintained with turret range-finders to keep both ends at the same temperature. Range-finders fitted with calcium chloride pots or calcium containers for drying out should be examined at least semi-monthly and the calcium chloride or metallic calcium renewed when it becomes damp. A neglected pot or container accelerates corrosion.

70. Dirt or grit should be removed from outside optical surfaces with a camel's hair brush to avoid scratching. Only clean soisette. lens paper or chamois should be used for wiping optical parts. Since alcohol and ether dissolve the balsam used for cementing lenses, cleaning with these fluids should be restricted to experienced repair personnel. Any repairs affecting the adjustment of the optical systems should be accomplished by the repair ship.

71. When no drills are held for protracted periods, as during overhaul, small exposed range-finders should be stowed away in their packing boxes. In handling, the range-finder should always be supported by and rested on its bearing rings. Its crate or case should always be used-for transportation.

Rangefinder Checking

72. Due to the possible instrumental errors, including those of the internal adjuster system, it is necessary to check each range-finder with some external object so that all range-finders are in agreement at the same range. Satisfactory performances can be attained only by frequent checking. Opportunities to check range-finders at long and extreme ranges on satisfactory targets are very rare; each such opportunity should be utilized.

73. The following methods are used for checking range-finders:


Checking at Infinity.

74. Checking the range-finder at infinity range, on the moon, or a star, for example, should prove satisfactory if the halving or height adjustment is carefully made. However accurate coincidence on a target at very great distance is exceedingly difficult to make and in coincidence range-finders the use of the astigmatizer lenses is involved.

Instrumental Comparison.

75. Instrumental comparison consists in comparing a rangefinder with another of longer base or of greater accuracy. It is applicable at any time and is particularly advantageous for checking short base instruments. Ranges for comparison can be selected at will. The disadvantages of this methhod are: (1) If the comparing range-finder is on a different ship, a common time must be used or good communication is essential, (2) it is dependent on the accuracy of the range-finder used as the standard.

Known Range.

76. All range-finders are checked on "known" ranges on land before leaving the factory and after repairs ashore. There are good checking ranges on land at Guantanemo and San Pedro. The advantages of checking on land are: (1) The range is more accurately known, (2) The range-finder is absolutely steady.

The disadvantages are: (1) Danger of derangement in transportation, (2) Difference in temperature and atmospheric conditions from those at sea.

77. The Hydrographic Office has prepared range-finder testing charts for the chief bases and harbors. These charts are drawn to large scale and have the ranges of various well defined objects marked on them in concentric circles. The position of the ship is ascertained by observing sextant angles of various navigational marks shown on the chart and cutting in with the threearm protractor. This position must be frequently checked during the test. This method may be used to check several range-finders simultaneously but is subject to inaccuracy due to errors in plotting the ship's position and changes in position between plots.

Base-Line Method.


78. The base-line method involves the use of two targets on a base line of known length, this base line being parallel to the base line of the range-finder, for example two stacks on a destroyer presenting a 90° or 270° target angle. It involves making coincidence on each of the targets and in bringing one target into coincidence with the other. The method and necessary computations are completely described in O.P. 105, pp. 53-58 and in U.S. Naval Institute Proceedings, vol. 56, No. 324 of February, 1930. "A Method of Calibrating Range Finders at Sea", by Comdr. R. C. Parker, U.S.N. This method has the advantages: (1) It is almost everywhere available at sea; (2) several range-finders can be checked simultaneously, (3) under service condition; (4) no time comparison is necessary. Disadvantages: (l) Checking is not accomplished at maximum ranges; (2) for satisfactory results very accurate scale readings are required.

Checking Procedure;

79. The procedure in checking a range-finder (plotting method) is:

80. For checking coincidence range-finders, only well-defined vertical targets should be used. When checking range-finders simultaneously the talker of each range-finder should be on a separate telephone line, as 1 JW for One Range, 2 JD for Two Range, 2 JW for Three Range, 2 JE for Four Range, 1 JT for Five Range, 3 JW for Six range.


81. Two curves are used on connection with range-finders, Curve A and Curve B. These are constructed from the data from


(h), Par. 69. The A curve is pip-t/bed using as ordinates range in thousands of yards, as abscissae" the value in yards of one unit of the adjuster scale. A full curved should be faired in for three or more points from observed data. A dotted curve should be drawn in, computed from-the formula:

S' = S (R')2/R

where S is the observed value in yards of one unit of the adjuster scale at range R
and S' is the value in yards of one unit at range R'.

The B curve is plotted using as ordinates range in thousands of yards, and as abscissae personal index correction or off-set of internal adjuster. B curves should be obtained for each operator for each range-finder. Experience has shown that B curves vary for different localities and for different atmospheric conditions in each locality. Accordingly they should be constructed as data is obtained for the various weather conditions in each locality. Each curve should be properly marked as to location, time of year, weather, temperature, etc. range-finder no. and name of observer.

It is essential that range finder curves be kept up-to-date.

Application of Checking Data.

82. Checking data should be applied from the plotting room. After operations have cast loose the range-finders they should report the adjuster setting obtained and a range. The rangefinder offer should consult the B curves and obtain the proper index correction for the range, weather conditions, locality, etc. for each instrument, apply these to the reported adjuster settings, and send-each-instrument the corrected adjuster setting for the operator to use.

During plotting if a range-finder reads consistently low or high with respect to the mean range line, the range-finder officer may "spot" it on from the A curve for that instrument. In this respect it is important that the range-finder officer thoroughly understand the natural "spread" of the range-finder, the theoretical limits of accuracy and what may be expected of the instrument under the existing sea and weather conditions.

The range-finder officer should always keep in mind the significance of the basic formula for all range-finders:

e = ± B2 Δ/B M

This formula emphasizes the need for using high magnification if conditions permit. It shows that the range error varies as the angular alignment error. Under poor conditions this may easily be ten times the value used for the "unit of error". Lastly the formula emphasizes that the range error varies as the square of the range.


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