German Coaxial Sighting Telescope, WWII Tactical and Technical Trends, No. 8, September 24, 1942 (Lone Sentry)

The examination of a German sighting telescope TZF 5b manufactured by L. Leitz, Etzlar, and taken from a German Mark IV tank, where it was used for the coaxially mounted 75-mm gun and the 7.92-mm machine gun 34, revealed the following particulars:

a. Dimensions

Length 32 in

Weight 25 lbs

Diameter of objective (mounted) 23 mm

Diameter of eye-lens (mounted) 48 mm

Maximum elevation of telescope +36°

Maximum depression of telescope -22°

b. Optical Constants

Diameter of effective aperture of objective 14.5 mm

Diameter of exit pupil 6.0 mm

Magnification 2.4 times

Field of view (angular extent) 23.5°

Apparent field (field x power) 56°

Eye relief (1) mechanical 21 mm

(2) optical 30 mm

Plane of entrance pupil 48 mm

Axial light transmission 20.1%

"Veiling glare" index 14.5

"Eye relief" refers to the distance between the eyepoint and (a) the eyepiece mount (mechanical) or (b) the eye-lens (optical).

The "veiling-glare index" is a measure of the amount of scattering of the light in its passage through the telescope, (in other words, the loss of contrast in the image) and is a function of the optical design and the cleanliness of the glass surfaces. A clean binocular usually has a value between 1 and 2, while a value of 5 gives a noticeably misty appearance. It is probable that the high veiling-glare index of this telescope is inherent in the optical design, involving as it does some 30 air-glass surfaces.

c. Optical Performance

The captured sight was tested for optical performance with the following results:

With focus adjusted to give the best possible performance at the center of the field of view, definition was good over an angular field of 8 degrees and fair over a field of 15 degrees. Definition near the margin was poor. Eye freedom was found to be good, and correction for color, curvature, and astigmatism, satisfactory.

d. General Construction

The telescope, which is of the fixed eye-piece type, consists of three main parts:

(1) Part one, (which moves with the gun) includes the objective (with protecting glass), the graticule, and a single-reflecting prism. The objective is arranged in a tube (6A) (see accompanying sketches for numbered references hereinafter) mounted on the front half of the graticule box (4A), and the prism in a casing behind the graticule box.

(2) Part two includes the eye-piece and a second single-reflecting prism, which are mounted in a stationary eye-piece tube (2A) fixed to the turret of the tank.

(3) Part three includes a double reflecting prism, which is arranged in an offset box (3A), and connects the optical systems of parts one and two. The optical and mechanical hinge of the telescope is located at this part.

e. Optical Hinge

The single reflecting prisms on the pivoting ends of parts one and two transmit the line of vision to and from the respective reflecting surfaces of the double reflecting prism, through hollow tubes which form the bearings of the mechanical hinge mechanism.

f. Mechanical Hinge

The mechanical link between parts one and two is formed by a small, flat, steel plate pierced near each end by two large circular holes. Short tubes projecting at right angles on the fixed and moving sections engage with these holes, and form two bearings, allowing the sections to move in a vertical plane relative to each other.

[German Sighting Telescope TZF 5b]

Gears fixed to each short tube mesh beneath the plate and equalize the movement at each bearing point, thus forming a virtual hinge midway between the two bearings. These bearings are adjustable by tapered, flanged sleeves, which are threaded on to the projecting tubes, and which form the bearing surface in the connecting plate. The tapered flanges on the sleeves can thus be drawn by the screw thread into countersunk recesses in the plate and any slack taken up.

A light metal cover beneath the connecting plate forms the seal to exclude dust from the bearings and gear wheels; an aluminum casting, containing the double reflecting prism attached by screws, forms the outer cover.

g. Focusing Adjustment

The focus can be adjusted by rotating a milled collar (1A) on the eye-piece. This collar bears a scale showing single diopters and is figured at zero with 5 divisions on each side of zero, the + and - signs being also engraved.

h. Range-Setting Gear

Ranges are set on the sight by means of a range-setting wheel (6B) on the left of the eye-piece. This range-setting wheel is coupled by a shaft drive including two universal joints (4B) to the mechanism in the graticule box which controls the relative positions of the graticule plates. A limiting stop a spring-loaded brake (5B) are provided for maintaining the selected range setting.

i. Graticule Box

Two glass plates are arranged in the graticule box: a circular range plate which is mounted to rotate and a plate engraved with sighting marks, which moves vertically.

The scales on the range plate consist of small circles, numbered, as shown in the accompanying sketch, in hundreds of meters. The sighting marks on the other plate consist of a central triangle with three smaller triangles on each side of it. The row of triangles covers an angle of about 1 1/2 degrees in the object space. A fixed transparent pointer (7C) (figures 3 and 4) indicates the range on the scales of the range plate.

To set a given range, the range-setting wheel is rotated, causing the range plate to revolve until the required range marking is opposite the pointer, and causing the sliding plate to move up or down so that the triangular sighting marks indicate a position of the field in accordance with the scale reading.

The range plate (1D) (figure 5) is carried in the center of a cam ring (2D) which is rotated by a gear wheel (3D) meshing with a gear behind it. A radius arm (4D) rests on the cam ring and engages with a vertical sliding block (5D), which is thus raised or lowered on rotation of the cam ring. The block (5D) engages a block (3C) (figure 4) on the vertical slide (2C) which contains the plate (1C) engraved with the sighting marks. The block (3C) is kept constantly pressed against the block (5D) by a return spring (4C).

Some appropriate measurements of the relative positions of the central triangle for various scale settings gave the following results. (The column headed "position of triangle" gives the approximate angular distance between the central triangle and the circle marking the scale setting.)

MG scale
setting       Position of
triangle       75-mm scale
setting       Position of
triangle
0 6° 25' 0 6° 25'
2 6° 32' 4 7° 0'
4 6° 38' 8 7° 42'
6 6° 45' 12 8° 22'
8 7° 0' 14 8° 50'

j. Adjustment of the sight

The sighting point can be adjusted vertically by turning a key (1E) (figure 5) and laterally by turning a key (6C), (figure 4).

Rotation of the key (1E) causes movement of a transverse slide (6D) (figure 5), in which the vertical sliding block (5D) is mounted, thus moving the block (5D) across the block (3C) and hence raising or lowering the sighting point. A spring (7D) constantly exerts pressure against the slide (6D) to take up any play or backlash.

Similarly, rotation of the key (6C) cause movement of a traverse slide (5C) in which the slide (2C) is mounted. At the same time, the block (3C) moves across the block (5D), giving a resultant diagonal motion, the vertical component of which may have to be compensated by turning the key (1E).

Both adjustments are made to fit a square key and the housings are internally threaded, presumably for dust caps, which however were missing in the sight examined.

k. Night Illumination

The mounting for the range-scale plate is drilled on the edge, so that the scale can be illuminated by the bulb holder (8C) (figure 4) through a glass window on the side of the graticule box. A variable shutter in the bulb holder varies the degree of illumination provided.

l. Miscellaneous

A 10-mm armor plate (3B) (figure 2) is mounted behind the graticule box to give added protection in the case of a direct hit.