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T8 Periscopic Sights

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"Periscopic T8 sight

The principle of the T8 periscope does not differ from previously examined periscopes. It combines an observation periscope with a telescopic system. This design is a further step towards the perfection and development of these types of sights.

The significant difference between the T8 sight and other sights of the first and second group is that the degree of magnification is significantly increased, and a collimator device is with a sighting grid is introduced. Thanks to this, the T8 sight and other sights of its type can be called a variable magnification sight.

Characteristics of the sight


1x optical system

6x optical system

Magnification

1x

6x

Field of vision (horizontal)

56 degrees

11 degrees 20 minutes

Field of vision (vertical)

10 degrees

11 degrees 20 minutes

Length

365 mm

Width

178 mm

Thickness

57 mm


Sight optics

The observation block, or 1x optical system, consists of a main rectangular prism and lower half-silvered mirror. Protective glass is installed in front of the mirror.

A collimator device is installed outside of the observation block, consisting of a two-lens condenser aluminium mirror, a collimator lens, and an illumination light. A grid is marked on the flat part of the upper lens of the condenser.

The converted beam of light given by the condenser is reflected by the aluminium mirror and, passing through the collimator lens, hits the lower half-silvered mirror. Due to the fact that the half-silvered mirror allows focused light to pass through, the grid image is overlaid on the image observed through the sight. The focal plane of the collimator lens coincides with the flat side of the upper lens of the condenser, and therefore the grid and the observed image are seen the same distance away, i.e. there is no parallax between the grid and far away objects. The 6x optical system consists of a lens, a prismatic rotation system (Porro prism 1st type), a gird, and an Erfle eyepiece. The main prism of the 1x optical system is used as the upper reflector.

Fig. 10: Optical system of the T8 sight.


Design of the sight

The sight (fig. 11) consists of: the head, the case, optical components, optical component attachments, adjustment mechanisms, and grid illumination mechanisms.

The head of the sight includes: a plastic case, a rectangular prism, and prism attachment parts. The lower part of the case has a metallic base with tabs that the head clips onto. The slanted part of the head contains a table for writing down calibration markings.

The case consists of two parts, firmly connected by bolts. The upper part of the case contains a half-silvered mirror and the lens of the 6x system. The collimator lens and the protective glass are installed between the upper and lower part of the case. The lower part of the case holds the rectangular prisms of the rotating system, installed on a special bridge, the grid of the 6x system, a three part eyepiece, the condenser of the collimator device, and two sockets for lightbulbs. The cap attached between the upper and lower halves of the casing from the outside holds the aluminium mirror of the collimator device. The upper part of the casing has two rotating eccentric clips, similar to the ones on the M4A1 sight. Below the clips are two cylindrical holders that the periscope mounts on in the mount.

The adjustment of the T8 periscope differs from those described earlier. It is performed by moving the grids that are located in the holders, which allows moving the grids in perpendicular directions. The adjustment mechanisms are similar to those used on the M4A1 sight.

Fig. 11: cutaway of the T8 sight.

The small magnification of the T8 periscope is compensated by the addition of the 6x device, tactically sufficient for modern gun systems, as it allows the recognition of targets from up to 3 km away. This corrects the main drawback of earlier sights. In addition, the presence of a grid in the 1x observation block makes it easy to fire on targets up to 1 km away. The sight, like earlier sights, is simple and comparatively cheap.

The grid of the 6x sight (fig. 12) is marked in hectoyards. The vertical dashes and distances between them correspond to 200 yards and are marked for up to 4200 yards. The distances between horizontal dashes correspond to 400 yards. The length of the horizontal dashes and distances between them correspond to 5 mils. The grid of the 1x sight has the same image, but markings up to 1200 yards.

Fig. 12: Grid of the 6x optical system of the T8 sight.

Conclusions

The T8 sight is becoming the primary optical sight of American tanks. These sights have a number of positive qualities:
  1. Good combination of wide observation angle and high magnification.
  2. Relative simplicity of design.
  3. Provide for quick and easy replacement of the vulnerable component (upper head).
  4. Provide a good solution to the problem of clearing the upper window.
Drawbacks include:
  1. The need to have a conversion table for firing different types of shells. Grids are provided for one type only (usually armour piercing).
  2. Poor shape of the grids. In the T8 sight the grid obscures the vision through the sight, although it is convenient to correct fire side to side and the distances marked on the grid (200 yards) are acceptable.
  3. As with all telescopic sights, there is insufficient precision due to the use of a rod device. The latter drawback explains the need to equip all American tanks with a backup sight, a telescopic sight that is rigidly connected with the gun. This is why latest American tanks are equipped with two types of sights: a peritlescopic T8 sight and a telescopic sight that is rigidly connected to the gun."
Tank Industry Herald 10-11, 1945

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