Apart from its basic simplicity, the straight-through telescopic sight has the great advantage of being rigidly attached to the gun mounting, which eliminates the possibility of alignment errors that exists whenever there is relative motion between the sight and the gun. However, the straight-through telescopic sight is awkward to use because the eyepiece has to be at a considerable distance from its pivot, which is on the axis of the gun trunnions, and therefore moves up and down over a relatively large arc as the gun is elevated or depressed. In spite of this, straight-through telescopic sights were used in many tanks up to the early 1940s and in particular in those built in Britain, including the Comet of 1944.
Some of the early telescopic sights such as that mounted in the Renault FT and the Aldis No.22 telescope of the Vickers Medium Mark I provided no magnification, which reflected the short range of engagement of the contemporary tanks. However, later telescopes provided magnification which increased steadily with the effective range of tank guns. Thus, the No.57 telescopic sight of the Comet had a magnification of x3 and the much earlier French Char B and S-35 already had x4 magnification.
In the mid-1930s the original, straight-through telescopes began to be replaced by articulated telescopic sights. With this type of sight the objective is still attached to the gun mounting, but there is an optical hinge behind it which enables the rear portion of the telescope and in particular the eyepiece to the fixed to the turret roof. As a result, the gunner does not have to move his head up and down when the gun is elevated or depressed, as he has to do with a simple telescopic sight. At the same time the articulated telescope retains the advantage of the objective remaining fixed to the gun and, hence, of being free of alignment errors.
Articulated telescopic sights appear to have been developed originally in Germany by the Ernst Leitz company. The first Leitz sight of this type was probably the TZF 4, which was produced for the 1935-designed Pz.Kpfw.II. At about the same time articulated telescopic sights also began to be fitted in Soviet tanks, the first of them being probably the BT-7. The TMFD sight of this type with a magnification of x2.5 was adopted for both the KV heavy and T-34 medium tanks, first built in 1939 and 1940, respectively. Other articulated telescopic sights have been used in all Soviet battle tanks built since then until the introduction of the T-64 and T-72.
Concurrently with its adoption in Soviet tanks of the late 1930s, the articulated type of telescopic sight was also adopted in the Italian M13/40 and in the German Pz.Kpfw.III and IV. The TZF 5 telescopic sights of the two German tanks were produced by Leitz but they had the same x2.5 magnification as the Zeiss periscopic gunner's sights used in earlier German tanks. Both models of the Tiger heavy tank also had an articulated telescopic sight, the TZF 9, with a magnification of x2.5, as did the Panther. The TZF 12a sight of the Panther was also designed by Leitz and represented a further development of the telescopic sight in having dual x2.5 or x5 magnification.
After the Second World War dual x3.5 or x7 magnification was also incorporated in the TSh 2-22 articulated telescopic sight of the Soviet T-54 tank and later also in the sight of the T-62 tank. In contrast to their use in German, Soviet and Italian tanks, articulated telescopic sights were not installed in British and US tanks until 1959 or 1960, and then only as single-magnification auxiliary sights. The sights were the No.26 telescope with x7 magnification which was mounted in the Chieftain, and the M105 telescope with x8 magnification which was mounted in the M60 tank.
At about the same time the Zeiss TZF 1A articulated telescope with x8 magnification was adopted for the German Leopard 1 as a secondary sight and another articulated telescope with x8 magnification, the M271, was adopted as the gunner's primary sight in the French AMX-30. A similar telescope was mounted for the same purpose in the more recently built AMX-32 and AMX-40 and in the late 1970s Vickers decided to use the Vickers Instruments' L30 articulated telescope with x10 magnification as the gunner's primary sight in the Valiant prototype. An articulated telescopic sight was chosen in this case and that of the three French tanks mainly because its objective can be fixed to the gun and because of the consequent elimination of the alignment and sighting errors which are associated with other sights.
In the meantime British and US tanks were fitted with periscopic sights as their principal or only gunner's sights. The use of periscopic sights originated with the Leicht and Grosstraktoren built in Germany in 1929-1930, all of which had Zeiss periscopic sights with a head prism rotatable in elevation and linked mechanically with the gun. Similar Zeiss TWZF 3 periscopic sights with xl.75 magnification were fitted in the first Swedish tank, the Strv m/31, and then in other tanks built in Sweden by Landsverk. From the mid- 1930s onwards other sights of the same kind were also fitted in Soviet tanks such as the T-26B and BT-5 as well as BT-7. This was followed by the installation of the PT-47 periscopic sight with x2.5 magnification in the KV and T-34 tanks, but in their case only in addition to the TMFD telescopic sight.
However, during the Second World War Soviet tanks ceased to be fitted with periscopic sights. Thus the T-34-85 which was introduced in 1943 was no longer fitted with the PT-47 but only with the TMFD sight. But while they were being abandoned in Soviet tanks, periscopic gunner's sights were adopted in US M3 and M4 medium tanks. In the prototype of the M4 the sight was of the fixed type, like the original Zeiss periscopes, with a head mirror rotated in elevation by a linkage connected to the gun mounting. But when the M4 went into production this was replaced by the M4 periscope, which pivoted as a whole under the action of the linkage connecting it to the gun. A pendulous periscope of this kind linked to its 75mm gun had been adopted already in the M3 medium tank and similar periscopes continued to be used in US tanks until after the Second World War. During the intervening period they were improved optically to a considerable extent. In particular their magnification was increased from the inadequate level of x 1.44 of the M4 periscope to x6 of the M10 periscope, which was introduced in M4 tanks towards the end of the Second World War and which was still fitted in M46 tanks in 1948.
After they were abandoned in the United States pendulous periscopic sights continued to be used in British tanks. The first of them was the Centurion, which was originally fitted with the No. 1 sight with two channels providing the same x6 and x 1 magnification as the US M10 periscope. But as they were rearmed with the more powerful, 105mm guns, Centurions were fitted with sights with x8 magnification, and so were the early Chieftains. Subsequently, in keeping with the longer range capabilities of their 120mm guns, Chieftains and Challengers were provided with sights with x10 magnification.
Pendulous periscopes represent a relatively crude way of moving the line of sight in elevation but they are superior ergonomically at least to straight-through telescopic sights because their eyepieces move fore-and-aft rather than up-and down as guns are elevated or depressed, which requires less awkward head movements on the part of the gunner. They are also superior to the telescopic sights from the point of view of armour protection, as they do not require any aperture or 'ballistic window', in the frontal armour of turrets. They are also more robust than fixed periscopes with tilting head mirrors but they are inferior to them as well as to articulated telescopic sights from the point of view of ergonomics because the latter have eyepieces that do not move.
The ultimate verdict on the pendulous periscopes is the fact that since the Second World War there has been an increasingly strong revival of the use of the alternative, fixed type of periscopic sight with a pivoted head prism or mirror. The first of the new generation of such sights was the US T35, which was originally installed in the turret of the T42 tank and which then went into service in the M47 medium tank as the M20 gunner's periscope. Similar sights were fitted subsequently in the M48 and M60 tanks but while the M20 periscope had a magnification of x6 and x 1, like the M10 periscope, the M31 and M32 sights of the M48 A3 and M60A1 have had the magnification of their high power day channel increased to x8 in keeping with the general trend.
In addition to the M20 gunner's periscopic sight, the M47 tank was also fitted with the M12 stereoscopic rangefinder, which is operated by the gunner and can be used not only for determining the range of targets but also to lay the gun on them. The rangefinder provides x7.5 magnification and only one half of it is used as a sight, when it becomes the equivalent of a periscopic sight laid on its side. Because of its magnification and the wide, 1.52m separation of its objectives, the rangefinder also provides the gunner with a very powerful binocular observation instrument which enhances stereoscopic effects and makes distant objects stand out in greater relief.
The development of an instrument which could be used by itself for rangefinding, gun-laying and binocular observation represented a major advance in the optical equipment of tank gunners. Nevertheless only Leopard 1 followed the example of the M47 tank in having a rangefinder operated by the gunner which was also the primary gun sight as well as being a powerful observation binocular. In this case the TEM 2A rangefinder has had an even greater magnification of x 16 and an optical base of 1.72m. In other tanks optical rangefinders have not been allocated to gunners but to commanders, in spite of the fact that they distract the latter from their primary command functions. Moreover, most of the rangefinders have been of the coincidence type, which means that they are monocular rather than binocular instruments and therefore less effective for general observation. But the merits of the alternative approach of providing gunners with an optical instrument capable of performing three different functions, like the M12 and TEM 2A stereoscopic rangefinders, were invalidated when all optical rangefinders began to be superseded by laser rangefinders which are considerably easier to install as well as being much more accurate.
However, one feature of the installation of the M23 rangefinder in the M47 tank, namely its use as a gunner's periscopic sight with the head projecting out of the side of the turret, reappeared in the XM-1 prototype designed by General Motors, which had the gunner's primary sight mounted at the right side of its turret. This avoided the need to partially compromise the frontal armour of the turret or, alternatively, to accept a large projection out of the turret roof, which would interfere with the tank commander's vision. The side mounting of the sight was made particularly attractive by the size to which gunner's sights had grown by then. In particular, their entrance windows had become large, to allow for the movement of the two-axis stabilised head prisms or mirrors which began to be incorporated in them. The windows were made larger still by the requirements of night vision, especially since the latter became based on thermal imaging and consequently required a germanium window separate from the glass window of the daylight vision and laser rangefinder channel.
But the example of the General Motors' XM-1 prototype has not been followed in other tanks with equally sophisticated and large gunner's sights. Instead, the sights have been fitted into the front of the turret, as in the Leopard 2, or mounted in the turret roof, as in the M1 tank produced by General Dynamics.
Apart from incorporating laser rangefinders and thermal imagers as well as stabilised head mirrors, the latest periscopic gunner's sights also provide greater magnification than earlier sights. Thus, the sight developed by Delco Electronics for the MBT-70, which was the first of the new generation of periscopic gunner's sights, incorporated a zoom telescope with magnification varying from x7 to x14. Zoom telescopes were not used in the sights developed subsequently by Delco Electronics for the XM-803 and the XM-1, which had telescopes with a dual magnification of x8 or x12.5 and x4 and x10, respectively. Dual magnification of x3 or x10 is also provided by the sight of the M1 tank but production versions of Leopard 2 have sights with a single magnification of x12, in spite of its prototypes having sights with dual magnification.
The magnification quoted for the different sights indicates the optical characteristics which have come to be required of gunner's sights. Their power has obviously grown over the years but the x10 magnification which has been adopted in the latest of them is not as high as that of some of the sights which preceded them. This is due to it being a better compromise between the conflicting requirements of speed of detection and of detection at long ranges. In general, the time to detect a target increases with the magnification of the sight, because it takes longer to scan a search field when the magnification is high and the field of view is inevitably small than when the magnification is low and the field of view is correspondingly large. However, low magnification makes it difficult to detect targets at long ranges. Thus, unit power is the most effective, overall, at short ranges but to recognise targets at the maximum range of tank guns and beyond it is necessary to use a magnification of at least x8 and preferably x10.
A number of the simpler periscopic sights provide this degree of magnification and unit power viewing for target searching as well as general observation. This makes them superior in this respect to the telescopic sights with a single high magnification but still leaves much to be desired because of the big difference between the fields of view associated with magnifications of x10, or even x8, and x1. Thus, x10 magnification is associated with a field of view of about 6° while unit power is associated with fields of view of at least 24° and this can make it difficult or time consuming to switch from it to high power and still retain sight of the target. This is particularly true when, as is usually the case, the unit power window is separate from the high power eyepiece.
It is much more effective therefore to incorporate an intermediate degree of magnification, of x3 or x4, as has been done in the most advanced of the gunner's sights. This provides capabilities which overlap with those provided by the x10 magnification and make switching from one to the other relatively easy. Since they are associated with a field of view of 15° to 20°, magnifications of this order are suitable for observation and for target detection at up to the maximum range of tank guns and they provide sufficient resolution for the engagement of targets at other than long ranges. It is of interest to note that sights with similar dual magnification of x3 and x10 or x12 have also been developed for missile-armed helicopters.
But, for all the advances incorporated in them, even the latest gunner's sights are not as good as they might be in at least one respect, namely in their suitability for general surveillance. This calls for instruments that can be used for prolonged periods and therefore with a minimum of eye strain, which implies the use of both eyes. As it is, all the sights with two-axis stabilised head prisms or mirrors are monocular and although sights of this kind can not be made truly binocular they can, at least, be provided with biocular eyepieces.
Binocular vision can be provided, with all that this implies not only in terms of the fully relaxed use of both eyes but also of depth perception but at the price of stabilising the head prism only in elevation. This has been demonstrated by the Jungner sights developed for the S-tank but its consequence is that the stabilisation of the head mirror, or prism, in azimuth is not better than that of the cupola or turret in which the sight is mounted. A head mirror stabilised only in elevation is actually incorporated in the gunner's sight of the US M1 tank but, unlike the Jungner sights, it is monocular.
In the other sights the head elements have been independently stabilised in azimuth as well as elevation. This has done much to isolate the line of sight from both horizontal and vertical disturbances caused by tanks moving over rough ground. In consequence, gunner's capabilities are protected to a considerable extent against degradation by line of sight jitter caused by vehicle motion. The independent two-axis stabilisation of the sights' head mirrors or prisms also allows much smoother target tracking than that which can be achieved by the turret traverse drives by themselves, particularly at low tracking rates.
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