Telescopic sight
From Wikipedia, the free encyclopedia
A telescopic sight, commonly called a scope, is a device used to give an accurate point of aim for weapons such as firearms, airguns and crossbows. Other sighting systems are iron sights, red dot sights, and laser sights.
This article will concentrate on firearm scopes; the principles described are equally applicable to any device which needs aiming. For the sake of brevity, the term gun will be used to indicate any device aimed by a telescopic sight.
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[edit] Types
Telescopic sights are classified in terms of the optical magnification and the objective lens diameter, e.g. 10×50. This would denote 10 times magnification with a 50 mm objective lens. In general terms, larger objective lens diameters are better (collect more light and give a wider field of view), the magnification power should be chosen on the basis of the intended use. There are also adjustable objectives sights where the magnification can be changed by manually turning one part; the syntax is the following: minimal magnification — maximum magnification × objective lens, for example, 3–9×40.
Telescopic sights come with a variety of different reticles, ranging from the traditional crosshairs to complex reticles designed to allow the shooter to estimate accurately the range to a target, to compensate for the bullet drop, and to compensate for the windage required due to crosswinds. Perhaps most flexible is the "mil-dot" reticle, which consists of duplex crosshairs with small dots at milliradian intervals in the field of view. (A milli-radian equates to 3.43775 MOA, that is, approximately 21.6 inches at 600 yards; each MOA equates to 1.0472 inch at 100 yards, often rounded to 1 inch at 100 yards for fast mental calculations.) A trained user can estimate the range to objects of known size, the size of objects at known distances, and even compensate for both bullet drop and wind drifts at known ranges with a reticle-equipped scope.
For example, with a typical Leupold brand duplex 16 MOA reticle (of a type as shown in image B) on a fixed power scope, the distance from post to post (that is, between the heavy lines of the reticle spanning the center of the scope picture) is approximately 32 inches at 200 yards, or, equivalently, approximately 16 inches from the center to any post at 200 yards. With a known target of a diameter of 16 inches that fills just half the distance from scope center to post, the distance to target is approximately 400 yards. With a known target of a diameter of 16 inches that fills the entire sight picture from post to post, the range is approximately 100 yards. Other ranges can be similarly estimated accurately in an analog fashion for known target sizes through proportionality calculations. Holdover, for estimating vertical point of aim offset required for bullet drop compensation on level terrain, and horizontal windage offset (for estimating side to side point of aim offsets required for wind effect corrections) can similarly be compensated for through using approximations based on the wind speed (from observing flags or other objects) by a trained user through using the reticle marks. The less-commonly used holdunder, used for shooting on sloping terrain, can even be estimated by an appropriately-skilled user with a reticle-equipped scope, once the slope of the terrain and the slant range to target are both known.
[edit] Mounting
As very few firearms come with built-in telescopic sights (military bullpup designs such as the Steyr AUG being the primary exception) mounting a scope to a firearm requires additional equipment. Equipment is available to mount scopes on most production firearms. A typical scope mounting system consists of two parts, the scope base and the scope rings. By picking the appropriate combination of scope base to fit the firearm and scope rings to fit the scope, a wide range of scopes may be mounted to most firearms.
[edit] Scope bases
The base is attached to the rifle, usually with screws, and is often designed to have a low profile, and to allow use of the iron sights if the scope is not present. Some manufacturers provide integral bases on many of their firearms; an example of such a firearm is the Ruger Super Redhawk revolver. The most commonly encountered mounting systems are the 3/8 inch (9.5 mm) and the 11 mm dovetail mounts, commonly found on rimfires and air guns, the Weaver type base, and the STANAG 2324 base. Ruger uses a proprietary scope base system, though adapters are available to convert the Ruger bases into Weaver type bases.
[edit] Scope rings
In addition to needing the right type of connector to attach to the desired base, scope rings must be used to hold the scope to the mount. The rings must be of the proper size to fit the scope; common sizes are 3/4 inch (19 mm), 1 inch (25 mm), and 30 mm. Red dot sights commonly are found in larger sizes, such as 40 mm, and these often use ringless mounting systems designed to fit dovetail or Weaver type bases. Rings are also available in a variety of heights and materials. Ring height is chosen to place the scope high enough to clear the firearm, and at a height comfortable for the shooter.
[edit] Mounting issues
Scopes for use on light recoiling firearms, such as rimfires, can be mounted with a single ring, and this method is not uncommon on handguns, where space is at a premium. Most scopes are mounted with two rings, one in the front half of the scope and one on the back half, which provides additional strength and support. The heaviest recoiling firearms, such as Thompson Center Arms Contender pistols in heavy recoiling calibers, will use three rings for maximum support of the scope. Use of too few rings can result not only in the scope moving under recoil, but also excessive torque on the scope tube as the gun rolls up under recoil.
Scopes on heavy recoiling firearms and spring piston airguns (which have a heavy "reverse recoil" caused by the piston reaching the end of its travel) suffer from a condition called scope creep, where the inertia of the scope holds it still as the firearm recoils under it. Because of this, scope rings must be precisely fitted to the scope, and tightened very consistently to provide maximum hold without putting uneven stress on the body of the scope. Rings that are out of round, misaligned in the bases, or tightened unevenly can warp or crush the body of the scope.
[edit] Uses
Telescopic sights have both advantages and disadvantages relative to iron sights. Standard doctrine with iron sights is to focus the eye on the front sight and align it with the resulting blur of the target and the rear sight; most shooters have difficulty doing this, as the eye tends to be drawn to the target, blurring both sights. Telescopic sights allow the user to focus on both the crosshair and the target at the same time, as the lenses project the crosshair into the distance (50 to 100 yards or meters for rimfire scopes, 150 or more for centerfire calibers). This, combined with telescopic magnification, clarifies the target and makes the target stand out against the background. The main disadvantage of magnification is that the area to either side of the target is obscured by the tube of the sight. The higher the magnification, the narrower the field of view in the sight, and the more area that is hidden. Rapid fire target shooters use red dot sights, which have no magnification; this gives them the best field of view while maintaining the single focal plane of a telescopic sight. Telescopic sights are expensive, and require additional training to align. Sight alignment with telescopic sights is a matter of making the field of vision circular to minimize parallax error.
[edit] Military use
Until the 1990s, military use of telescopic sights was restricted to snipers because of the fragility and expense of optical components, though they had been used as early as the American Civil War on rifles, and even earlier for other jobs. The glass lenses are prone to breakage, and environmental conditions such as condensation, precipitation, dirt, and mud obscure external lenses. The scope tube also adds significant bulk to the rifle. Snipers generally used moderate to high magnification scopes with special reticles that allow them to estimate range to the target.
Telescopic sights provide some tactical disadvantages. Snipers rely on stealth and concealment to get close to their target. A telescopic sight can hinder this because sunlight may reflect from the lens and a sniper raising his head to use a telescopic sight might reveal his position. The famous Finnish sniper Simo Häyhä preferred to use iron sights rather than telescopic sights to present less of a target. Harsh climate can also cause problems for telescopic sights as they are less rugged than iron sights. Many Finnish snipers in WWII used iron sights heavily because telescopic sights did not cope with very cold Finnish winters.
The market for military telescopic sights intended for military long-range shooting is highly competitive. Several high end optics manufacturers are constantly adapting and improving their telescopic sights to fulfil specific demands of military organizations. Two European companies that are active this field are Schmidt & Bender and Zeiss.
The Israeli military began widespread use of telescopic sights by ordinary infantrymen to increase hit probability (especially in dim light) and extend effective range of standard issue infantry rifles. Palestinian militants in the al Aqsa Intifada likewise found that adding an inexpensive scope to an AK-47 increased its effectiveness.
Today, several militaries issue telescopic sights to their infantry, usually compact, low-magnification sights suitable for snap-shooting, like red dot sights. American GIs in Iraq frequently purchase their own and carry them from home. The British army fields the SA80 rifle with a 4× optical sight as standard issue to allow average shooters to fire more accurately. The Canadian Forces standard C7 rifle has a 3.4× optical sight. Both Austria and Australia field variants of the Austrian Steyr AUG which has built an integral 1.5x optical sight since its deployment in the late 1970s, and the German Army's G36 has a more or less built in 3.0x optical sight, as well as a red dot sight with no magnification.
