Telescopic sight
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A telescopic sight, commonly referred to as 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.
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" at 100 yards, often rounded to 1" 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.
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.Military use
Until the 1990s, military use of telescopic sights was restricted to snipers because of the fragility 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, and a telescopic sight can hinder this. 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.
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 fielded 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 3.7× optical sight.
External links
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