Telescopic sight


A telescopic sight, commonly called a scope informally, is an optical sighting device based on a refracting telescope. Sights are equipped with a referencing pattern mounted in a focally appropriate position in its optical system to provide an accurate point of aim. Telescopic sights are classified in terms of the optical magnification and the objective lens diameter.
The first experiments directed to give shooters optical aiming aids go back to the early 17th century. For centuries, different optical aiming aids and primitive predecessors of telescopic sights were created that had practical or performance limitations. Most early telescopic sights were fixed-power and were in essence specially designed viewing telescopes. Telescopic sights with variable magnifications appeared later, and were varied by manually adjusting a zoom mechanism behind the erector lenses. Other types of scopes include prism sights and low-power variable optics.
Telescopic sights have both advantages and disadvantages relative to iron sights. They are built to various specifications and use a variety of adjustment controls, reticles, features, technologies, and mounting systems. These specifications are often selected based on the intended use of the sight.

Description

A telescopic sight is an optical sighting device based on a refracting telescope. It is equipped with some form of a referencing pattern – known as a reticle – mounted in a focally appropriate position in its optical system to provide an accurate point of aim. Telescopic sights are used with all types of systems that require magnification in addition to reliable visual aiming, as opposed to non-magnifying iron sights, reflector sights, holographic sights or laser sights, and are most commonly found on long-barrel firearms, particularly rifles, usually via a scope mount. Similar devices are also found on other platforms such as artillery, tanks and even aircraft. The optical components may be combined with optoelectronics to add night vision or smart device features.
Telescopic sights are classified in terms of the optical magnification and the objective lens diameter. For example, "10×50" would denote a fixed magnification factor of 10×, with a 50 mm objective lens. In general terms, larger objective lens diameters, due to their ability to gather a higher luminous flux, provide a larger exit pupil and hence provide a brighter image at the eyepiece.

History

The first experiments directed to give shooters optical aiming aids go back to the early 17th century. For centuries, different optical aiming aids and primitive predecessors of telescopic sights were created that had practical or performance limitations.
In the late 1630s, English amateur astronomer William Gascoigne was experimenting with a Keplerian telescope and left it with the case open. Later he found that a spider had spun its web inside the case, and when he looked through the telescope he found that the web was in focus with distant objects. Gascoigne realised that he could use this principle to make a telescopic sight for use in his astronomical observations.
In 1776, Charles Willson Peale collaborated with David Rittenhouse to mount a telescope to a rifle as a sighting aid, but was unable to mount it sufficiently far forward to prevent the eyepiece impacting with the operator's eye during recoil. In the same year, James Lind and Captain Alexander Blair described a gun which included a telescopic sight.
The first rifle sight was created in 1835–1840. In the book The Improved American Rifle, written in 1844, British-American civil engineer John R. Chapman described a sight made by gunsmith Morgan James of Utica, New York. Chapman worked with James on the concepts and design of the Chapman-James sight. In 1855, optician William Malcolm of Syracuse, New York began producing his own telescopic sight, used an original design incorporating achromatic lenses such as those used in telescopes, and improved the windage and elevation adjustments. These Malcolm sights were between 3× and 20× magnification. Malcolm's sights and those made by Vermont jeweler L. M. Amidon were the standard sharpshooter equipment during the American Civil War. Other telescopic sights of the same period were the Davidson and the Parker Hale.
An early practical refracting telescope based telescopic sight was built in 1880 by August Fiedler, forestry commissioner of German Prince Reuss. Later telescopic sights with extra long eye relief became available for use on handguns and scout rifles. A historical example of a long-eye relief telescopic sight is the German ZF41 which was used during World War II on Karabiner 98k rifles.
An early example of a man-portable sight for low visibility/night use is the Zielgerät 1229, also known by its code name Vampir. The ZG 1229 Vampir was a Generation 0 active infrared night vision device developed for the Wehrmacht for the StG 44 assault rifle, intended primarily for night use. The issuing of the ZG 1229 Vampir system to the military started in 1944 and it was used on a small scale in combat from February 1945 until the final stages of World War II.

Types

Most early telescopic sights were fixed-power and were in essence specially designed viewing telescopes. Telescopic sights with variable magnifications appeared later, and were varied by manually adjusting a zoom mechanism behind the erector lenses. Variable-power sights offer more flexibility when shooting at varying distances, target sizes and light conditions, and offer a relatively wide field of view at lower magnification settings.
The syntax for variable sights is the following: minimal magnification – maximum magnification × objective lens, for example "3-9×40" means a telescopic sight with variable magnification between 3× and 9×, and a 40 mm objective lens. The ratio between the maximum and minimum magnifications of a variable-power sight is known as its zoom ratio.
Confusingly, some older telescopic sights, have a different classification where the second part of the designation refers to light-gathering power. In these cases, a 4×81 sight would be presumed to have a brighter sight picture than a 2.5×70, but the objective lens diameter would not bear any direct relation to picture brightness, as brightness is affected also by the magnification factor. Typically, objective lenses on early sights are smaller than modern sights. In these examples, the 4×81 would have an objective 36 mm diameter and the 2.5×70 should be approximately 21 mm.

Prismatic telescopic sight

A relatively new type of telescopic sight, called prismatic telescopic sight, prismatic sight or prism scope, replaces the image-erecting relay lenses of a traditional telescope with a roof prism design commonly found in compact binoculars, monoculars and spotting scopes. The reticle is etched onto one of the prism's internal reflection surfaces, which allows an easy way to illuminate the reticle even when active illumination is turned off. Being optical telescopes, prism sights can focally compensate for a user's astigmatism.
Prismatic sights are lighter and more compact than conventional telescopic sights, but are mostly fixed-powered in the low magnification ranges, suitable for shooting at short/medium distances. One of the best known examples is the battle-proven Trijicon ACOG used by the USMC, US Army, and USSOCOM, although variable-magnification prism sights do also exist, such as the ELCAN Specter DR/TR series used by the Canadian Army.

Low-power variable optic

Variable-zoom telescopic sights in the low magnification range are known as low-power variable optics or LPVOs. These telescopic sights are often equipped with built-in [|reticle illumination] and can be dialed down to 1× magnification. As low magnifications are mostly used in close- and medium ranges, LPVOs typically have no [|parallax compensation] and have a completely cylindrical shape ahead of the eyepiece, since the image illuminance is often sufficient without needing an enlarged objective bell to enhance light-gathering. Most LPVOs have reticles mounted at the second focal plane, but recently first-focal plane LPVOs have become popular, especially those with high zoom ratios above 6×.
LPVOs are also informally referred to as "AR scopes" or "carbine scopes", due to the recently increasing popularity of modern sporting rifles and compact "tactical"-style semi-automatic rifles used among the law enforcement, home defense and practical shooting enthusiasts crowd.

Specifications

Optical parameters

Telescopic sights are usually designed for the specific application for which they are intended. Those different designs create certain optical parameters. Those parameters are:
  • Magnification – The ratio of the focal length of the objective divided by the focal length of the eyepiece gives the linear magnifying power of telescopes. A magnification factor of 10, for example, produces an image as if one were 10 times closer to the object. The amount of magnification depends upon the application the telescopic sight is designed for. Lower magnifications lead to less susceptibility to shaking. A larger magnification leads to a smaller field of view.
  • Objective lens 'diameter – The diameter of the objective lens determines how much light can be gathered to form an image. It is usually expressed in millimeters.
  • Field of view – The field of view of a telescopic sight is determined by its optical design. It is usually notated in a linear value, such as how many meters in width will be seen at, or in an angular value of how many degrees can be viewed.
  • Exit pupil – Telescopic sights concentrate the light gathered by the objective into a beam, the exit pupil, whose diameter is the objective diameter divided by the magnifying power. For maximum effective light-gathering and brightest image, the exit pupil should equal the diameter of the fully dilated iris – for a youthful dark-adapted human eye about 7 mm, reducing with age. If the cone of light streaming out of the eyepiece is larger than the pupil it is going into, any light shining outside the pupil is considered "wasted" in terms of providing visual information.
  • Eye relief' – Eye relief is the distance from the rear eyepiece lens to the exit pupil or eye point. It is the optimal distance the observer must position their eye behind the eyepiece to see a non-vignetted image. The longer the focal length of the eyepiece, the greater the eye relief. Typical telescopic sights may have eye relief ranging from to over, but telescopic sights intended for scout rifles or handguns need much longer eye relief to present a non-vignetted image. Telescopic sights with relatively long eye relief are favorable to avoid periorbital lacerations and eye injuries caused by recoil-induced collision with the metal eyepiece, especially in instances where it is difficult to keep the stock steady. Eye relief is important for eyeglasses wearers, as the presence of an eyeglass in front of the eye shortens the available physical space before colliding with the eyepiece, so a longer eye relief is necessitated.