Telescopic sight

[1] 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.

[1] "This is that admirable secret, which, as all other things, appeared when it pleased the All Disposer, at whose direction a spider's line drawn in an opened case could first give me by its perfect apparition, when I was with two convexes trying experiments about the sun, the unexpected knowledge...if I .... placed a thread where that glass [the eyepiece] would best discern it, and then joining both glasses, and fitting their distance for any object, I should see this at any part that I did direct it to ..."— William Gascoigne[1]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 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.

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.

Confusingly, some older telescopic sights, mainly of German or other European manufacture, have a different classification where the second part of the designation refers to light-gathering power.

As low magnifications are mostly used in close- and medium ranges, LPVOs typically have no parallax compensation (though a few rare models do) 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.

Determined by the optical properties of the lenses used and intended primary use of the telescopic sight, different coatings are preferred, to optimize light transmission dictated by the human eye luminous efficiency function variance.

[17] Maximal light transmission around wavelengths of 555 nm (green) is important for obtaining optimal photopic vision using the eye cone cells for observation in well-lit conditions.

The windage and elevation adjustment knobs (colloquially called "tracking turrets") often have internal ball detents to help accurately index their rotation, which provide a crisp tactile feedback corresponding to each graduation of turn, often accompanied by a soft but audible clicking sound.

The crosshair lines geometrically resemble the X- and Y-axis of the Cartesian coordinate system, which the shooter can use as a simple reference for rough horizontal and vertical calibrations.

There can be additional features such as enlarged center dot (frequently also illuminated), concentric circle (solid or broken/dashed), chevron, stadia bars, or a combination of the above, that are added to a crosshair to help with easier aiming.

[citation needed] To allow methodological uniformity, accurate mental calculation and efficient communication between spotters and shooters in sniper teams, mil-based sights are typically matched by elevation/windage adjustments in 0.1 mil increments.

Some milling reticles have additional marking patterns in the bottom two quadrants, consisting of elaborate arrays of neatly spaced fine dots, "+" marks or hashed lines (usually at 0.2 mil or ½ MOA intervals), to provide accurate references for compensating bullet drops and wind drifts by simply aiming above (i.e. "hold [the aim] over" the target) and upwind of the target (i.e. deflection shooting, or "Kentucky windage").

Such aiming technique can quickly correct for ballistic deviations without needing to manually readjust the sight's zero, thus enabling the shooter to place rapid, reliably calibrated follow-up shots.

Normally these impact shifts are insignificant, but accuracy-oriented users, who wish to use their telescopic sight trouble-free at several magnification levels, often opt for FFP reticles.

The Trijicon Corporation, famous for their ACOG prism sights that are adopted by various assault infantry branches of the United States military, uses tritium in their combat and hunting-grade firearm optics.

Bullet drop compensation (BDC, sometimes referred alternatively as ballistic elevation) is a feature available on some telescopic sights, usually those used by more tactically oriented semi-automatic and assault rifles.

The feature provides pre-determined reference markings for various distances (referred to as "bullet drops") on the reticle or (much less commonly) on the elevation turret, which gives reasonably accurate estimations of potential gravitational deviation upon the bullet in flat-firing scenarios, so the shooter can proactively adjust their aim to compensate without needing to trial with missed shots or dealing with complex ballistic calculation.

The BORS module is in essence an electronic Bullet Drop Compensation (BDC) sensor/calculator package intended for long-range sniping out to 2,500 m (2,700 yd) for some telescopic sight models made by Leupold and Nightforce.

[35] The SAM (Shooter-supporting Attachment Module) measures and provides aiming and ballistic relevant data and displays this to the user in the ocular of the Zeiss 6–24×72 telescopic sight it is developed for.

Some modern scopes have a transparent display built inside the eyepiece, which allows digital data from a microprocessor to be superimposed over the optical target image to create an augmented reality.

A totally different approach recently developed, which has been applied in the ELCAN DigitalHunter series and the ATN X-Sight series, essentially uses a video camera system to digitally capture, process and display a virtual reality image of the target into a small flat panel display built inside the eyepiece, often with additional built-in rangefinder, ballistic calculator, signal filters, memory card and/or wireless access smart device interface to create a "smart scope" that can store/share data with other mobile devices.

When mounting a telescopic sight, dovetail-interfaced scope rings can be slid onto the rail at any desired position, and friction-fastened via set screws, or clamped firm with screw-tightened plates called "grabbers".

Due to the relative ease of machining a reliably straight metal bar, dovetail rails pretty much eliminated the misalignment concerns of the screw-and-hole scope rings.

The more recent proprietary systems mainly offer aesthetic advantages for people who have problems with redundant drill holes in the sight in case it is used on different guns.

The heaviest-recoiling firearms, such as Thompson Center Arms Contender pistols in heavy-recoiling calibers, will use three rings for maximum support of the telescopic sight.

For maximum effective light-gathering and brightest image, the exit pupil should equal the diameter of the fully dilated iris of the human eye—about 7 mm, reducing with age.

Several high end optics manufacturers are constantly adapting and improving their telescopic sights to fulfill specific demands of military organizations.

The former Warsaw Pact members produce military telescopic sights for their designated marksmen and developed a range finding reticle based on the height of an average human.

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.