Red dot sight

A red dot sight is a common classification for a type of non-magnifying reflector (or reflex) sight for firearms that gives the user an aimpoint in the form of an illuminated red dot. A standard design uses a red light-emitting diode (LED) at the focus of collimating optics which generates a dot style illuminated reticle that stays in alignment with the weapon the sight is attached to regardless of eye position (nearly parallax free). They are considered to be fast acquisition and easy to use gun sights for target shooting, hunting, and in police and military applications.

Description
The typical configuration for a red dot sight is a tilted spherical mirror reflector with a red light-emitting diode (LED) at its off axis focus. The mirror has a partially silvered multilayer dielectric dichroic coating designed to reflect just the red spectrum allowing it to pass through most other light. The LED used is usually deep red 670 nanometre wavelength since they are very bright, are high contrast against a green scene, and work well with a dichroic coating since they are near one end of the visible spectrum. The size of the dot generated by the LED is controlled by an aperture hole in front of it made from metal or coated glass. The LED as a reticle is an innovation that greatly improves the reliability and general usefulness of the sight. There is no need for other optical elements to focus light behind a reticle. And the LED itself is solid state and consumes very little power, allowing battery powered sights to run for hundreds and even tens of thousands hours. Using a "dot" shaped reticle also greatly simplifies the sight since the small diameter image does not require a sophisticated optical reflector to focus it. More complex reticle patterns such as cross hairs or concentric circles can be used but need more complex aberration free optics.

Like other reflector sights, the collimated image of the red dot is only truly parallax free at infinity, with an error circle equal to the diameter of the collimating optics for any target at a finite distance. This is compensated for by keeping the dot in the middle of the optical window (sighting down the sight's optical axis). Some manufacturers modify the focus of the LED/optical collimator combination, making models with the optical collimator set to focus the dot at a finite distance. These have a maximum amount of parallax due to eye movement, equal to the size of the optical window, at close range, diminishing to a minimal amount at the set distance (somewhere around a desired target range of 25-50 yards).

Sights may also use a more sophisticated optical system that compensates for off axis spherical aberration, an error that can cause the dot position to diverge off the sight's optical axis with change in eye position. The optics used is a type of mangin mirror system, consisting of a meniscus lens corrector element combined with the semi-reflective mirror, sometimes referred to in advertising as a "two lens" or "double lens" system. Although these are referred to as "parallax free" sights, the system only keeps the aiming dot in alignment with the sight itself and does not compensate the inherent parallax errors induced by a collimated sight.

Red dot sights generally fall into two categories, "tube" or "open" designs. Tube sights look similar to a standard telescopic sight, with a cylindrical tube containing the optics. Tube sights offer the option of fitted dust covers and the ability to add filters, such as polarizing or haze filters, and glare reducing sunshades. Since a reflector sight only really needs a single optical surface, the "reflector", the tube is not needed. This allows for non-tubed open sights that consist of a flat base, with a single loop of material to support the reflective optics.

Most red dot sights have either active or passive adjustments for the dot brightness, allowing a very bright dot for high visibility in bright conditions, and a very dim dot to prevent loss of night vision in low light conditions.

History
The idea of attaching a reflector (or reflex) sight to a firearm has been around since the sight's invention in 1900. Many different types of reflector sights specifically designed for firearms have been marketed over the past half century (see Reflector sight: History), some lit by batteries and some lit by ambient light. One, the Weaver Qwik-Point, even presented the viewer with a red aiming dot due to the red plastic “light pipe” used to collect ambient light. All had the reticle illumination drawback common with reflector sights small enough for a firearm: proper ambient lighting could not be depended on and incandescent light bulbs could drain a battery in a few hours.

In 1975 the Swedish optics company Aimpoint AB marketed the first "electronic" red dot sight combining a reflecting curved mirror and a light-emitting diode, based on a design by Helsingborg engineer John Arne Ingemund Ekstrand. The sight was called the "Aimpoint Electronic" and had a closed tube design that could be mounted similar to a telescopic sight. The LED could run for 1500 to 3000 hours on mercury batteries. Other manufacturers soon followed with over a dozen offering models today. Newer generation red dot sights were produced with lower power consumption LEDs and power saving electronics, allowing them to run for years without being turned off. In 2000 the US military introduced a red dot sight into field use, the Aimpoint CompM2, designated the “M68 Close Combat Optic”.

Reticles
Red dot sight reticles are measured in minutes of angle, or "MOA". MOA is a convenient measure for shooters using English units, since 1 MOA subtends approximately 1.0472 inches at a distance of 100 yards (91.44 m). This is generally rounded to 1 inch at 100 yards, which makes MOA a handy unit to use in ballistics. One of the most common reticles used in red dot sights is a small dot, covering 5 MOA (1.5 mrad). The 5 MOA (1.5 mrad) dot is small enough not to obscure most targets, and large enough to quickly acquire a proper "sight picture". For many types of action shooting, a larger dot is preferred; 7 (2.0 mrad), 10 (2.9 mrad), 15 (4.4 mrad) or even 20 MOA (5.8 mrad) dots or rings are used; often these will be combined with horizontal and/or vertical lines to provide a level reference.

Uses
Red dot sights place the target and the reticle on nearly the same optical plane, allowing a single point of focus. This makes them fast acquisition and easy to use sights, allowing the user to keep their attention on the field of view in front of them. They are common in speed shooting sports such as IPSC. Military units and police forces have also adopted them. Red dot sights are also popular among paintball and airsoft players for similar reasons.

Because there is no magnification, the shooter need not worry about parallax or eye relief. The long eye relief makes red dot sights appropriate for firearms with heavy recoil that might drive a conventional short eye relief telescopic sight into the shooter's eye. Since dot sights can be mounted at any distance from the shooter's eye with no issues of focus, military rifle mounts usually place the sight in any mechanically-convenient mounting position, such as the carrying handle of the M16 rifle, or on a rail system (typically a Picatinny rail) on top of the rifle. This leaves plenty of room for night vision equipment to be used with the red dot sight.