Reticle



A crosshair or reticle is a shape superimposed on an image that is used for precise alignment of a device, most notably that of a telescopic sight. Crosshairs are most commonly represented as intersecting lines in a "+" shape, though many variations exist, including dots, posts, circles, scales, chevrons, or a combination of these. Most commonly associated with telescopic sights for aiming firearms, crosshairs are also common in optical instruments used for astronomy and surveying, and are also popular in graphical user interfaces as a precision pointer. The crosshair was invented by Robert Hooke, and dates to the 17th century.

The word reticle is from the Latin meaning "net", a network or grid of lines displayed in an optical instrument. The minimum reticle consists of simple "cross-hairs", as described above.

Firearms
Telescopic sights for firearms, generally just called scopes, are probably the device most often associated with crosshairs. Motion pictures and the media often use a view through crosshairs as a dramatic device, which has given crosshairs wide cultural exposure.

Reticle shape
While the traditional thin crossing lines are the original and still the most familiar crosshair shape, they are really best suited for precision aiming at high contrast targets, as the thin lines are easily lost in complex backgrounds, such as those encountered while hunting. Thicker bars are much easier to discern against a complex background, but lack the precision of thin bars. The most popular types of crosshair in modern scopes are variants on the duplex crosshair, with bars that are thick on the perimeter and thin out in the middle. The thick bars allow the eye to quickly locate the center of the reticle, and the thin lines in the center allow for precision aiming.

Wire crosshairs
The original crosshairs in fact used hair or spiderweb, as it was thin and strong. Many modern scopes use wire crosshairs, which can be flattened to various degrees to change the width. These wires are usually silver in color, but appear black when backlit by the image passing through the scope's optics. Wire reticles are by nature fairly simple, as they require lines that pass all the way across the reticle, and the shapes are limited to the variations in thickness allowed by flatting the wire; duplex crosshairs, and crosshairs with dots are possible, and multiple horizontal or vertical lines may be used. The advantage of wire crosshairs is that they are fairly tough and durable, and provide no obstruction to light passing through the scope.

Etched reticles
The first suggestion for etched glass reticles was made by Philippe de La Hire in 1700. His method was based on engraving the lines on a glass plate with a diamond point. Many modern crosshairs are actually etched onto a thin plate of glass, which allows a far greater latitude in shapes. Etched glass reticles can have floating elements, which do not cross the reticle; circles and dots are common, and some types of glass reticles have complex sections designed for use in range estimation and bullet drop and drift compensation (see external ballistics). The disadvantage of glass reticles is that they are less durable than wire crosshairs, and the surface of the glass reflects some light, lessening transmission through the scope.

Illuminated reticles
Either type of reticle can be illuminated for use in low light. Illumination is usually provided by a battery powered LED, though a radioactive element containing Tritium may be used for autonomous illumination for approximately 11 years without using a battery, via radioactive decay, like in the British SUSAT sight for the SA80 (L85) assault rifle, or the American ACOG (Advanced Combat Optical Gunsight). The light is projected forward through the scope, and reflects off the back surface of the reticle. Red is the most common color used, as it is the least destructive to the shooter's night vision, but some products use green or yellow dots, either as a single colour or changeable via user selection.

Focal plane
The reticle may be located at the front or rear focal plane of the scope. On fixed power scopes there is no significant difference, but on variable power scopes the front plane reticle remains at a constant size compared to the target, while rear plane reticles remain a constant size to the user as the target image grows and shrinks. Front plane reticles are slightly more durable, but most American users prefer that the reticle remain constant as the image changes size, so nearly all modern variable power scopes are rear focal plane designs. European high end optics manufacturers often leave the customer the choice between a front or rear focal plane reticle.

Collimated reticles
Collimated reticles give the viewer an image of the reticle superimposed over the field of view. Collimated reticles are created using refractive or reflective optical collimators to generate a collimated image of an illuminated or reflective reticle. The most common implementations use beam splitters to allow the viewer to see the field of view and a reflection of the projected reticle simultaneously. These devices, often called sights, are used on surveying/triangulating equipment, to aid celestial telescope aiming, and as sights on firearms. Reflex sights such as the Aimpoint_CompM2 are widely fielded by the U.S. Military. Historically they were used on larger military weapon systems where the operator needed a wide field of view to track and range a moving target visually (i.e. weapons from the pre laser/radar/computer era).

Holographic reticles
Bushnell has recently introduced the Holosight, which is a sight that uses a holographic reticle and a laser diode for illumination. The Holosight is able to render a three-dimensional reticle image. The downside to the Holosight is the cost and weight; it is more expensive and significantly heavier than other reflex sights.

Surveying and astronomy
In older instruments, reticle crosshairs and stadia marks were made using threads taken from the cocoon of the brown recluse spider. This very fine, strong spider silk makes for an excellent crosshair.

Surveying
In surveying, reticles are designed for specific uses. Levels and theodolites would have slightly different reticles. However, both may have features such as stadia marks to allow distance measurements.

Astronomy
For astronomical uses, reticles could be simple crosshair designs or more elaborate designs for special purposes. Telescopes used for polar alignment could have a reticle that indicates the position of Polaris relative to the north celestial pole. Telescopes that are used for very precise measurements would have a filar micrometer as a reticle; this could be adjusted by the operator to measure angular distances between stars.

For aiming telescopes, sights are popular, often in conjunction with a small telescope with a crosshair reticle. The reflex sight, such as the Telrad, make aiming the telescope on a Astronomical object or a region of the sky quite easy.

The constellation Reticulum was designated to recognize the reticle and its contributions to astronomy.