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Areticle orreticule,[1][2] also known as agraticule orcrosshair, is a pattern of fine lines or markings built into theeyepiece of anoptical device such as atelescopic sight,spotting scope,theodolite,optical microscope or thescreen of anoscilloscope, to providemeasurement references duringvisual inspections. Today, engraved lines or embedded fibers may be replaced by adigital image superimposed on a screen or eyepiece. Both terms may be used to describe any set of patterns used for aiding visualmeasurements andcalibrations, but in modern usereticle is most commonly used forweaponsights, whilegraticule is more widely used for non-weaponmeasuring instruments such asoscilloscope display,astronomic telescopes,microscopes andslides,surveying instruments and other similar devices.
There are many variations of reticle pattern; this article concerns itself mainly with the most rudimentary reticle: thecrosshair. Crosshairs are typically represented as a pair ofperpendicularly intersecting lines in the shape of a cross, "+", though many variations of additional features exist including dots,posts,concentriccircles/horseshoes,chevrons,graduated markings, or a combination of above. Most commonly associated with telescopic sights for aimingfirearms, crosshairs are also common in optical instruments used forastronomy andsurveying, and are also popular ingraphical user interfaces as a precisionpointer. The reticle is said to have been invented byRobert Hooke, and dates to the 17th century.[3] Another candidate as inventor is the amateur astronomerWilliam Gascoigne, who predated Hooke.[4]
The termreticle comes from the Latinreticulum, meaning small net.
Telescopic sights for firearms, generally just calledscopes, 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.
While the traditional thin crossing lines are the original and still the most familiar cross-hair 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 cross-hair in modern scopes are variants on theduplex cross-hair, 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. The thin bars in a duplex reticle may also be designed to be used as a measure. Called a 30/30 reticle, the thin bars on such a reticle span 30 minutes of arc (0.5º), which is approximately equal to 30 inches at 100 yards or 90 centimeters at 100 meters. This enables an experienced shooter to deduce, on the basis of the known size of an object in view, (as opposed to guess or estimate) the range within an acceptable error limit.
Originally crosshairs were constructed out of hair or spiderweb, these materials being sufficiently thin and strong.[5] 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 flattening 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.
The first suggestion for etched glass reticles was made byPhilippe de La Hire in 1700.[6] His method was based on engraving the lines on a glass plate with adiamond point. Many modern crosshairs are actuallyetched onto a thin plate ofglass, which allows a far greater latitude in shapes. Etched glass reticles can havefloating elements, which do not cross the reticle; circles and dots are common, and some types of glass reticles have complex sections designed for use inrange estimation and bullet drop and drift compensation (seeexternal ballistics). A potential disadvantage of glass reticles is that the surface of the glass reflects some light (about 4% per surface on uncoated glass[7]) lessening transmission through the scope, although this light loss is near zero if the glass is multicoated (coating being the norm for all modern high quality optical products).
Reticles may be illuminated, either by a plastic orfiber opticlight pipe collecting ambient light or, in low light conditions, by abattery poweredLED. Some sights also use the radioactive decay oftritium for illumination that can work for 11 years without using a battery, used in the BritishSUSAT sight for theSA80 (L85) assault rifle and in the AmericanACOG (Advanced Combat Optical Gunsight). Red is the most common color used, as it is the least destructive to the shooter'snight vision, but some products use green or yellow illumination, either as a single colour or changeable via user selection.
Another term for reticle isgraticule, which is frequently encountered in British and British military technical manuals. It came into common use duringWorld War I.[8]
The reticle may be located at the front or rear focal plane (First Focal Plane (FFP) or Second Focal Plane (SFP))[9] of the telescopic sight. On fixed power telescopic sights there is no significant difference, but on variable power telescopic sights 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 focal plane reticles are slightly more durable, but most American users prefer that the reticle remains constant as the image changes size, so nearly all modern American variable power telescopic sights are rear focal plane designs.[citation needed] American and European high end optics manufacturers often leave the customer the choice between a FFP or SFP mounted reticle.
Collimated reticles are produced by non-magnifying optical devices such asreflector sights (often calledreflex sights) that give the viewer an image of the reticle superimposed over the field of view, and blindcollimator sights that are used with both eyes. Collimated reticles are created usingrefractive orreflectiveoptical collimators to generate acollimated image of an illuminated or reflective reticle. These types of sights are used on surveying/triangulating equipment, to aid celestial telescope aiming, and assights onfirearms. Historically they were used on larger military weapon systems that could supply an electrical source to illuminate them and where the operator needed a wide field of view to track and range a moving target visually (i.e. weapons from the prelaser/radar/computer era). More recently sights using low power consumption durablelight emitting diodes as the reticle (calledred dot sights) have become common on small arms with versions like theAimpoint CompM2 being widely fielded by the U.S. Military.
Holographic weapon sights use aholographic image of a reticle at finite set range built into the viewing window and acollimatedlaser diode to illuminate it. An advantage to holographic sights is that they eliminate a type ofparallax problem found in some optical collimator based sights (such as thered dot sight) where the spherical mirror used inducesspherical aberration that can cause the reticle to skew off the sight'soptical axis. The use of a hologram also eliminates the need for image dimming narrow band reflective coatings and allows for reticles of almost any shape ormil size. A downside to the holographic weapon sight can be the weight and shorter battery life. As with red dot sights, holographic weapon sights have also become common on small arms with versions like theEotech 512.A65 and similar models fielded by the U.S. Military[10] and various law enforcement agencies.
In older instruments, reticle crosshairs and stadia marks were made using threads taken from thecocoon of thebrown recluse spider. This very fine, strong spider silk makes for an excellent crosshair.[11][12]
In surveying, reticles are designed for specific uses.Levels andtheodolites would have slightly different reticles. However, both may have features such asstadia marks to allow distance measurements.
For astronomical uses, reticles could be simple crosshair designs or more elaborate designs for special purposes. Telescopes used forpolar alignment could have a reticle that indicates the position ofPolaris relative to the north celestial pole. Telescopes that are used for very precise measurements would have afilar micrometer as a reticle; this could be adjusted by the operator to measure angular distances between stars.
For aiming telescopes,reflex sights are popular, often in conjunction with a small telescope with a crosshair reticle. They make aiming the telescope at anastronomical object easier.
The constellationReticulum was designated to recognize the reticle and its contributions to astronomy.
