Crosshair - AbsoluteAstronomy.com

A reticle is a net of fine lines or fibers in the eyepiece of a sighting device, such as a telescope, a telescopic sight

Telescopic sight

A telescopic sight, commonly called a scope, is a sighting device that is based on an optical refracting telescope. They are equipped with some form of graphic image pattern mounted in an optically appropriate position in their optical system to give an accurate aiming point...

, a microscope, or the screen of an oscilloscope. The word reticle comes from the Latin "reticulum," meaning "net." Today, engraved lines or embedded fibers may be replaced by a computer-generated image superimposed on a screen or eyepiece. The term graticule is the synonymous term from French, it comes from the Latin craticula for gridiron

Gridiron (cooking)

A gridiron is a metal grate with parallel bars typically used for grilling meat, fish, vegetables, or combinations of such foods. It may also be two such grids, hinged to fold together, to securely hold food while grilling over an open flame.-Development:...

. Both may be used to describe any set of lines used for optical measurement, but in modern use the term reticle is most commonly used for gunsights and such, while graticule is more widely used for the covers of oscilloscope

Oscilloscope

An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...

s and similar roles.

There are many variations of reticles; this article concerns itself mainly with a simple reticle: crosshairs. Crosshairs are most commonly represented as intersecting lines in the shape of a cross, "+", though many variations exist, including dots, posts

Vertical bar

The vertical bar is a character with various uses in mathematics, where it can be used to represent absolute value, among others; in computing and programming and in general typography, as a divider not unlike the interpunct...

, circles

Concentric

Concentric objects share the same center, axis or origin with one inside the other. Circles, tubes, cylindrical shafts, disks, and spheres may be concentric to one another...

, scales, chevrons, or a combination of these. Most commonly associated with telescopic sights for aiming firearm

Firearm

A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

s, crosshairs are also common in optical instruments used for astronomy and surveying, and are also popular in graphical user interface

Graphical user interface

In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...

s as a precision pointer

Cursor (computers)

In computing, a cursor is an indicator used to show the position on a computer monitor or other display device that will respond to input from a text input or pointing device. The flashing text cursor may be referred to as a caret in some cases...

. The reticle is said to have been invented by Robert Hooke

Robert Hooke

Robert Hooke FRS was an English natural philosopher, architect and polymath.His adult life comprised three distinct periods: as a scientific inquirer lacking money; achieving great wealth and standing through his reputation for hard work and scrupulous honesty following the great fire of 1666, but...

, and dates to the 17th century.

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 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 the duplex 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 inches at 100 yards when the scope's power is at 4x. This enables an experienced shooter to deduce (as opposed to guess or estimate) the range within an acceptable error limit.

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 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.

Etched reticles

The first suggestion for etched glass reticles was made by Philippe de La Hire

Philippe de La Hire

Philippe de La Hire was a French mathematician and astronomer. According to Bernard le Bovier de Fontenelle he was an "academy unto himself"....

in 1700. His method was based on engraving the lines on a glass plate with a diamond

Diamond

In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

point. Many modern crosshairs are actually etched onto a thin plate of glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

, 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

External ballistics

External ballistics is the part of the science of ballistics that deals with the behaviour of a non-powered projectile in flight. External ballistics is frequently associated with firearms, and deals with the behaviour of the bullet after it exits the barrel and before it hits the target.-Forces...

). A potential disadvantage of glass reticles is that they are less durable than wire crosshairs, and the surface of the glass reflects some light (about 4% per surface on uncoated glass) 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).

Illuminated reticles

Reticles may be illuminated, either by a plastic or fiber optic light pipe collecting ambient light or, in low light conditions, by a battery

Battery (electricity)

An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

powered LED

LEd

LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....

. Some sights also use the radioactive decay of tritium

Tritium

Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

for illumination that can work for 11 years without using a battery, used in the British SUSAT

SUSAT

The Sight Unit Small Arms, Trilux, or SUSAT, is a 4× telescopic sight with tritium-powered illumination, utilised at dusk or dawn. The full name of the current model is the SUSAT L9A1. The sight is not designed as a sniper sight, but is rather intended to be mounted on a variety of rifles and to...

sight for the SA80

SA80

The SA80 is a British family of 5.56mm small arms. It is a selective fire, gas-operated assault rifle. SA80 prototypes were trialled in 1976 and production was completed in 1994....

(L85) assault rifle and in the American ACOG (Advanced Combat Optical Gunsight)

Advanced Combat Optical Gunsight

Advanced Combat Optical Gunsights are a series of telescopic sights manufactured by Trijicon. The ACOG is designed to be used on the M16 rifle and M4 carbine, but Trijicon has developed ACOG accessories for certain other firearms...

. Red is the most common color used, as it is the least destructive to the shooter's night vision

Night vision

Night vision is the ability to see in low light conditions. Whether by biological or technological means, night vision is made possible by a combination of two approaches: sufficient spectral range, and sufficient intensity range...

, but some products use green or yellow illumination, either as a single colour or changeable via user selection.

Graticule

A graticule is another term for reticle, frequently encountered in British and British military technical manuals, and came into common use during World War One.

Reticle focal plane

The reticle may be located at the front or rear focal plane (First Focal Plane (FFP) or Second Focal Plane (SFP)) 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. European high end optics manufacturers often leave the customer the choice between a FFP or SFP mounted reticle.

Collimated reticles

Collimated reticles are produced by non-magnifying optical devices such as reflector sight

Reflector sight

A reflector or reflex sight is a generally non-magnifying optical device that allows the user to look through a partially reflecting glass element and see an illuminated projection of an aiming point or some other image superimposed on the field of view...

s (often called reflex sights) that give the viewer an image of the reticle superimposed over the field of view, and blind collimator sight

Collimator sight

A collimator sight is a type of optical sight that allows the user looking into it to see an illuminated aiming point aligned with the device the sight is attached to regardless of eye position...

s that are used with both eyes. Collimated reticles are created using refractive

Refraction

Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

or reflective

Reflection (physics)

Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

optical collimators to generate a collimated

Collimated light

Collimated light is light whose rays are parallel, and therefore will spread slowly as it propagates. The word is related to "collinear" and implies light that does not disperse with distance , or that will disperse minimally...

image of an illuminated or reflective reticle. These types if sights are used on surveying/triangulating equipment, to aid celestial telescope aiming, and as sights on firearm

Firearm

s. 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 pre laser

Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

/radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

/computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

era). More recently sights using low power consumption durable light emitting diodes as the reticle (called red dot sight
Red dot sight
A red dot sight is a common classification for a type of non-magnifying reflector sight for firearms that gives the user an aimpoint in the form of an illuminated red dot...

s) have become common on small arms with versions like the Aimpoint CompM2

Aimpoint CompM2

The CompM2 is a battery powered non-magnifying red dot type of reflex sight for firearms manufactured by Aimpoint AB. In the U.S. military, it is designated M68 Close Combat Optic and is also known as M68 Aimpoint and is designed to meet United States military standards...

being widely fielded by the U.S. Military.

Holographic reticles

Holographic weapon sight

A holographic weapon sight or holographic diffraction sight is a non-magnifying gun sight that allows the user to look through a glass optical window and see a reticle image superimposed at a distance on the field of view...

s use a holographic image of a reticle at infinity built into the viewing window and a collimated

Collimated light

laser diode

Laser diode

The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...

to illuminate it. The downside to the holographic weapon sight is the cost, weight and shorter battery life.

Surveying and astronomy

In older instruments, reticle crosshairs and stadia marks were made using threads taken from the cocoon of the brown recluse spider

Brown recluse spider

The brown recluse spider or violin spider, Loxosceles reclusa, is a member of the family Sicariidae . The spider has a venomous bite....

. This very fine, strong spider silk makes for an excellent crosshair.

Surveying

In surveying, reticles are designed for specific uses. Level

Dumpy level

A dumpy level, builder's auto level, leveling instrument, or automatic level is an optical instrument used to establish or check points in the same horizontal plane...

s and theodolite

Theodolite

A theodolite is a precision instrument for measuring angles in the horizontal and vertical planes. Theodolites are mainly used for surveying applications, and have been adapted for specialized purposes in fields like metrology and rocket launch technology...

s would have slightly different reticles. However, both may have features such as stadia mark

Stadia mark

Stadia marks, also called stadia lines or stadia hairs, are crosshairs on the reticle of a theodolite or other surveying instrument that allow stadiametric rangefinding.-Etymology:...

s 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

Polar Alignment

Polar alignment is the act of aligning the rotational axis of a telescope's equatorial mount in parallel with that of the Earth. There are various ways to achieve this.- Alignment Methods :...

could have a reticle that indicates the position of Polaris

Polaris

Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....

relative to the north celestial pole. Telescopes that are used for very precise measurements would have a filar micrometer

Filar micrometer

A filar micrometer is a device used in astronomical telescopes for astrometry measurements. The word filar derives from Latin filum, a thread...

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. The reflex sight, such as the Telrad, make aiming the telescope on a Astronomical object

Astronomical object

Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...

or a region of the sky quite easy.

The constellation Reticulum

Reticulum

Reticulum is a small, faint constellation in the southern sky. Its name is Latin for a small net, or reticle—a net of crosshairs at the focus of a telescope eyepiece that is used to measure star positions...

was designated to recognize the reticle and its contributions to astronomy.

External links

Chuck Hawks' article on firearm scope reticles
An explanation of firearm rangefinding reticles from a maker of ballistics software
Article on Cabela's Alaskan Guide scopes, which use a first focal plane reticle
User Guide for Mil-Dot Equipped Optics, Remington Military Products Division

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.