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A crosshair or reticle is a shape superimposed on an image that is used for precise alignment of a device. Crosshairs are most commonly a "+" shape, though many variations exist, including dots, posts, circles, and chevrons. 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.

Contents 1 Use in firearms 1.1 Reticle shape 1.2 Wire crosshairs 1.3 Etched reticles 1.4 Illuminated reticles 1.5 Focal plane 1.6 Collimated reticles 1.7 Holographic reticles 2 Use in Surveying and Astronomy 3 References 4 External Links

[edit] Use in 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.

[edit] 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.

[edit] Wire crosshairs

The original crosshairs in fact used hair, 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.

[edit] Etched reticles

The first suggestion for etched glass reticles was made by Philippe de La Hire in 1700.^[1] 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.

[edit] Illuminated reticles

Either type of reticle can be illuminated for use in low light. Illumination is usually provided by a battery powered LED, though other sources can be used. 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.

[edit] 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 users prefer that the reticle remain constant as the image changes size, so nearly all modern variable power scopes are rear focal plane designs.

[edit] Collimated reticles

Also known as a reflex sight. Red dot sights use a significantly different method of operation than normal telescopic sights. The red dot sights rarely provide any magnification, but are instead designed for the widest practical field of view, and high speed aiming. The red dot sight uses a partially reflecting concave surface that acts as a reflective but not a refractive lens. The image of the target passes through relatively undistorted, while the back surface reflects a collimated image of the reticle back to the user. As the name implies, the most common reticle is a simple red dot, but other colors and shapes are possible by passing the light through an etched glass reticle. Some models even have a dial containing multiple reticles etched at different point, allowing the user to change the shape of the reticle by rotating the dial.

[edit] Holographic reticles

Bushnell has recently introduced the Holosight, which is similar to a red dot sight, but using a holographic reticle and a laser diode for illumination. The Holosight is able to render a 3-D reticle image. The downside to the Holosight is the cost and weight; it is more expensive and significantly heavier than most red dot sights, which are its nearest competition.

[edit] Use in Surveying and Astronomy

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.

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, reflex sights are popular, often in conjunction with a small telescope with a crosshair reticle. The reflex sight, such as the Telrad,^[2] make aiming the telescope on a celestial object or a region of the sky quite easy.

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.^[3][4]

[edit] References

1. ^ Maurice Daumas, Scientific Instruments of the Seventeenth and Eighteenth Centuries and Their Makers, Portman Books, London 1989 ISBN 978-0713407273
2. ^ Telrad information including history of the device.
3. ^ Raymond Davis, Francis Foote, Joe Kelly, Surveying, Theory and Practice, McGraw-Hill Book Company, 1966 LC 64-66263
4. ^ Berenbaum, May R., Field Notes - Spin Control, The Sciences, The New York Academy Of Sciences, September/October 1995

[edit] External Links

• A list of firearm scope reticle styles offered by various manufacturers
• Chuck Hawks' article on firearm scope reticles
• A firearm scope with a Christmas tree type reticle
• 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