Crosshair
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- "Reticle" redirects here. For the pattern used in photolithography, see photomask.
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.
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[edit] Use in firearms
Telescopic sights for firearms, generally just called scopes, are probalby 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
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 different, 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
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.
