Rear Sight
How A Red Dot Sight Works
The optical collimator makes the reflex sight possible. This takes the light from a source and aligns it in parallel Refractive and reflective collimators are employed. The collimator takes the image of the reticle illuminated by a radioactive or fiber-optic light source or an LED and projects it to infinity. This image is directed onto a dichroic mirror or beam splitter which uses a special reflective coating so as to reflect only the exact colour of the reticle whilst allowing all other colours to pass through. The result is a perfect image of the target with the aiming mark apparently floating on top of it.
Holographic reflex sights use a laser hologram to project the image of the reticle into the target plane. This has the benefit that the complete reticle is free from parallax and not just its centre. It also makes the unit less sensitive to head position relative to the sight, allowing for greater variety in shooting position. This makes the sight all but immune to all but total obscuration or removal of the aiming window as the reticle is still visible if any part of the window is visible. Laser diodes do drain batteries much faster than LEDs though, so that you can expect a battery life of perhaps 500 hours rather than 50,000.
The shape of the sight is largely a result of the position of the collimator. If it is mounted beneath you can have a unit that is less bulky, with the sighting screen taking the form of a mini head up display. This can put the sight line high above the bore – a perfect configuration with M16s or other rifles or support weapons with an inline configuration, but less advantageous with others. It can also result in a less robust sight as the protruding display can be vulnerable. Side mounted collimators enable a tube to be employed to contain the workings of the sight. Whilst heavier, this is immensely robust and allows much lower mounting. Some sights – the Aimpoint CompM2 for example – can be mounted with the adjusters on the preferred side just by rotating the unit in its mount.
Reflex or red dot sights are usually non magnifying – this enables them to be employed with both eyes open for very fast target acquisition and engagement. This is best accomplished with a red dot or circle reticle, the dot size being a compromise between fast acquisition (larger dot better) and precision (smaller dot better). However, magnifying red dot sights exist which take advantage of reflex operation to combine a large illuminated aiming point for fast target engagement with a traditional ballistically calibrated ladder reticle for long range precision. A bright illuminated aiming mark is used for close ranged shooting, backed by a traditional reticle for longer ranged engagements. The Trijicon ACOG is the best example of this philosophy. 1X magnification sights can be turned into low powered scopes by the simple addition of a detachable or flip to side magnifier. The ubiquitous ACOG can attain similar versatility with the mounting of peep battlesights and/or a small rear mounted red dot sight on their housings.
It is almost universal for reflex sights to use internal adjustment. The sight is mounted firmly to the weapon and the optics move internally to move the point of impact. These are calibrated and, almost universally feature audible/tactile clicks. The calibrations vary from manufacturer to manufacturer and model to model. The CompM4s from Aimpoint (the most recent incarnation of the US Army M68) for example uses clicks that are 16mm at 100 meters or 1/2″ at 80 yards whilst the US Marines’ AN/PVQ-31B has clicks that move the point of impact 0.33″ at 100 yards.
The sights have to cope with different brightnesses of daylight and thus employ variable reticle brightness settings. Some have night vision compatible ranges of settings. Some units will adjust automatically, either employing sensors or a fiber-optic light gatherer that pulls in light from its surroundings. That can create problems – for example when shooting into brightly lit terrain from a darkened room or bunker. The brightness of the image can be varied in some models by rotating a polarised filter against another one – this results in anything from full brightness to total black-out. As the dichroic mirror or beam splitter is itself sometimes polarised, fitting one such filter can have a similar effect. Manufacturers now select their polarisations with care so as not to cause problems with those used on eyewear.
Co-witnessing iron sights with non magnifying red dot sights has become popular. Some shooters struggle to get the aiming mark of their reflex sight to sit on top or directly in line with their iron sights. This is aesthetically pleasing, but pointless as it is only vital for both systems to be zeroed, not to appear exactly synchronised with each other. A popular method is to have the iron sights in the bottom 1:3 of the sight picture and manufacturers like Eotech are starting to make sights with optional risers to facilitate this; for example their Model 557.
Copyright Chris Pieterman 2010
AR15 Buyers Guide: Rear Sights