The optical collimator makes the reflex sight possible. The collimator aligns a stream of particles from a source into a parallel stream. The collimator can be refractive or reflective. An image of the reticle, generated by the collimator and illuminated by a fiber-optic or radioactive light source or LED, is projected 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. This produces a perfect image of the target with the aiming mark projected to infinity.
Holographic reflex sights use a laser hologram to project the image of the reticle into the target plane. This produces an aiming mark that is optically centered in its entirety. This in turn avoids parallax errors caused by differences in eye position relative to the aiming window. It also means partial obstruction of the aiming window will not prevent the shooter seeing the reticle in its entirety. However, laser diodes drain batteries far faster than LEDs - more than a hundred times faster in some cases, meaning that holographic sights are designed to turn off automatically - typically after four or eight hours.
The position of the collimator dictates the configuration of the sight. The bottom mounted collimator makes for a less bulky sight with a small protruding screen. This can put the sight line well above the bore - a perfect configuration with M4s 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 window can be vulnerable. Side mounted collimators lend themselves to tube construction, like a scope. Tubes are inherently very strong and make for immensely robust sights with adjusters that can be switched from right to left hand operation by simply turning the sight in its mount.
Reflex sights tend to be 1X magnification as this is best for close quarters, both eyes open shooting. The reticle or red dot size is dictated by the application - a large dot will be easy to acquire and place on target, but it will also obscure much of the target and thus be hard to center accurately, especially at longer ranges. Magnifying reflex sights have become ubiquitous in the War on Terror where target identification is at a premium. A bright illuminated aiming mark is used for close ranged shooting, backed by a traditional 'ladder' reticle for more distant targets. This philosophy is exemplified by the Trijicon ACOG, the choice of the US and British armed forces. However 1x magnification or 'unitary' reflex sights can be boosted by with detachable or flip to side magnifiers to provide a very versatile combination. Trijicon have gone the opposite route by attaching peep battlesights and/or small red dot sights to the top of their most recent ACOGs.
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. The adjusters are calibrated so that you know how far a graduation or click will move the point of impact. The calibrations vary from manufacturer to manufacturer and model to model. The Aimpoint CompM4s (the latest US Army M68) for example utilizes clicks that are 16mm at 100 meters or 1/2" at 80 yards whilst the US Marines' Trijicon TA31RCO / AN/PVQ-31B ACOG has clicks that adjust the point of impact 0.33" at 100 yards.
Brightness of the reticle can be adjusted to suit varying light conditions. This is particularly important in those sights intended for use with night vision. The adjustment can be automatic - either electronic or via a fiber optic light gatherer that draws in light from its surroundings. That can create problems - for example when shooting into brightly lit terrain from a darkened room or bunker. A polarising filter can be employed on some models to reduce the brightness of the image - rotating two polarised lenses can graduate the image from complete brightness to complete black-out. As the sight itself often incorporates a polarizing filter, mounting a single adjustable polarizing filter can have the same effect. This could be problematic with polarized eyewear, but the leading makes all now choose polarizations that do not conflict with sunglasses.
Shooters now typically 'co-witness' their iron sights with their non magnifying red dot sight. It is unnecessary to align the aiming mark to sit on top of the foresight or anywhere else - it only matters that both systems are zeroed on the target. 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 permit this; for example their Model 557.
Holographic reflex sights use a laser hologram to project the image of the reticle into the target plane. This produces an aiming mark that is optically centered in its entirety. This in turn avoids parallax errors caused by differences in eye position relative to the aiming window. It also means partial obstruction of the aiming window will not prevent the shooter seeing the reticle in its entirety. However, laser diodes drain batteries far faster than LEDs - more than a hundred times faster in some cases, meaning that holographic sights are designed to turn off automatically - typically after four or eight hours.
The position of the collimator dictates the configuration of the sight. The bottom mounted collimator makes for a less bulky sight with a small protruding screen. This can put the sight line well above the bore - a perfect configuration with M4s 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 window can be vulnerable. Side mounted collimators lend themselves to tube construction, like a scope. Tubes are inherently very strong and make for immensely robust sights with adjusters that can be switched from right to left hand operation by simply turning the sight in its mount.
Reflex sights tend to be 1X magnification as this is best for close quarters, both eyes open shooting. The reticle or red dot size is dictated by the application - a large dot will be easy to acquire and place on target, but it will also obscure much of the target and thus be hard to center accurately, especially at longer ranges. Magnifying reflex sights have become ubiquitous in the War on Terror where target identification is at a premium. A bright illuminated aiming mark is used for close ranged shooting, backed by a traditional 'ladder' reticle for more distant targets. This philosophy is exemplified by the Trijicon ACOG, the choice of the US and British armed forces. However 1x magnification or 'unitary' reflex sights can be boosted by with detachable or flip to side magnifiers to provide a very versatile combination. Trijicon have gone the opposite route by attaching peep battlesights and/or small red dot sights to the top of their most recent ACOGs.
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. The adjusters are calibrated so that you know how far a graduation or click will move the point of impact. The calibrations vary from manufacturer to manufacturer and model to model. The Aimpoint CompM4s (the latest US Army M68) for example utilizes clicks that are 16mm at 100 meters or 1/2" at 80 yards whilst the US Marines' Trijicon TA31RCO / AN/PVQ-31B ACOG has clicks that adjust the point of impact 0.33" at 100 yards.
Brightness of the reticle can be adjusted to suit varying light conditions. This is particularly important in those sights intended for use with night vision. The adjustment can be automatic - either electronic or via a fiber optic light gatherer that draws in light from its surroundings. That can create problems - for example when shooting into brightly lit terrain from a darkened room or bunker. A polarising filter can be employed on some models to reduce the brightness of the image - rotating two polarised lenses can graduate the image from complete brightness to complete black-out. As the sight itself often incorporates a polarizing filter, mounting a single adjustable polarizing filter can have the same effect. This could be problematic with polarized eyewear, but the leading makes all now choose polarizations that do not conflict with sunglasses.
Shooters now typically 'co-witness' their iron sights with their non magnifying red dot sight. It is unnecessary to align the aiming mark to sit on top of the foresight or anywhere else - it only matters that both systems are zeroed on the target. 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 permit this; for example their Model 557.
About the Author:
Chris Pieterman is a gun enthusiast with 30 years experience of red dot sights. For more information on Eotech sights please visit Combat Optics Reviews Dot Com which has more information on the AN/PVQ-31B ACOG.
No comments:
Post a Comment