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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 be used by all infantrymen. A similar device is the Advanced Combat Optical Gunsight.
SUSAT is the primary sighting system for the British Army's SA80 series weapons and L108 and L110 light machine guns. It is also used in the armies of Cameroon, Oman, Spain and Sweden, in assault rifles such as the Swedish Ak5B and the Spanish CETME LV, although after the replacement of the latter by the HK G36, those sights have been employed in Rheinmetall MG3 machine guns.
A similar unit known as the Sight Unit, Infantry, Trilux (SUIT) sight was used on the L1A1 SLR.
The reticle of the SUSAT is of unusual design. Unlike the traditional crosshair layouts commonly used, which are in essence a cross intersecting the target, the SUSAT has a single obelisk-shaped post protruding from the bottom edge of the sight. This type of reticle is sometimes referred to as the "German Post". This obscures the target at long range and the foreground. The reticle is tritium-illuminated for low-light condition aiming. The radioactive tritium light source has to be replaced every 8–12 years, since it gradually loses its brightness due to radioactive decay. The SUIT sight uses a similar single post to the SUSAT, but protrudes from the top edge of the sight down to the middle of the field.
The SUSAT sight was developed in the United Kingdom by Royal Armaments Research Development Establishment (RARDE) and is manufactured by United Scientific Instruments and Avimo, now known as Thales Optics.
- Overall dimensions: (L x W x H): 145 x 60 x 55 mm
- Weight: 417 gram
- Magnification: 4×
- Field of view: 10 degrees (177 mils)
- Objective diameter: 25,5 mm
- Exit pupil: 6.375 mm
- Eye relief: 25 mm
- Light permeability: >80%
- Reticle illumination: Red tritium, glass ampoule
- Illumination strength: Adjustable
- Tritium ampoule lifetime: 8–12 years
- Focus: −0.75 to −1.25 dioptres
- Operational temperature: −46 to +71 °C
- Range Settings: 100 to 600 meters (SUSAT L9A1) or 300 to 800 meters (SUSAT L12A1) in 100 m intervals
- NATO Stock Number (NSN): 1240-99-967-0947 (Sight Unit Small Arms Trilux (SUSAT) L12A1)