The SUSAT (Sight Unit Small Arms, Trilux) scope is a scope with x4 magnification with tritium-powered illumination developed by Royal Armaments Research Development Establishment (RARDE) based in Great Britain. The scope is found in a variety of location from vendors, stashes to some stalkers mounting the add-on their weapons. It can only be mounted on weapons with a Picatinny rail more specifically NATO class weaponry. Despite the x4 magnification on a real SUSAT scope, the ones in the S.T.A.L.K.E.R. games never magnify more than x2, except the integrated one on the Gauss Rifle.
The SUSAT Scope can be mounted to NATO weapons in all three S.T.A.L.K.E.R. games. In Clear Sky and Call of Pripyat, you may have to apply upgrades in order to mount the scope.
The SUSAT is comparatively more common then its Warsaw counterpart, the PSO-1. Nearly every Mercenary encountered throughout the series has a SUSAT attached to their rifle, and most Freedom members with LR-300s have them as well.
Call of Pripyat
Nearly every Merc encountered in Call of Pripyat has a standard x2 SUSAT attached to their rifles, giving the player an ample supply of them right from the very first level. Most Stalkers, not just Freedomers, will attach SUSATs to their own weapons as well. Like the PSO-1, Call of Pripyat introduces several variations that can be bought from traders or found in stashes. Most interestingly, the SUSAT can be attached to the Protecta shotgun, the only shotgun capable of taking any kind of attachment.
x4 SUSAT Scope - The scope from Clear Sky. Very common, as it is the only version other Stalkers and Mercs make use of. The lens portion is colored blue in the inventory for easy identification. Despite its 4x label, its magnification is only 2x.
x1.6 SUSAT Scope - The 'classic' scope from Shadow of Chernobyl. Lower resolution, but quicker turning when scoped in. Only a strip of the lens is colored blue.
x4 SUSAT NVG Scope - A standard x4 scope with second-generation night-vision. Rather rare scope, Owl may sell them after doing his missions. Despite its 4x label, its magnification is only 2x.
x4 SUSAT "Dusk" Scope - A standard x4 scope with a special tinted lens, which increases the image contrast - making it more useful than night-vision scopes during the early morning and late night. Poor visibility in pure darkness. The lens, due to a glitch may appear green in inventory, like the NVG, whereas it should be yellow. Very rare scope. Despite its 4x label, its magnification is only 2x.
SUSAT "Custom" Scope - An adjustable scope, can be adjusted all the way from x1.6-x2, even though the game claims it is a x1.6-x4 scope. Both ends of the SUSAT are colored blue in the inventory for identification. Very rare.