EOTech CNVD-T (SU-232/PAS) Clip-On Night Vision Device - Thermal - Manual (Page 22)

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2.4.2 Eyepiece . The eyepiece assembly may be fitted with
one or both of the following components:
2.4.2.1 When installed, the night vision spectrum filter
prevents fogging of the eyepiece, and reduces backlighting
and loss of night vision in low light environments. The filter /
shield is installed as follows:
a. Place the filter / shield over the eyepiece with the
threaded side down.
b. Carefully turn the filter / shield clockwise to screw it
into the eyepiece assembly.
c. Tighten until snug.
2.4.2.2 When attached, the eyecup reduces emission of
stray light from the eyepiece. To install, stretch the base of
the eyecup into place over the eyepiece assembly. Rotate
the eyecup to obtain a proper eye socket and cheek weld.