The sextant was used with its counterpart (the scanning telescope) to determine the Command Module’s position and attitude with relation to stars or landmarks. This data was then supplied to the onboard computer to calibrate the spacecraft’s Inertial Measurement Unit (IMU).
The sextant was a highly accurate optical device capable of measuring the included angle between two targets. Angular sightings of two targets were conducted through the fixed beam splitter and movable mirror located in the sextant head. The sighting head assembly is rotatable to 270 degrees in shaft axis position in either direction, from the zero reference point. The sextant lens provides 1.8-degree true field of view with 28X magnification. The movable mirror is capable of sighting a target to 57 degrees line of sight from the shaft axis. Both the fixed and moveable mirrors are fabricated of beryllium; the beryllium is Kanigen coated and aluminized to obtain maximum reflectivity. The sextant target optics include an indexing mirror assembly and a beam splitter, telescope, and eyepiece assembly.
Block II metal cased unit measures approximately 7 x 6 x 6, with parts having serial numbers SN EE-1 and SN EE-2. The sextant was designed by MIT Instrumentation Lab, and manufactured by Kollsman Instrument Corp.
Part of the DFJ Space collection. We just did a promo video on it too =)
Other angles 
And a spare lens, acquired separately: 
numbered 2011796 AKICS3, measuring 4″ long
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