Prototype of a Long Eye Relief Eyepiece for the Apollo Command Module's Optical Unit Assembly (OUA) scanning telescope, measuring approximately 5″ x 3.5″ x 3″, labeled on the main cylinder: "Display Telescope, Scanning, M5H3XA-041511, Tinsley Laboratories, Inc., 420018, U.S." This was designed to be a long eye relief telescope eyepiece—one that could be used by an astronaut wearing a helmet or eyeglasses (the Apollo and other program astronauts did have sunglasses issued to them). It incorporates a focusing mechanism that covers a large range and the lens closest to the eye is of large diameter.
This has a different mounting arrangement from the Block II quick-change system, likely because it would have attached to a dual-prism assembly that did have this quick-change attachment. This also allowed for other eyepieces to be attached to the SCT without having to carry a separate dual-prism assembly for each one. For example, the 'standard' SCT eyepiece was not a long eye relief one. Both eyepieces were attached to the dual-prism assembly with thumbscrews.
The OUA had two optical systems in it: the sextant (SXT), a 28-power high-accuracy optical system; and a scanning telescope (SCT), a one-power optical system. The SXT was on the left side and the SCT on the right. The SCT acted very much like the 'finder scope' that astronomers use to locate astronomical features for closer observation. The astronaut using the optical unit would use the SCT to locate a star or horizon and by placing the object in the reticle crosshairs; a joystick was used to drive motors that positioned a prism for viewing areas of the sky. The electronic link from the SCT to the SXT would drive the SXT optics by way of servo motors to place the object at, or very near, the center of the field of view of the sextant. The sextant showed a fixed line of sight (aligned with the spacecraft) and a movable line of sight. This allowed for angular measurements from the Earth's (or moon's) horizon to a known star, or the angle between two known stars. From a catalog of stars in the Apollo computer, the computer could then calculate a 'fix' and update the spacecraft position.
