Eyepiece for the Apollo Command Module (Block II) Optical Unit Assembly (OUA) sextant, measuring approximately 8.5″ x 4.5″ x 4″, marked on the base with a Kollsman part number: "2012891-011A, S/N 2009." 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. This SXT eyepiece has a heater assembly to prevent fogging of the optics.
The OUA and its eyepieces were manufactured by Kollsman Instrument Company, at the time based in Elmhurst, NY. From Block I to Block II, NASA changed the design of the mounting system. This eyepiece has a quick-change system that allowed the astronauts to remove or install the eyepieces with a quarter-turn. The Block I system used thumbscrews which were difficult to handle with suit gloves on.
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.
NASA is currently studying sextant designs for the Mars missions. The use of a sextant for navigation continues a long history from ancient mariners to modern-day astronauts.
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