Optical Unit
The CM had a fixed sextant the AOT, which could measure angles between stars, and Earth or Moon landmarks and planetary horizon. The unit included a scanning telescope for star sightings, and could be used to determine position and orientation in space. In contrast, the LM had an Alignment Optical Telescope, and could only determine the craft's orientation. The outer element of the AOT was a sun-shielded prism that could be rotated to one of three fixed positions relative to the LM, in order to cover a large portion of the lunar sky. When rotated, the AOT's position was readable by the AGC; by pointing the reticule at several different stars, the computer could determine the craft's orientation .
Read more about this topic: Apollo PGNCS
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