page_title: Apollo PGNCS
section_title: Optical units
hierarchical_section_title: Apollo PGNCS / Optical units
caption_attribution_description: English: Optical unit assembly used on the Apollo Command Module to measure the apparent positions of stars in order to align the inertial measurement unit and to measure the angle between stars and landmarks on the earth or Moon to determine spacecraft position.
context_page_description: The Apollo primary guidance, navigation, and control system was a self-contained inertial guidance system that allowed Apollo spacecraft to carry out their missions when communications with Earth were interrupted, either as expected, when the spacecraft were behind the Moon, or in case of a communications failure. The Apollo command module and lunar module, were each equipped with a version of PGNCS. PGNCS, and specifically its computer, were also the command center for all system inputs from the LM, including the alignment optical telescope, the radar system, the manual translation and rotation device inputs by the astronauts as well as other inputs from the LM systems.
  PGNCS was developed by the MIT Instrumentation Laboratory under the direction of Charles Stark Draper. The prime contractor for PGNCS and manufacturer of the inertial measurement unit was the Delco Division of General Motors. PGNCS consisted of the following components:
  an inertial measurement unit
  the Apollo Guidance Computer
  resolvers to convert inertial platform angles to signals usable for servo control
  optical units, one for the CM and a different one for the LM
context_section_description: The CM optical unit had a precision sextant (SXT) fixed to the IMU frame that could measure angles between stars and Earth or Moon landmarks or the horizon. It had two lines of sight, 28X magnification and a 1.8º field of view. The optical unit also included a low-magnification wide field of view (60º) scanning telescope (SCT) for star sightings. The optical unit could be used to determine CM position and orientation in space. 
  The LM instead had an alignment optical telescope (AOT), essentially a periscope. The outer element of the AOT was a sun-shielded prism that could be rotated to one of six fixed positions relative to the LM, in order to cover a large portion of the lunar sky. Each position had a 60º field of view. When rotated, the AOT's position was readable by the AGC; by pointing the reticule at two different stars, the computer could determine the craft's orientation.
  The sun shade was added late in the program, in 1967, after tests and modeling determined that the astronauts might not be able to see stars on the lunar surface due to direct sun light or light scattered by near-by parts of the LM impinging on the outside prism. Adding the sun shade also allowed increasing the number of view positions from three to six.
caption: Apollo CM optical unit
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