with many identical modules that could be replaced with spares, if necessary, in flight. However high humidity conditions inside the crew compartments and accidents in handling body fluids during the Gemini 7 mission made having unsealed electrical connections undesirable. The repairability goal was eliminated in Block II and all units and electrical connections were sealed. The fatal Apollo 1 fire reinforced this concern. Inertial measurement unit The IMU was gimbaled on three axes. The innermost part, the stable member (SM), was a 6-inch beryllium cube, with three gyroscopes and three accelerometers mounted in it. Feedback loops used signals from the gyroscopes by way of the resolvers to control motors at each axis. This servo system kept the stable member fixed with respect to inertial space. Signals from the accelerometers were then integrated to keep track of the spacecraft's velocity and position. The IMU was derived from the guidance system developed by Draper for the Polaris missile. Inertial guidance systems are not perfect and Apollo system drifted about one milliradian per hour. Thus it was necessary to realign the inertial platform periodically by sighting on stars. Optical units 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, 28× magnification and a 1.8° field of view. The optical unit also included a low-magnification wide field of view (60°) 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 60° 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. Apollo 11 Command Module Pilot Michael Collins noted that the visibility through the optics was sub-standard, and it was difficult to see through in certain lighting conditions. 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