The Command and Service Module on the Apollo Spacecraft

The guidance platform on the Apollo spacecraft is a complex device, which working with the onboard computer enables the crew and mission control to know where they are in space. It's also known as the Inertial Measurement Unit, or IMU, and there's one for the Command Module and another in the Lunar Module. You can think of the platform as a three-dimensional compass needle, providing each spacecraft with a kind of true north. For the guidance platform to work, it needs to be properly aligned against fixed points in space, specific stars in particular constellations. To do that, the astronauts ideally needed to maneuver the vehicle and find some stars through a special telescope mounted through the roof