The aperture door mechanism protects the interior telescope hardware and critical optical surfaces from particulate and molecular contamination during ground operations and on-orbit SOHO attitude control maneuvers. This subassembly, mounted on the forward face of the TSU end cap assembly, consists primarily of the entrance aperture, housing, door and drive sub-assemblies. The forward surface of the housing mounts the entrance aperture, a gold-plated frame with an integral serrated external occulter edge. The occulter edge and plate possess match-drilled interfaces to allow a proper alignment of the instrument occulting system. The remaining aperture edges are also mounted and aligned onto an aperture plate. The door consists of a beryllium structural frame supporting a clear teflon window. Although the primary purpose of this window is for contamination protection, this window also allows 90% of the nominal thermal energy (with door open) to pass into the instrument sunlight trap and maintain a proper thermal balance when the door is closed on-orbit. The door is actuated by a stepper motor gearing network such that when it is open, the door hardware lies completely behind the entrance aperture plane to prevent extraneous light scatter into the aperture. When the door is closed, link springs and two door latches are engaged to lock the door into its closed position. Redundant external microswitches provide a dual indication through instrument telemetry of the door actuation from and to either its open or closed positions.
The baseline door open or close operation requires 45 seconds. Both slower and faster rates are also commandable. For example, the fastest door rate (door closing in about 15 seconds) would be implemented upon receipt of the emergency Sun reacquisition flag signal from the SOHO Spacecraft command system.