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ACIS Detector Assembly
The primary components of the Detector Assembly as shown in Figure 2.7 are the titanium collimator, the door and door drive mechanism, aluminum camera body, the connector back plate assembly, the CCD focal plane, and the focal plane support assembly (FSA). The Venting Subsystem, which is not shown in the diagrams, provides a low conductance vent path to the outside of the Spacecraft.
The titanium collimator is used primarily to support the camera body which is 13 inches away from the SIM interface. Titanium was chosen because of its mechanical and thermal properties. It has a high Modulus of Elasticity, low coefficient of expansion, and low thermal conductivity which makes it an ideal choice for this application. The collimator is a one-piece component machined out of a solid billet of titanium and provides for a first mode resonance of the Detector Assembly of 200 hertz. This is significantly above the 50 hertz requirement. The collimator attaches to the SIM with six 0.25" diameter bolts.
The collimator supports the door drive mechanism and includes the door, seal retainer, linkages, drive shaft, bushings and the actuator. The two-bar linkage provides an over-center locked position to hold the door closed during launch. A swing link provides for a uniform seating of the door onto the seal retainer. An access cover provides access to the mechanism components during assembly. An externally mounted paraffin actuator rotates the drive shaft and door to the open position once the AXAF is on orbit. To open the door on-orbit, a command is received from the RCTU by the PSMC which turns on an internal heater in the actuator. The PSMC monitors the limit switches and turns off the heater when the door is latched open. Actuator case temperature is also monitored and will turn the actuator heater off in the event this temperature reaches 135 C.
The camera body, which is machined from a single block of aluminum, provides a isothermal and clean environment for the focal plane and Optical Blocking Filters (OBF). O-rings seal all mechanical interfaces so that a vacuum can be maintained inside the detector during ground testing. The vacuum keeps the focal plane and OBF clean and prevents acoustic energy from being transmitted to the OBF during test and at launch. The side walls are at least 0.5 inches thick to provide radiation shielding for the CCDs. Internal surfaces of the camera body are gold plated. The top of the camera body is a precision ground surface for the FSA standoffs, Alignment Reference Mirrors, collimator top flange and the fiducial lights. The camera body attaches to the collimator with eleven #6 screws.
The camera back plate provides a vacuum feedthrough to the CCDs for the interconnect cabling from the DEA. The back plate is approximately 0.9 inches thick, is gold plated on inside surfaces, and has O-ring seals. Flex cables attach the electrical feed-throughs to the CCDs. Nineteen #6 screws between the camera body and the back plate assembly provide uniform clamping pressure on the O-ring.
The collimator, camera back plate and other surfaces visible to the CCD are coated with gold. This gold coating is sufficiently thick to absorb fluorescent photons produced within the titanium (collimator) and aluminum (camera body), which would otherwise provide a non-celestial source of background when struck by high energy charged particles or gamma rays. The gold is thin enough to produce very few fluorescent emissions.
The beryllium focal plane provides a light weight, isothermal and stable environment for the CCDs. (Beryllium is particularly useful for having no characteristic X-ray lines which can be fluoresced by incident charged particles and reach the CCD sensitive regions.) It is held in place in the camera body with the focal plane support assembly consisting of thermal standoffs, cold fingers and cold stubs. The four thermal standoffs are made of a high strength plastic called Torlon. Each standoff consists of a 0.75" diameter tube which is 1.5" long with .035" walls. A flange with (3) #6 screws attaches the standoff to the camera body. A 0.5" barrel nut clamps the top of the standoff to the cold finger and cold stub flanges. Torlon has excellent mechanical properties including high modulus, low conductivity, good fatigue properties, and low mass. Each standoff also has two O-ring interfaces which seal the standoff flange to the camera body and the top of the standoff to the cold fingers and cold stubs. Two +Z standoffs provide a vacuum tight feed- through for the copper cold fingers. Each cold finger/stub flange attaches to the focal plane with three #6 screws.
A Venting Subsystem is connected to the camera body with a 5/8" ID stainless steel vent tube that attaches to the Large Vent Valve mounted to the SIM. The valve is powered open and closed using the same model Starsys actuator as is used on the door mechanism. A bellows on each side of the large vent valve provides stress relief for installation stresses and isolates the large vent valve from dynamic movement during launch. Low conductance vent valves are used to vent any remaining pressure which is in the camera once it is on orbit. A light shield mounts to the vent tube outside the spacecraft to prevent light from getting into the camera. If necessary the vent tube can be reclosed to eliminate any possibility of light reaching the CCDs through this tube.
Thermal Control Subsystem (TCS)
The primary components of the Thermal Control Subsystem are the Warm and Cold Thermal Straps, the Warm and Cold Radiators, the Radiator Standoffs, the Sun and Telescope Shades, and the Shade Support Posts. Figures 2.7 and 2.8 show each of these components.
Each Warm Thermal Strap is used to conductively tie the detector housing camera body to the Warm Radiator while maintaining structural isolation from the dynamic deflections of either the radiators or the detector housing. Each strap consists of eight silver plated copper braids soldered to a copper flange at each end. Two #8 screws provide good thermal and structural interfaces at the Camera Body and the Warm Radiator. Indium foil is used at the mechanical interfaces to improve the thermal interface conductance.
The Cold Thermal Straps are similar in construction to the warm straps. However, a battery clamp is used at the interface with the cold fingers. Two #6 screws provide the clamping force between the clamp and the cold finger. Indium foil is used between the flanges at the Cold Radiator interface with (2) #8 screws providing the clamping force.
The Warm Radiator is a light weight aluminum honeycomb panel approximately four square feet in area. The outer face-sheet is .060" thick aluminum sheet and the inner is .020" thick. The honeycomb between the face-sheets is also aluminum with a 1/4" cell size. Doublers are used at each of the Warm Strap mounting locations. The radiator is held off of the +Z panel of the SIM with (8) Torlon thermal standoffs. The radiator is electrically grounded to the +Z panel of the SIM though 1750 angstroms of sputtered gold on each of the standoffs. Martin Black is used on the outside surface to provide as high an emittance as possible.
The Cold Radiator is a solid aluminum panel with a nominal thickness of 0.030". Its radiation area is 2 square feet and also has a Martin Black outer surface. The radiator has been thickened at the Cold Strap mounting locations for structural support and heat spreading. It is held off of the Warm Radiator with (6) Torlon thermal standoffs which provide a electrical ground path to the warm radiator through the sputtered gold coating.