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Figure 4: Detector Housing and Radiator
Figure 4 shows the current design concept for the ACIS Detector Assembly. The Detector Assembly consists of the X-ray baffle, cover and cover mechanism, camera body, focal plane, interconnect board, and the optical blocking filter.
The Detector Assembly attaches to the SIM with the Titanium X-ray baffle. Titanium minimizes the thermal expansion/contraction and also acts as a thermal isolator between the SIM which is operating at 0 degrees C and the camera body which is operating at -70 degrees C. Titanium also has a high stiffness to weight ratio which minimizes dynamic deflections of the Detector Assembly during launch. Contamination of the focal plane is minimized during ground test and handling by a door with a redundant cover mechanism that will be opened once the AXAF is on orbit. The assembly will be designed to contain a positive pressure of dry nitrogen when not under vacuum conditions.
The camera body, which attaches directly to the X-ray baffle, will be constructed of aluminum to provide isothermal operation and maximum radiation shielding close to the CCDs. The camera body is attached to the warm radiator with a flexible thermal strap. This thermal strap will provide a thermal conduction path without coupling thermal stress, vibration, or acoustic loads into the camera.
The focal plane will be attached to one side of the camera body via the anchor plate as shown in section A-A. The anchor plate is an important part of the design since it supports the focal plane and cold radiator. It will be constructed from honeycomb with low conductivity face sheets. A single point attachment to the anchor plate provides an acceptably small heat leak between the camera body and focal plane operating at -120 degrees C. The focal plane also attaches to the cold radiator at the anchor plate.
Since CCDs are very sensitive to visible light as well as X-rays, an optical blocking filter is required to block the photons in the visible wavelength range. This light filter consists of 1000 Angstroms of aluminum over 800 Angstroms of Lexan. The filter also provides an indirect benefit of reducing the radiative heat load from the telescope to the CCDs. Since the Detector Assembly acts as a cavity, the exit of the X-ray baffle appears as a black surface at 10 degrees C. With the optical blocking filter in place, only the filter and inside surface of the X-ray baffle can see this warm surface. Therefore, this radiated heat load can more efficiently be rejected from the warm radiator and not the cold radiator as would be the case if this was a visible light CCD camera.
Each of the ten CCDs from the spectroscopy and imaging arrays are connected to an interconnect board inside the camera body via a 32 conductor flex cable and connector. These cables represent a majority of the conductive heat load to the focal plane. The length of these cables has been increased to the extent possible and the effective cross-section of copper conductors has been reduced using high thermal impedance metallic conductors where possible to decrease this heat leak. There will be several heaters and thermal diodes on the focal plane requiring another 8 wires to the interconnect board which will also be high impedance material. The electrical connection between the DEA and the interconnect board consists of up to 320 wires which must pass through the camera body. This cable is less of a thermal concern since the heat load from this cable can be rejected from the warm radiator.