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The ACIS Digital Processing Assembly (DPA) will provide overall instrument control, image processing, data formatting and observatory interface capabilities. It will, in addition, provide the necessary housekeeping functions, such as temperature and electrical sensing, to monitor the general condition of the instrument during the mission.
ACIS's DPA will consist of 5 independent processing subsystems, each responsible for the data from one of the five Detector Electronic Assembly (DEA) sections controlling two of the focal plane's CCDs. These subsystems will consist of a front-end processor section, to continually monitor the digitized CCD video data stream extracting events and providing control signals to the DEA. The back-end processor section will take those extracted events and determine if they are valid x-ray events, rejecting those caused by sources such as high-energy particles, gamma-rays or hot-pixels. The back-end processor will then prepare the data for transfer to the observatory's Command and Telemetry Unit (CTU) for transmission to the ground station.
Figure 5: ACIS Data Flow
Events are identified when a pixel's intensity level is above a defined threshold and a local maximum relative to that pixel's nearest neighbors. Information that is preserved about a valid x-ray event includes the position in the focal plane where the event took place, time of occurrence and energy levels of the pixel and it's neighboring pixels. The mode in which the instrument is operating will determine the amount of event information that is ultimately transferred back to the observatory's CTU for transmission.
The DPA will support the command and data flows for up to 6 CCDs operating simultaneously, the maximum number of detectors that can be placed in the observatory's field of view at one time. It will be able to process the data coming from the DEAs when the CCDs are running in a timed exposure mode or continuously, the latter representing the highest data rates that the front-end processor section will need to handle.
Operational parameters set in the different sections of the DPA will allow the instrument to run in various modes. For the most part, these different modes will determine the amount of information about the events to be transmitted back to the ground station. The trade off will be between spectral, position, and temporal resolution. It will be up to the observer to select which operating mode is appropriate for a given observation, and to send the necessary configuration commands to the instrument.
When AXAF-1 becomes operational, the ACIS instrument, in combination with the AXAF-I high angular resolution X-ray optics and the High Energy Transmission Grating, will represent a significant advancement in the field of X-ray astronomy. The ACIS instrument is being designed to take full advantage of the remarkable capabilities of the AXAF-I observatory in performing X-ray observations of faint celestial sources with a resolving power and a sensitivity never before possible. The ACIS CCD detector focal plane, along with its signal and data processing systems, make possible the unique opportunity to develop an instrument that will fully complement the powerful and forward looking observatory that AXAF-I represents.