3.3.1 Overview

ACIS has significant on-board-processing with a multitude of operating modes. The duties may be listed as:

• Instrument control, housekeeping and communication with observatory data system
• Timing and control for CCD and analog processing chains
• Event finding in CCD output data streams
• Diagnostic histograms
• Event processing and data formatting

Figure 3.2 gives an overview of the data flow through ACIS. The key data processing tasks are divided between the two types of processor sections.

The next two sections describe the on-board data processing flow. Two fundamentally different processing schemes apply, depending upon whether the CCD data is collected as timed exposures, or as continuous clocking. First the timed exposures will be described, followed by the continuous data processing.

 

Figure 3.2:   On-board data flow overview.

The basic division of responsibility between the Back and Front End Processors is as follows. The Back End Processors (BEP) must perform:

• Command Management
• Telemetry Management
• DEA Control
• Science Data Filtering and Formatting
• DEA Housekeeping Acquisition
• ACIS Software Housekeeping (including error management)
• Managing Front End Processor Code and Data Loads

The Front End Processors (FEP) must perform:

• Event Selection from Raw Pixel Data
• Pixel Bias Computation (including overclock calculations)