5.1.3 Level 0 Data Products

On the ground, the ASC performs a decommutation step to produce separate data streams for ACIS science data, for HRC science data, and for each of the many engineering signals that are commutated into the major frame. After this decommutation step, the ACIS science data stream is a simple byte stream since individual packets and the boundaries of science runs are not yet identified.

The ASC then performs a processing step which identifies packets in the science data stream, groups packets, and extracts data from packets in order to recover each of the data streams produced by ACIS (science run data, calibration data, command logs, software housekeeping, etc.).

These activities are known as Level 0 processing and result in Level 0 data products. Since science runs execute in one of several possible modes, Level 0 data products have formats specific to the mode of the science run. For example, a Timed Exposure Raw Mode science run produces Level 0 products that contain raw pixel data (images). A Timed Exposure Faint Mode science run produces Level 0 products that contain lists of X-ray events.

Again as noted in the discussion of operations the event data telemetered to the ground are generally raw pixel data. The appropriate bias must be subtracted on the ground, requiring that the bias be available on an event-by-event basis (in Timed Exposure with Bias mode) or be accumulated as a complete bias map which is telemetered at the start of a Science Run.