3.1.4 List of Routine Modes

  The delivered instrument and software contains the ability to operate in a number of modes, selectable to optimize the scientific performance for various types of astrophysical targets and scientific objectives.

These modes include the ability to process and telemeter data from a single CCD in either array, the entire spectroscopic array, or the entire imaging array, and the ability to select a rectangular region of interest to either telemeter or exclude from telemetering. The efficiency of the processing after any of these choices is no worse than the minimum rates specified under the event processing modes.

Imaging

The four ACIS-I CCD chips are read-out and the pixel data stream is processed so as to recognize events from background. These events are then processed through event recognition and packetizing software for telemetry to the ground as a list of potential X-ray photon events. Typical auxiliary choices are Faint vs. Bright event packetization, 1, 2, 4 or 6 CCD mode readout and sub-array event selection windows. (6 CCD readout includes processing data from the closest S array chips to increase the field of view.)

PHA/Calibration

The CCD data are not processed by event recognition, but simply accumulated into pulse height bins. A histogram of pixel pulse heights is telemetered to the ground (as opposed to individual X-ray events). This mode is mostly used during HRC science runs when ACIS is accumulating data from the external calibration sources.

For sources too bright to do photon-counting of dispersed spectra, data are compressed by applying a read-region mask, and only processing those data.

Diagnostic Full Frame

All individual pixels are telemetered without processing in this mode. The processing will be adequate to process data up to the normal CCD clocking rate. Ordinarily this mode is not used for data collection. It is likely to only be used as part of a diagnostic investigation of camera performance or optimization.