3.1 Modes

  The CCD is a powerful tool for simultaneous imaging and spectral data collection. It differs from many previous X-ray detecting instruments in that it must integrate images (rather than responding promptly) and no trigger signal is generated unequivocally indicating the presence of an X-ray or other event in a pixel. Thus the pixel data stream normally requires on-board processing to recognize X-ray events and extract them, otherwise the volume of data would be too large to telemeter to the ground.

The event processing and data packetization post processing offer many parameters or choices which must be made before data can be sent to the ground. The large number makes testing and definition of each possible combination of choices impractical.

Instead we define the various processing choices which can be made under the section `On-board Data Processing Choices' (Section 3.1.3), and a set of standard choices which we expect to be commonly employed under `List of Routine Modes' (Section 3.1.4).

Conceptually the easiest way to understand the selection of mode switches is to consider the choices required in order to specify ACIS operation. There are four primary decisions:

• Array imaging point?
• Timed integration, or continuous readout?
• Which on-board data-processing program?
• Is a new bias frame required?