3.1.3.2 Telemetry Saturation Limits

When planning an observation it is necessary to consider how many events will be detected. ACIS's ability to process data is a function of the number of events which are detected, and the capacity of the telemetry to send those events to the ground. ACIS has buffering capability within the instrument, but if these limits are exceeded for any significant amount of time processing of additional CCD frames is stopped and data in subsequent frames are lost until the accumulated data are readout.

Processing and telemetry saturation will occur at the following situations.

• Each FEP can handle upwards of 14,000 threshold crossings per second. A threshold crossing that is a local maximum is reported to the BEP in an event record.
• The BEP can read a total of no more than  6,000 event records per second from all FEPs.
• The BEP applies a series of filters to determine whether to accept each event: according to its CCD location, grade, and graded pulse height.
• The number of surviving events that can be written to the 24 kbaud downlink depends on the BEP mode:

  
  

    

  Readout Mode Bits/event Events/sec^*

  CC graded 34 700

  CC faint 55 432

  TE graded 58 410

  TE faint 128 186

  TE faint+bias 236 100

  TE very faint 320 74

  
  
  ^* assumes only minimal DEA and S/W housekeeping.
  
  
  
• When the telemetry stream becomes saturated, a FEP will find that it is unable to write all event records from the current exposure frame to the BEP. It then skips one or more whole exposures until the BEP accepts more events. The result is that whole exposures are dropped.