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A wiring error in the HRC causes the time of an event to become disconnected from the rest of the data associated with that event. Using a proof-of-concept version of the HRC, the problem was traced to a backplane wiring error that sends an inverted logic signal to the latch that captures an event time. The proper transition of this signal therefore occurs late and results in the event time appearing with the next event's data. The IPI team has generated a short memo on this hardware problem. If every MCP event were telemetered, the full timing capability could be recovered in ground processing by shifting the time-tags on the events.
However, a further analysis of the wiring diagrams shows that the timing latch is being set for every HRC front end trigger, not just for valid events. This poses potential problems for timing analyses because the following event could be rejected by the on board vetoing. In addition, telemetry saturation (having more events generated than we have bandwidth to telemeter) can lead to the loss of events on-board. In order to avoid telemetry saturation the HRC-I is routinely operated with the Anti-Coincidence shield vetoing enabled, while for the HRC-S Edge Blanking is used. The vetos result in the on-board rejection of as many as 2/3 of the MCP events.
If the source is faint, and thus represents a small fraction of the total detected event stream, most of the times received in the standard imaging configuration will be incorrect due to intervening events which are rejected by the standard on board vetoing criteria. This degrades the accuracy from about 16 microseconds to a value equal to one over the total MCP event rate (typically about 4 milliseconds).
Eliminating the on-board vetoing, and thus telemetering all events does allow for the event times to be correctly correlated with position via software on the ground. However, in general the total event rate can often exceed the telemetry saturation level of 184 events/sec. This saturation reduces the overall efficiency of the detector and introduces various timing signatures associated with the details of how events are buffered and entered into the data stream. These complicate period analysis and may also add biases to light curve shapes.
The HRC-S, however, can be operated in an "imaging" mode where the outer segments are disabled. In this mode (with all on-board vetos disabled, SI_MODE = S_TIMING) the total rate is below the telemetry saturation limit except for observations with bright target sources or during times of high background, and all events can be assigned accurate times via ground based processing, i.e. using the time of the following event.
In S_TIMING mode, HRC should meet all of the timing performance specifications.
Last modified: 04/06/18
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