2/12/02 - M. Zombeck
Secondary Science Rate Double Counting
1 = 2
In addition to the primary science data for individual events, the rate of microchannel plate triggers (total rate) and triggers (valid rate) that pass on-board validity tests are telemetered to the ground. The valid rate is used to correct the primary rate for deadtime and telemetry saturation effects. As long as the primary rate is below saturation, the primary rate itself can be used to make the small (<1%) correction, since the event processing dead-time is known. However, when the event rate exceeds saturation, a not uncommon occurrence because of the flaring background from low energy protons, the valid rate is necessary to correct the event rate. Unfortunately, the total and valid event rates are overestimated by about 15% for normal operation of the HRC-S . In the HRC-S "imaging" (aka "timing") mode these rates are overestimated by a few percent, at most. The problem is caused by and overshoot in occasional large trigger pulses. This results in double counting in the total and valid event on-board scalers. The primary science event is not affected, since once event processing starts with the initial trigger pulse, a gate rejects further pulses until processing is complete. The HRC-I does not have the overshoot problem. We suspect that rise-time differences in the trigger pulses in I and S are the reason for the difference in the responses. We are studying ways of mitigating the problem. Raising the trigger threshold level reduces the double counting but we need to evaluate the consequence on detector QE and uniformity. The simplest and least "invasive" solution would be to apply a correction factor based on the fraction of large pulses determined from the pulse height distribution obtained during an observation. We are studying this solution.