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There are two new ME OBC modes which have been tested successfully during the recent Crab Nebula observation and are now fully operational. They are MHTR4, designed to provide extremely high time resolution over a selected energy band, and MHTR5, designed to replace MHTR3 for most observations.

MHTR4 provides intensity samples at 0.25 msec time resolution over a selectable range of energy channels. The ME energy samples are processed and a single bit is set to '0' or '1' per sample corresponding to the absence or presence of a photon within the defined energy range. With 0.25 msec time resolution, MHTR4 uses 58% of available telemetry. The deadtime is the usual deadtime associated with the 4K sampling rate (10% on a background observation, 40% on a 1 Crab source). There is no selection on detector ID and the ME should normally be co- aligned for maximum observing efficiency. MHTR4 can be operated in conjunction with MHER2, MHER4. or MHER5 in order to obtain simultaneous spectral information. The CPU usage should remain below its limit for all allowable count rates (ie. a limit of < 500 cts/s/detector based on detector safety considerations) unless either MHER4 or MHER5 is used in conjunction with LDIR2 and GHBL4 in which case the 14E count rate to be processed by the OBC must be below ~1500 cts/s. Note that it is possible to process only Ar or Xe data in the OBC to reduce CPU usage and deadtime. MHTR5 cannot easily be used to give a lower time resolution than 0.25 msec because at less frequent sampling rates the deadtime increases unacceptably.

MHTR5 is designed to replace MHTR3 because it requires half as much telemetry to provide the same information. It samples the qualified event counter, which can be configured to record the Ar, Xe or the sum of both sets of detectors, at various rates typically between 1 and 31 msec. MHTR5 gives a 4 bit intensity sample (0-15) rather than an 8-bit sample (0-255) thereby reducing by a half the telemetry usage. This means that the maximum time resolution at which MHTR5 can be operated is 1 msec for 60% telemetry usage compared to 2 msec with MHTR3. At 2 msec time resolution MHTR5 uses 30% of available telemetry etc. if more than 15 qualified events are detected during the sampling interval the program sets the value 15 which therefore indicates 15 or more counts. For long integration intervals, especially if Xe is being sampled, it is sometimes advisable to use MHTR3 to avoid overflows. The deadtime associated with the qualified event counter is small and can be ignored for most purposes.

A.N. Parmar

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