Specific HESSI instrument performance parameters are given in Table 2.

3.1 Data Acquisition

      The primary science data will be returned in event data packets, indicated in Table 3 along with other packet types. The contents of these packets include the time, energy and detector-segment identification of each detected event. The relative time resolution of 1 m s is sufficient that coincident events are indicated by identical time-tags, instead of conventional flags. Aspect data from SAS and RAS are included with sufficient time resolution that the instantaneous aspect associated with each detected event can be inferred.

      To help accommodate the large dynamic range expected in flare count rates as a function of time and energy, instrument logic will control the mechanical insertion of either or both of two x-ray attenuators between the lower grids and detectors. As an additional measure, in large flares, should the memory and/or data storage rates approach their limits, on-board instrument logic will apply increasingly stringent energy criteria to favor the retention of high energy photons, along with an unbiassed subset of the more numerous low energy photons. At exceptionally high rates, time-tagging of each detected event in the front segment detectors will be replaced by recording in Fast Rate Counters with sufficient time resolution to permit imaging, albeit with lower spectral resolution. In all cases, Monitor Rates with lower time resolution are used to provide an overview of detector performance and as input to support on-board decision-making.