The instrument

In order to separate different sources, an indirect imaging technique similar to STIX is adopted.  A pair of parallel grids is placed in front of each detector.  Their shadows, projected on the sensor, produce a moiré pattern which is sampled with a relatively coarse pixelization.  This is sufficient to measure the phase and amplitude of the moiré pattern, which then are used to estimate the source size and direction with an angular resolution much smaller than the pixel size.

In order to reduce systematic uncertainties, MiSolFA will measure the photon spectrum adopting CdTe detectors as STIX.  CEA Saclay, where the Caliste-SO units of STIX have been developed, has new detectors of the same family, called Caliste-HD and Caliste-O, with 16×16 pixels with different size (625 µm and 800 µm).  In addition, a brand new detector called MC2 is under development, with finer pixels (250µm). With respect to the coarse pixels of STIX, these detectors offer a better energy resolution and a much better spatial resolution, which makes them more robust against possible deviations from the nominal moiré patterns produced by the imager.  Similarly to STIX, weak radioactive sources will be used to constantly monitor the energy calibration.

Detector data are collected by the Detector Interface Units, equipped with a very fast FPGA under development at NASA Goddard, that outputs single-hit information consisting of channel identifier, energy (in ADC counts) and time stamp.  Hits are then handled by the Data Processing Unit, under development at CBK Warsaw, which during a flare populates a memory buffer, to be processed later by a CPU that will compute low-latency coarse results (to be used e.g. to send a flare alert to the space weather community) and will take care of data compression and formatting.

The main technological challenges of the instrument is the grid production and the development of the new MC2 detectors, although nothing is actually simple in space.  Because the grids distance is significantly shorter than in STIX, achieving a similar angular resolution is only possible with significantly smaller periods, and this implies that a different fabrication technique is necessary.  A study is currently in progress to develop grids with the necessary small periods.  In parallel, lot of work is ongoing on the detector side.  Check the news to stay tuned!

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