The Front-End Assembly contains the detectors, two of the filters, and the first stage of amplifiers.
The job of the FEA is to support the detectors in the correct position (at the
focus of the X-ray mirrors) while conducting a minimum of heat to them. It also
houses some electronics, including the first stage of amplification.
Here is a photo of the detectors mounted in the Flight Model FEA. The central section (with the detectors) is kept at 60mK. It is attached to the outer mounting box (which runs at 1.3 Kelvin) by thin Kevlar strands at the corners. The Kevlar supports are just visible in this photo, but are clearer in the larger version.
The electrical connections to the detectors are made by the rows of wires on the near and far edges. The gold wires are actually copper-beryllium springs which keep the wires taut. In this photo, they have not yet been soldered to the external connections.
Here is the FEA with the Calorimeter Thermal Sink (CTS) lid, including the coldest of the five filters
between the outside world and the detectors. This
filter is at the temperature of the detectors (60 mK), to minimize the amount of thermal radiation
it emits.
The CTS lid also contains one radioactive calibration source. This is a small piece of 55Fe, which produces 6 keV X-rays aimed at the dedicated calibration pixel. These X-rays have a fixed (and well-known) energy, and can be used to help determine how to convert the measured pulseheight to the energy of the incoming X-rays.
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