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The radiation will be re-imaged onto the three detector arrays using a pair of mirrors arranged in a "Gaussian beam telescope" configuration (see figure at right), which corrects for aberrations in the main telescope, and provides a flat focal plane.
The second conic mirror in this configuration has a focal length equal to its distance from the aperture, ensuring that all of the detectors in the array have their illumination centered on the Lyot stop. The off-axis angle of the final re-imaging mirror allows us to place dichroic beam splitters in front of the focal plane, making possible simultaneous measurements by three arrays operating at 250, 350, and 500 μm.
The duration of flight demands that the cryostat operate for at least 14 days with the detectors at 300 mK. This requires 37 liters of liquid helium, a separate miniature pumped 4He pot, and a 3He recycling refrigerator.
The receiver consists of an optical cavity inside a long-hold-time liquid nitrogen and liquid helium cryostat.