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A schematic of the HAWC+ optical design is shown in Figure 10-1. Light enters the set of warm fore-optics mounted outside the cryostat, reflecting from a folding mirror and a field mirror that images the SOFIA pupil at the cold pupil inside the HAWC cryostat. After the fore-optics, light enters the cryostat through a 7.6 cm (3.0 in) diameter HDPE window, followed by near-infrared blocking filters, then the cold pupil on a rotatable carousel. The pupil carousel contains eight aperture positions, four of which contain HWPs, one with a clear aperture whose diameter is matched to the SOFIA pupil, and three other aperture options meant only for instrument alignment tests. Light then passes through a stack of band-pass filters and plate-scale-defining lenses mounted on a six position movable carousel; filters and lenses for the five bandpasses are mounted here.
The pupil carousel and filter/lens wheel are at an approximate temperature of 10 K. After this point the light passes through a wire grid that reflects one component of linear polarization and transmits the orthogonal component. This grid is heat-sunk to the HAWC+ 1 Kelvin stage. The grid directs the two components (often referred to as the R and T components) to the two detector arrays.
Each of the two 64x40 HAWC+ detector arrays is composed of two co-mounted 32x40 subarrays from NASA/GSFC and NIST. The detectors are superconducting transition-edge sensor (TES) thermometers on membranes with wide-band absorber coating. The detector array is indium bump bonded to a matched array of superconducting quantum interference device (SQUID) amplifiers, all cooled to a base temperature of ~0.1- 0.2 K.
The optical design allows HAWC+ to measure solely the total unpolarized intensity, or simultaneously the polarized intensity. In both cases the incoming radiation is split into two orthogonal polarizations by the wire grid and measured simultaneously by the two detector arrays. In the case of polarimetry, the half-wave plate (HWP) matched to the band-pass is selected and rotated to modulate the incident polarization states. The total intensity is given simply by summing the signal in the two orthogonal arrays. Alternatively, unpolarized intensity can be measured without polarization by removing the HWPs from the optical path (utilizing the open pupil position). The resulting sensitivity improvement is not yet measured, but is expected to be small.