10.1.1 Design

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 high-density polyethylene (HDPE) window, followed by near-infrared blocking filters to define each bandpass and lenses designed to optimize the plate scale, then the cold pupil on a rotatable carousel. The pupil carousel contains eight aperture positions, four of which contain half wave plates (HWPs) for HAWC+ bands, an open aperture whose diameter is matched to the SOFIA pupil, and three aperture options meant only for instrument alignment tests.

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. The grid directs the two components (often referred to as the R and T components) to the two detector arrays and the grid itself is heat-sunk to the HAWC+ 1 Kelvin stage

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.  Alternatively, unpolarized intensity can be measured without polarization by removing the HWPs from the optical path (utilizing the open pupil position) and simply summing the polarization states.

While HAWC+ is designed for two 64x40 detector arrays, one full array (for R) and one half array (for T) are currently installed.  The 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.

Schematic of the HAWC+ optics

Figure 10-1: Schematic of the HAWC+ optics. Light from the SOFIA telescope is incident from the right. The field and folding mirrors are mounted on the HAWC+ cryostat but extend into the SOFIA Nasmyth tube. At the polarizing grid the light is split into two orthogonal components of linear polarization and detected at the two separate arrays.

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