The Primordial Inflation Polarization Explorer (PIPER): optical design

The balloon-borne PIPER instrument will observe the polarization in the cosmic microwave background (CMB) at 200, 270, 350, and 600 GHz. Two co-pointed telescopes are placed inside a 3000 L liquid helium dewar and cooled to 1.5 K. The telescopes are arranged so that one measures Stokes parameters Q and V while the other measures U and V . Each telescope consists of a variable-delay polarization modulator (VPM) as the primary optical element, two off-axis mirrors, a folding flat, and re-imaging optics with off-axis lenses that focus each of the orthogonal linear polarization signals through an analyzer grid and onto two bolometer arrays (one for each polarization state). A cold Lyot stop is imaged onto the VPM to define the entrance pupil of the telescope. Each telescope has a 6° × 4.7° field-of-view.

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