PROCEEDINGS ARTICLE | July 10, 2018
Proc. SPIE. 10708, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy IX
KEYWORDS: Bolometers, Telescopes, Polarization, Cameras, Sensors, Superconductors, Physics, Receivers, Microwave radiation, Optical instrument design
(On behalf of the SPT-3G collaboration)
The SPT-3G receiver was commissioned in early 2017 on the 10-meter South Pole Telescope (SPT) to map the Cosmic Microwave Background (CMB) with arcminute resolution. The new instrument significantly improves on previous SPT camera designs, offering unprecedented sensitivity to the CMB’s temperature and polarization anisotropies while maintaining a tight control on systematic effects.
A restructured optics chain features new secondary and tertiary mirrors to increase the receiver’s field of view by a factor of four in area, coupling light through a set of four cryogenic 720 mm alumina elements, which re-image the Gregorian focus inside the receiver. The broad-band (70-270 GHz) anti-reflective coated lenses are paired with a Lyot stop and cooled to 4 K using a dedicated two-stage pulse-tube cooler (PTC) to minimize in-band attenuation, internal radiative loading, and stray light on the considerably larger focal plane, which is populated with ~2,700 diffraction-limited pixels.
Each independent pixel incorporates a lenslet-coupled broadband sinuous antenna, which routes signals through superconducting transmission lines and in-line band-defining filters to six transition-edge sensors, each measuring one of three bands (90, 150, and 220 GHz) and two polarizations, yielding a total of ~16,000 bolometers distributed across ten 150 mm monolithic wafers on a 430 mm image plane. A robust mechanical and thermal architecture supports and cools all detectors across this sizable focal plane to ~250 mK using a closed-cycle Helium adsorption refrigerator backed by a PTC, allowing a duty cycle greater than 90%.
This order of magnitude increase in detector count relies on an improved digital frequency-domain SQUID readout system which integrates superconducting inductor-capacitor resonators wired in series with each bolometer to achieve a 68x multiplexing factor while mitigating noise and crosstalk effects.
Cumulatively, SPT-3G’s new optics, detector, and readout strategies boast a potential factor of ~20 improvement in mapping speed over its predecessor, SPT-Pol, offering the potential to improve our understanding of inflationary physics, astroparticle physics, and growth of structure. We summarize the new receiver’s core design principles, outline technology characterization results, and report on plans for future upgrades.
