The proposed SPT-3G+ receiver is designed to make sky maps at a rate nearly an order of magnitude higher than the current SPT-3G instrument, according to a new arXiv preprint. The receiver is still being developed, so that figure is a future design target rather than a measured result.
A large, tightly packed instrument
The design calls for 14 detector arrays with 90/150 GHz dichroic, polarization-sensitive pixels and 24,080 transition-edge sensor detectors in total. The arrays would sit at the ends of optics tubes about 240 millimeters wide and 772 millimeters long, arranged in a close-packed hexagonal layout aimed at a 4-degree field of view.
To fit the South Pole Telescope cabin, the cryostat uses three shells: a 300 K vacuum shell and radiation shields held at 40 K and 4 K. It is designed to house the 14 optics tubes while preserving optical quality and keeping blockage at the edges of the view negligible.
The package is described as 1,550 kilograms, with a 1.1-meter vacuum window and a height and length of 1.75 meters. Its cooling plan pairs pulse-tube refrigeration at 40 K and 4 K with a dilution refrigerator at 1 K and 100 millikelvin.
The cold stages leave modeled room
Thermal-budget modeling puts the predicted total load at 25.8 W on the 40 K stage, 0.72 W at 4 K, 5.3 mW at 1 K and 29.8 µW at 100 mK. The listed cooling capacities are 55 W, 2 W, 15 mW and 250 µW respectively, leaving modeled capacity above the load at every stage.
Computer modeling predicts that the L1 lenses will operate closer to 50 K even though the first radiation shield is at 40 K. A separate structural analysis predicts roughly 6.5 millimeters of vacuum-shell deflection, while retaining about 10 millimeters of clearance before the first optics-tube element; the paper says this would not significantly affect the optical design.
Thousands of detectors, fewer cables
The cryogenic readout uses microwave SQUID multiplexing, a scheme designed to place nearly 1,000 detectors on a single radio-frequency line. For 14 detector wafers, the design requires 28 RF input and output lines and more than 900 direct-current lines through the cryogenic stages.
The optics also serve a cooling purpose. Alumina, silicon and nylon lenses are intended to filter infrared radiation and reduce the radiative load reaching the cold stages.
Ambitious targets still await testing
The authors connect the proposed design to a mapping speed nearly an order of magnitude above current SPT-3G and to a future measurement target of σ(r) = 0.001. Those are prospective goals for the receiver, not results already obtained.
The planned schedule calls for cryostat shells by the end of 2026, optics-tube component testing at the start of 2027, integration during 2027 and installation on the South Pole Telescope in the 2028–29 austral summer before the 2029 winter observing season.
The document is an arXiv version 1 preprint dated 20 August 2026. Its thermal, mechanical and optical figures are predictions from the proposed design, leaving laboratory and commissioning tests to establish how closely the finished receiver matches them.
Paper data and sources
Original title: The SPT-3G+ receiver design
Authors: H. Athreya, Z. Ahmed, J. E. Austermann et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-20
DOI: Not available
Original paper · Full text