An arXiv preprint lays out SPT-3G+, a planned receiver for the South Pole Telescope, and forecasts that it could make cosmic microwave background (CMB) maps at a mapping speed nearly an order of magnitude larger than the current SPT-3G system. The paper is a design and methods study, not a report of observations from a deployed SPT-3G+ receiver. Its central targets include a six-year survey overlapping BICEP, a combined 90/150 GHz map depth of 0.5 µK-arcmin, improved lensing reconstruction, delensing, galaxy-cluster searches and approximately daily monitoring of transient sources across the planned field.
SPT-3G+ is described as the next survey receiver, planned for installation in early 2029 on the 10-meter South Pole Telescope. The proposed survey would cover 800 square degrees for six years. In the introduction, the authors set a separate target of 0.7 µK-arcmin temperature depth at both 90 and 150 GHz over that area.
A large receiver for a focused patch of sky
The planned detector array would contain 6,020 polarization-sensitive dichroic pixels and 24,080 detectors operating in the 90 GHz and 150 GHz bands. The detectors are designed to operate at 100 mK. Those specifications describe the scale of the proposed focal plane and the frequency coverage used for the survey forecasts.
Its optical design would give the receiver a 4-degree-diameter field of view divided among 14 optics tubes. The tubes would contain cryogenic lenses made from alumina, silicon and nylon. Together, the field and tube arrangement define how the planned receiver would view the survey area.
The readout plan is similarly detailed. Each wafer would use 1,756 TES bolometers, including 1,720 optical bolometers and 36 dark detectors. The design calls for two 910-channel multiplexers operating in the 4–6 GHz band, with 300 K SMuRF electronics. These are planned hardware specifications, rather than measurements from a completed instrument.
A full focal-plane-module prototype is currently in production. The prototype status reinforces the paper’s stage of development: the receiver is being described through engineering choices and performance calculations, while prototype and packaging decisions remain subject to laboratory or commissioning measurements.
The two map-depth figures in the paper should not be treated as identical. The abstract refers to a combined 90/150 GHz depth of 0.5 µK-arcmin after the six-year overlapping survey. The introduction instead gives a 0.7 µK-arcmin temperature-depth target at each of the two bands over 800 square degrees. The different wording indicates that the figures refer to different reported metrics.
What the models predict
The mapping-speed result comes from a model of the receiver’s noise performance. Mapping speed is a way of describing how quickly an instrument can build a map at a given noise level. The authors calculated the noise-equivalent temperature of each observing band with the publicly available jbolo software, which is based on BoloCalc. The resulting figures therefore depend on the planned design and the assumptions used in the calculation.
On that basis, the paper forecasts CMB mapping speed nearly an order of magnitude larger than SPT-3G. That is an expected improvement in survey efficiency, not a measured gain from an installed SPT-3G+ receiver.
The paper also models the reconstruction of gravitational lensing in the CMB maps. Its lensing-noise forecast uses an iterative algorithm, with temperature modes included up to ℓmax = 3,500 and polarization modes up to ℓmax = 4,000. For scales labelled L < 500, the forecast gives SPT-3G+ lensing signal-to-noise per mode at approximately nine times the level of the Simons Observatory comparison.
Those lensing maps feed into a delensing forecast. Delensing means removing lensing B modes from the polarization data before estimating the inflationary signal. The paper forecasts a σ(r) limit near 0.001 using data through 2034 after removing those lensing B modes. The figure is a projected limit, not a measured tensor-to-scalar ratio or a reported detection of inflationary B modes.
For galaxy clusters, the forecast uses a tSZ selection at a signal-to-noise ratio of 5 and a purity threshold greater than 99.5%. It reaches a mass threshold of M500c ≥ 8 × 10^13 M⊙ near z ≈ 2, and predicts twice as many clusters per square degree as SPT-3G. These figures describe the expected reach of the planned survey rather than an observed cluster catalogue.
Transient monitoring is another part of the proposed survey. The paper forecasts approximately daily coverage of the full 800-square-degree field, together with a factor-of-several improvement in instantaneous sensitivity, for monitoring and discovering transient sources. The cadence is presented as a planned capability of the survey design.
A blueprint, not a completed result
The paper’s scientific case rests on an anticipated receiver and a forecast survey. It reports engineering specifications and model-based performance, with SPT-3G as the main comparison for mapping and cluster projections and the Simons Observatory baseline used for the selected lensing comparison. No completed SPT-3G+ observing dataset is reported.
The performance numbers are presented as forecasts or targets, and the supplied analysis reports no uncertainty intervals or statistical error bars for the mapping-speed, lensing, delensing, cluster or transient projections. That makes the figures useful for describing the intended scale of the project, but not for establishing how closely the final receiver will match them.
The distinction matters especially for the proposed σ(r) result. A limit near 0.001 after lensing B-mode removal is conditional on the planned survey, the assumed instrument performance and the forecast reconstruction. It does not show that the experiment has already measured that limit, detected inflationary B modes or established a tensor-to-scalar ratio.
The engineering work will also have to be tested under final operating conditions. The paper reports a focal-plane prototype in production, while the wider design includes detector cooling at 100 mK, multiplexed readout, cryogenic optics and the proposed 90 and 150 GHz detector bands. Laboratory, commissioning and on-sky measurements will determine whether those choices deliver the forecast map, lensing, cluster and transient performance.
For now, the result is a detailed plan for what SPT-3G+ is intended to do: map a deep, BICEP-overlapping field, reconstruct CMB lensing, support delensing and search for clusters and changing sources. The preprint does not demonstrate the forecast map depth, mapping-speed gain, lensing signal-to-noise, cluster yield, transient discoveries or σ(r) constraint.
The supplied document is arXiv preprint version 1, dated 20 August 2026. Whether the receiver is installed as planned in early 2029, and whether its realized performance meets the paper’s forecasts, remain open questions.
Paper data and sources
Original title: SPT-3G+: A Cosmic Microwave Background Experiment for the South Pole Telescope
Authors: T. Natoli, Z. Ahmed, H. Athreya et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-20
DOI: Not available
Original paper · Full text