Physical Review Letters set a limit on the tensor-to-scalar ratio of . The BICEP2 affair forms the subject of a book by Brian Keating. Keck Array Immediately next to the BICEP telescope at the Martin A. Pomerantz Observatory building at the South Pole was an unused telescope mount previously occupied by the Degree Angular Scale Interferometer. The Keck Array was built to take advantage of this larger telescope mount. This project was funded by $2.3 million from W. M. Keck Foundation, as well as funding from the National Science Foundation, the Gordon and Betty Moore Foundation, the James and Nelly Kilroy Foundation and the Barzan Foundation. The Keck Array project was originally led by Andrew Lange. The Keck Array consists of five polarimeters, each very similar to the BICEP2 design, but using a pulse tube refrigerator rather than a large liquid helium cryogenic storage dewar. The first three started observations in the austral summer of 2010–11; another two started observing in 2012. All of the receivers observed at 150 GHz until 2013, when two of them were converted to observe at 100 GHz. Each polarimeter consists of a refracting telescope (to minimise systematics) cooled by a pulse tube cooler to 4 K, and a focal-plane array of 512 transition edge sensors cooled to 250 mK, giving a total of 2560 detectors, or 1280 dual-polarization pixels. In October 2018, the first results from the Keck Array (combined with BICEP2 data) were announced, using observations up to and including the 2015 season. These yielded an upper limit on cosmological B-modes of (95% confidence level), which reduces to in combination with Planck data. In October 2021, new results were announced giving (at 95% confidence level) based on BICEP/Keck 2018 observation season combined with Planck and WMAP data. BICEP3 Once the Keck array was completed in 2012, it was no longer cost-effective to continue to operate BICEP2. However, using the same technique as the Keck array to eliminate the large liquid helium dewar, a much larger telescope has been installed on the original BICEP telescope mount. BICEP3 consists of a single telescope with the same 2560 detectors (observing at 95 GHz) as the five-telescope Keck array, but a 68 cm aperture, providing roughly twice the optical throughput of the entire Keck array. One consequence of the large focal plane is a larger 28° field of view, which will necessarily mean scanning some foreground-contaminated portions