Keck Array

Keck Array
Name	Keck Array
Location	South Pole Telescope site, Amundsen–Scott South Pole Station
Altitude	2,835 m
Established	2010
Operational	2011–present
Wavelength	Microwave (CMB, ~90–270 GHz)
Diameter	Multiple 26 cm refracting telescopes
Detectors	Transition-edge sensor bolometers

Keck Array is a ground-based cryogenic microwave polarimeter designed to measure polarization anisotropies in the cosmic microwave background (CMB). The instrument targeted primordial B-mode polarization to test models of cosmic inflation and complement measurements from experiments such as BICEP2, Planck (spacecraft), and WMAP. Deployed at the Amundsen–Scott South Pole Station, the Keck Array formed part of a global effort including facilities like the South Pole Telescope and collaborations with institutions such as Harvard University, Caltech, and the Jet Propulsion Laboratory.

Overview

The Keck Array comprised multiple compact refracting telescopes mounted on a single azimuthal mount optimized for low-foreground, deep-field observations of the southern sky near the southern celestial pole. Its scientific goals linked to theoretical frameworks from Alan Guth and Andrei Linde on inflation, and observational programs tied to datasets produced by Planck Collaboration, BICEP/Keck Collaboration, and the ACT (Atacama Cosmology Telescope). Operations at the Amundsen–Scott South Pole Station provided access to a stable atmosphere and long integration times comparable to other polar facilities like Dome C and the Atacama Desert sites used by POLARBEAR and Simons Observatory. The instrument architecture and observing campaigns were influenced by earlier experiments including DASI, CBI, and MAXIMA.

Instrument Design and Detectors

Design elements included multiple 26 cm aperture refractors employing silicon and alumina optics, cryogenic cooling stages developed with expertise from NASA JPL and laboratory groups at University of Chicago and University of California, Berkeley. Detector arrays used transition-edge sensor (TES) bolometers read out by superconducting quantum interference device (SQUID) multiplexers pioneered in programs at NIST and SRON Netherlands Institute for Space Research. Frequency bands around 95 GHz, 150 GHz, and 220 GHz enabled foreground separation against polarized emission from Galactic Center regions, Large Magellanic Cloud, and diffuse dust characterized by missions like IRAS and Herschel Space Observatory. Polarization modulation schemes drew on polarization analysis techniques from POLARBEAR and instrument characterization relied on calibration sources such as Jupiter and the Crab Nebula. Cryogenics incorporated pulse-tube coolers and dilution refrigerators similar to systems used at CERN detector labs and Brookhaven National Laboratory facilities.

Observing Strategy and Site

Stationed at the Amundsen–Scott South Pole Station, the Keck Array executed continuous scans of low-foreground fields overlapping with the BICEP footprint near the southern celestial pole and regions observed by Planck (spacecraft) and SPTpol. The observing strategy emphasized deep integration on selected fields to beat down sample variance and instrumental noise, coordinated with seasonal campaigns and logistics managed by Antarctic Support Contractor teams and the National Science Foundation (United States). The polar environment enabled stable precipitable water vapor conditions similar to high-elevation sites like Cerro Tololo and Chajnantor Plateau, reducing atmospheric contamination that affects mid-latitude observatories including Mauna Kea Observatories.

Data Processing and Analysis

Data reduction pipelines combined time-ordered data cleaning, map-making, and power-spectrum estimation. Teams applied component-separation methods informed by algorithms used by Planck Collaboration, foreground modeling referencing studies of interstellar medium polarization and dust physics associated with E. R. Louis and A. Kogut-style analyses. Statistical inference employed likelihood frameworks and Markov chain Monte Carlo techniques widely adopted in cosmology by groups working on CosmoMC and CLASS codes. Cross-correlation analyses compared Keck Array maps with datasets from BICEP2, Planck, SPTpol, and ACT to quantify lensing B-modes from large-scale structure described by Lewis & Challinor formalism and to constrain tensor-to-scalar ratio parameters in inflationary models formulated by Starobinsky and Mukhanov.

Scientific Results

Keck Array results contributed to increasingly stringent upper limits on primordial gravitational waves expressed via the tensor-to-scalar ratio r, refining constraints initially suggested by the BICEP2 announcement and revised after joint analyses with Planck Collaboration. The instrument measured degree- and sub-degree-scale B-mode power consistent with expectations from gravitational lensing by large-scale structure characterized in studies by Lewis & Challinor and provided polarization spectra that improved understanding of polarized galactic dust emission modeled by teams including Finkbeiner, Davis & Schlegel. Keck Array data were incorporated into joint likelihoods used to bound inflationary parameter space in combination with temperature and polarization results from WMAP and Planck (spacecraft), influencing theoretical interpretation in works by Guth, Linde, and Albrecht & Steinhardt.

Collaborations and Funding

The Keck Array was developed and operated by the BICEP/Keck Collaboration, a consortium including researchers from Caltech, Harvard University, Stanford University, University of Minnesota, University of British Columbia, Princeton University, University of Chicago, and other institutions. Funding and logistical support were provided by agencies such as the National Science Foundation (United States), with technical contributions from NASA laboratories and academic cleanroom facilities at partner universities. International collaboration and data-sharing involved coordination with teams from European Space Agency, SRON, and observatory operations at the South Pole Telescope site.

Category:Cosmic microwave background experiments Category:Telescopes in Antarctica Category:Polarimetry experiments