Atacama Cosmology Telescope (ACT)

The Atacama Cosmology Telescope (ACT) is a six-meter diameter telescope on Cerro Toco in the Atacama Desert of northern Chile (image above by Jon Ward). It is designed to make high-resolution measurements of the cosmic microwave background (CMB) temperature and polarization anisotropies and detect massive galaxy clusters via the thermal Sunyaev Zel'dovich (tSZ) effect. Reconstruction of the CMB lensing potential will play a key role in the cosmological detection of neutrino mass, and measuring the imprint of primordial gravitational waves in B-mode polarization could provide unique insights into the early universe and quantum nature of gravity.


A first-generation receiver, the Millimeter Bolometric Array Camera (MBAC), mapped the CMB temperature anisotropies over an area of around 1000 square degrees from 2008 to 2010 with spectral bands centered at 148, 218 and 270 GHz and an angular resolution of 1.4 arcminutes at 148 GHz. Science highlights from these observations include:

  • The first measurement of the power spectrum of the CMB lensing potential (Das et al. 2012), providing evidence for dark energy from the CMB alone (Sherwin et al. 2012)
  • The first statistical detection of the kinematic Sunyaev Zel'dovich effect (Hand et al. 2012)
  • Measurements of primary and secondary CMB temperature anisotropy power over 600 square degrees (Sievers et al. 2013)
  • A catalog of tSZ-selected galaxy clusters and associated cosmological constraints (Hasselfield et al. 2013)
  • Detection of the tSZ effect in 1013 solar mass halos by stacking with FIRST and NVSS catalogs (Gralla et al. 2013)
  • Discovery of El Gordo, an extreme high-redshift merging galaxy cluster (Menanteau et al. 2012).


The second-generation, polarization-sensitive receiver ACTPol has observed since 2013. Four regions of sky covering a total area of 270 square degrees were mapped at 146 GHz with an angular resolution of 1.3 arcminutes over three months in 2013. Science highlights from these initial observations include:

  • Measurement of the TE and EE power spectra (Naess et al. 2014)
  • Reconstruction of the CMB lensing potential using temperature and polarization data in cross-correlation with the
    cosmic infrared background (van Engelen et al. 2014).
A service of the HEASARC and of the Astrophysics Science Division at NASA/GSFC
Goddard Space Flight Center, National Aeronautics and Space Administration
HEASARC Director: Dr. Alan P. Smale
LAMBDA Director: Dr. Eric R. Switzer
NASA Official: Dr. Eric R. Switzer
Web Curator: Mr. Michael R. Greason