Atacama Cosmology Telescope
|Part of||Llano de Chajnantor Observatory|
|Wavelength||28, 41, 90, 150, 220 GHz (1.07, 0.73, 0.33, 0.20, 0.14 cm)|
|First light||22 October 2007|
|Telescope style||cosmic microwave background experiment|
|Related media on Commons|
The Atacama Cosmology Telescope (ACT) is a six-meter diameter telescope located on Cerro Toco in the Atacama Desert in the north of Chile, near the Llano de Chajnantor Observatory. ACT makes high-sensitivity, high-resolution (arcminute angular scales), microwave-wavelength surveys of the sky in order to study the cosmic microwave background radiation (CMB), the relic radiation left by the Big Bang process. At an altitude of 5,190 metres (17,030 ft), it is one of the highest permanent, ground-based telescopes in the world.[a]
High sensitivity observations of the cosmic microwave background radiation allow precision measurements of cosmological parameters, detection of galaxy clusters among other scientific goals, probing the early and late stages in the history of the evolution of the universe.
Erected in the (austral) autumn of 2007, ACT saw first light on 22 October 2007 with its science receiver, the Millimeter Bolometer Array Camera (MBAC). ACT has had two major receiver upgrades which enabled polarization sensitive observations: ACTPol (2013-2016) and Advanced ACT (2017-). ACT is funded by the US National Science Foundation.
Water vapor in the atmosphere emits microwave radiation which contaminates measurements of the CMB, for this reason CMB telescopes benefit from arid, high-altitude locations. ACT is located in the dry and high (yet easily accessible) Chajnantor plateau in the Andean mountains in the Atacama Desert in northern Chile. Due to the exceptional observing conditions of the Atacama Desert and its accessibility by road and nearby ports, several other observatories are located in the region, including CBI, ASTE, Nanten, APEX and ALMA. These astronomical observatories and telescopes form the Llano de Chajnantor Observatory, a cluster of astronomical telescopes primarily in millimeter and sub-millimeter wavelengths.
The ACT is an off-axis Gregorian telescope. This off-axis configuration is beneficial to minimize artifacts in the point spread function. The telescope reflectors consist of a six-metre (236 in) primary mirror and a two-metre (79 in) secondary mirror. Both mirrors are composed of segments, consisting of 71 (primary) and 11 (secondary) aluminum panels. These panels follow the shape of an ellipsoid of revolution and are carefully aligned to form a joint surface. Unlike most telescopes which track the rotating sky during observation, the ACT observes the sky by keeping the telescope oriented at a constant elevation and by scanning back and forth in azimuth at the relatively rapid rate of two degrees per second. The rotating portion of the telescope weighs approximately 32 tonnes (35 short tons), creating a substantial engineering challenge. A ground screen surrounding the telescope blocks contamination from microwave radiation emitted by the ground. The design, manufacture and construction of the telescope were done by Dynamic Structures in Vancouver, British Columbia.
ACT can accommodate three instrument cameras simultaneously. Over time these cameras have been upgraded from the original MBAC design to the current Advanced ACT instrument progressively adding more features like polarization sensitivity and the ability to sense multiple frequencies in one instrument module. Each camera in ACT consists of a three lens system, the Gregorian focus is reimaged into a detector focal plane, a Lyot stop reimages the primary mirror allowing stray light mitigation.
The three lenses in ACT are made of cryogenically cooled anti-reflection coated silicon, a desirable material for instruments in the millimeter due to its high index of refraction (n=3). Anti-reflection coatings in ACTPol and AdvACT are made of sub-wavelength structured metamaterial silicon, an innovation in ground based CMB telescopes at the time. The optical components and the detector module are kept at a vacuum with a plastic window. A stack of filters reject infra-red radiation which is detrimental for mm-wavelength observations.
Radiation is thermally coupled to transition-edge sensor bolometers, which are read out using an array of SQUIDs.
Observations are made at resolutions of about an arcminute (1/60th of a degree) in three frequencies: 145 GHz, 215 GHz and 280 GHz. Each frequency is measured by a 3 cm × 3 cm (1.2 in × 1.2 in), 1024 element array, for a total of 3072 detectors. The detectors are superconducting transition-edge sensors, a technology whose high sensitivity allows measurements of the temperature of the CMB to within a few millionths of a degree. A system of cryogenic helium refrigerators keep the detectors a third of a degree above absolute zero.
ACT has had three generations of cameras. Each camera is the result of the development of specialized detector technology which has been optimized through the years. These cameras take advantage of superconducting transition edge sensor arrays to achieve high sensitivity.
The first array of cameras to populate the ACT focal plane (MBAC) consisted of three cameras where each one was sensitive to its own band and had no polarization sensitivity. The second generation of cameras (ACTPol) added polarization sensitivity and the first camera to be sensitive to two bands (dichroic). The third generation of cameras (AdvACT) incorporated the advances achieved in ACTPol, which allowed all cameras to be sensitive to two bands.
|Phase||Arrays||Freq. (GHz)||Sens. (µK√s)||Pol.||Years||Patches|
|ACTPol||pa1||150||17-29||Yes||2013-2015||D2 D5 D6 D56 D8 BN|
|AdvACT||pa4||150||18.2||Yes||2017-2021||AA Day‑N Day‑S|
Measurements of cosmic microwave background radiation (CMB) by experiments such as COBE, BOOMERanG, WMAP, CBI and many others have greatly advanced our knowledge of cosmology, particularly the early evolution of the universe. The South Pole Telescope is a similar, but complementary, telescope.
ACT released results measuring the statistical properties of the temperature of the CMB down to arcminute scales in January 2010. It found signals that were consistent with un-resolved point sources and the SZ effect. In 2011, ACT made the first detection of the power spectrum of gravitational lensing of the microwave background, which, combined with the WMAP results, for the first time provided evidence for dark energy from the CMB alone. Measurements of the CMB power spectrum from the South Pole Telescope were subsequently released  that were later also shown to provide evidence for dark energy from the CMB alone.
ACT has collaborators at Princeton University, Cornell University, the University of Pennsylvania, NASA/GSFC, the Johns Hopkins University, the University of British Columbia, NIST, the Pontificia Universidad Católica de Chile, the University of KwaZulu-Natal, Perimeter Institute for Theoretical Physics, the Canadian Institute for Theoretical Astrophysics, Stanford University, Stony Brook University, Cardiff University, Argonne National Laboratory, Haverford College, Rutgers University, the University of Pittsburgh, UC Berkeley, University of Southern California, the University of Oxford, the University of Paris-Saclay, University of Illinois at Urbana-Champaign, SLAC National Accelerator Laboratory, Caltech, McGill University, the Center for Computational Astrophysics, Arizona State University, Columbia University, Carnegie Mellon University, the University of Chicago, Haverford College, Florida State University, West Chester University, Yale University, and the University of Toronto. 
- BICEP and Keck Array
- Llano de Chajnantor Observatory
- List of highest astronomical observatories
- Lists of telescopes
- Fowler, J. W.; Niemack, M. D.; Dicker, S. R.; Aboobaker, A. M.; Ade, P. A. R.; Battistelli, E. S.; Devlin, M. J.; Fisher, R. P.; Halpern, M.; Hargrave, P. C.; Hincks, A. D. (10 June 2007). "Optical design of the Atacama Cosmology Telescope and the Millimeter Bolometric Array Camera". Applied Optics. 46 (17): 3444–3454. arXiv:astro-ph/0701020. Bibcode:2007ApOpt..46.3444F. doi:10.1364/AO.46.003444. ISSN 0003-6935. PMID 17514303. S2CID 10833374.
- Marrone; et al. (2005). "Observations in the 1.3 and 1.5 THz Atmospheric Windows with the Receiver Lab Telescope". Sixteenth International Symposium on Space Terahertz Technology: 64. arXiv:astro-ph/0505273. Bibcode:2005stt..conf...64M.
- Niemack, M. D.; Ade, P. A. R.; Aguirre, J.; Barrientos, F.; Beall, J. A.; Bond, J. R.; Britton, J.; Cho, H. M.; Das, S.; Devlin, M. J.; Dicker, S. (16 July 2010). "ACTPol: a polarization-sensitive receiver for the Atacama Cosmology Telescope". In Holland, Wayne S.; Zmuidzinas, Jonas (eds.). Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy V. Vol. 7741. San Diego, California, USA. pp. 537–557. arXiv:1006.5049. doi:10.1117/12.857464. S2CID 27705474.
- Henderson, S. W.; Allison, R.; Austermann, J.; Baildon, T.; Battaglia, N.; Beall, J. A.; Becker, D.; De Bernardis, F.; Bond, J. R.; Calabrese, E.; Choi, S. K. (1 August 2016). "Advanced ACTPol Cryogenic Detector Arrays and Readout". Journal of Low Temperature Physics. 184 (3): 772–779. arXiv:1510.02809. Bibcode:2016JLTP..184..772H. doi:10.1007/s10909-016-1575-z. ISSN 1573-7357. S2CID 53411729.
- Fowler, J.; et al. (2007). "Optical Design of the Atacama Cosmology Telescope and the Millimeter Bolometric Array Camera". Applied Optics. 46 (17): 3444–54. arXiv:astro-ph/0701020. Bibcode:2007ApOpt..46.3444F. doi:10.1364/AO.46.003444. PMID 17514303. S2CID 10833374.
- Fowler, A.; et al. (ACT Collaboration) (2010). "The Atacama Cosmology Telescope: A Measurement of the 600 < >>>ℓ < 8000 Cosmic Microwave Background Power Spectrum at 148 GHz". The Astrophysical Journal. 722 (2): 1148–1161. arXiv:1001.2934. Bibcode:2010ApJ...722.1148F. doi:10.1088/0004-637X/722/2/1148. S2CID 8882912.
- Das, S.; et al. (ACT Collaboration) (2011). "The Atacama Cosmology Telescope: Detection of the Power Spectrum of Gravitational Lensing". Physical Review Letters. 107 (2): 021301. arXiv:1103.2124. Bibcode:2011PhRvL.107b1301D. doi:10.1103/PhysRevLett.107.021301. PMID 21797590. S2CID 16368279.
- Sherwin, B. D.; et al. (ACT Collaboration) (2011). "The Atacama Cosmology Telescope: Detection of the Power Spectrum of Gravitational Lensing". Physical Review Letters. 107 (2): 021302. arXiv:1105.0419. Bibcode:2011PhRvL.107b1302S. doi:10.1103/PhysRevLett.107.021302. PMID 21797591. S2CID 13981963.
- Keisler, R.; et al. (SPT Collaboration) (2011). "A Measurement of the Damping Tail of the Cosmic Microwave Background Power Spectrum with the South Pole Telescope". Astrophysical Journal. 743 (1): 28. arXiv:1105.3182. Bibcode:2011ApJ...743...28K. doi:10.1088/0004-637X/743/1/28. S2CID 46121987.
- van Engelen, A.; et al. (SPT Collaboration) (2012). "A measurement of gravitational lensing of the microwave background using South Pole Telescope data". Astrophysical Journal. 756 (2): 142. arXiv:1202.0546. Bibcode:2012ApJ...756..142V. doi:10.1088/0004-637X/756/2/142. S2CID 39214417.
- "ACT public webpage".