J Low Temp Phys (2016) 184:559–567 DOI 10.1007/s10909-016-1573-1 The First Multichroic Polarimeter Array on the Atacama Cosmology Telescope: Characterization and Performance S. P. Ho 1 · C. G. Pappas 1 · J. Austermann 2 · J. A. Beall 2 · D. Becker 2 · S. K. Choi 1 · R. Datta 3 · S. M. Duff 2 · P. A. Gallardo 4 · E. Grace 1 · M. Hasselfield 1 · S. W. Henderson 4 · G. C. Hilton 2 · J. Hubmayr 2 · B. J. Koopman 4 · J. V. Lanen 2 · D. Li 2 · J. McMahon 3 · F. Nati 5 · M. D. Niemack 4 · P. Niraula 1 · M. Salatino 1 · A. Schillaci 1 · B. L. Schmitt 5 · S. M. Simon 1 · S. T. Staggs 1 · J. R. Stevens 4 · J. T. Ward 5 · E. J. Wollack 6 · E. M. Vavagiakis 4 Received: 9 October 2015 / Accepted: 1 March 2016 / Published online: 15 March 2016 © Springer Science+Business Media New York 2016 Abstract The Atacama Cosmology Telescope Polarimeter (ACTPol) is a polarization sensitive receiver for the 6-m Atacama Cosmology Telescope (ACT) and measures the small angular scale polarization anisotropies in the cosmic microwave background (CMB). The full focal plane is composed of three detector arrays, containing over 3000 transition edge sensors (TES detectors) in total. The first two detector arrays, observing at 146 GHz, were deployed in 2013 and 2014, respectively. The third and final array is composed of multichroic pixels sensitive to both 90 and 146 GHz and saw first light in February 2015. Fabricated at NIST, this dichroic array consists of 255 pixels, with a total of 1020 polarization sensitive bolometers and is coupled to the telescope with a monolithic array of broad-band silicon feedhorns. The detectors are read out using time-division SQUID multiplexing and cooled by a dilution refrigerator at 110 mK. We present an overview of the assembly and characterization of this multichroic array B S. P. Ho spho@princeton.edu 1 Department of Physics, Princeton University, Princeton, NJ 08540, USA 2 NIST Quantum Devices Group, 325 Broadway, Mailcode 817.03, Boulder, CO 80305, USA 3 Department of Physics, University of Michigan, Ann Arbor 48103, USA 4 Department of Physics, Cornell University, Ithaca, NY 14853, USA 5 Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104, USA 6 Goddard Space Flight Center, National Aeronautics and Space Administration, Greenbelt, MD 20771, USA 123