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View toward the Dark Sector Laboratory as seen through Keck Array at 150 GHz. The image shown uses data from a single detector only. Compare the image taken by a typical optical camera. A 150 GHz calibration source is attached to the top of the tall mast.
Keck Array Instrument

The design of each Keck Array receiver matches BICEP2 closely. Both BICEP2 and Keck Array use the same bolometric detector technology and optical design. The focal plane unit of each receiver houses 256 spatial pixels distributed across four photolithographed tiles. Each pixel is composed of two linearly polarized phased arrays of slot antennas, spatially coincident but orthogonal in polarization. The antenna signal is coupled to a titanium transition edge sensor (TES) held at approximately 270 mK. The TES signal is amplified by several SQUIDs and then read out using a time-domain multiplexing scheme.

The observing strategy has been chosen carefully to meet the science goals. Keck Array's small aperture results in a large, ~37 arcminute FWHM beam size to prevent point source and lensing contamination. This equates to a wide, ~30 square degree field of view per tile. Although the Antarctic atmosphere is mostly clean, dry, and stable, a relatively fast 2.8 degree/second scan speed helps to filter away atmospheric noise. Pairwise subtraction of the two polarized signals per pixel help to remove the remaining unpolarized atmospheric noise. Stokes parameters Q and U are measured by rotating all receivers in the mount around the boresight.

The main differences from BICEP2 are Keck Array's new cryogenic system and half-wave plates. The pulse tube refrigeration system will help reduce the expense of importing liquid helium to the South Pole. Two of the three receivers also have stepped rotating half-wave plates along the optical chain. These may be used for additional Q/U modulation and serve as a field test of the half-wave plate instrumentation to be installed in Spider.

Keck Array is an ambitious project. With the deployment of the first three receivers during the 2010-2011 austral summer, Keck Array has become the first telescope to have over one thousand polarized detectors trained on the CMB. Stay tuned for more receivers to be added and the first science data to become publicly available. The collaboration would like to thank the National Science Foundation and the W. M. Keck Foundation for their generous support.