The Atacama Cosmology Telescope: The Receiver and Instrumentation

TL;DR

The paper provides a comprehensive description of the Atacama Cosmology Telescope and its Millimeter Bolometer Array Camera, detailing the optical design, site considerations, scanning strategy, cryogenics, detectors, readout, shielding, and data system. It presents how three 1024-element TES bolometer arrays operating at 148, 218, and 277 GHz are integrated into a single, cryogenic camera and how the system achieves precise pointing, stable operation, and well-characterized beams. Key contributions include the architecture of three independent cold optics tubes with silicon/Lens-based optics, a robust SQUID-based readout coupled to a sophisticated data acquisition and synchronization system, and a fully described alignment and beam-measurement program using planetary calibrators. Together, these elements enable ACT to produce high-fidelity CMB and SZ measurements, with practical impact for arcminute-scale cosmology and cluster studies.

Abstract

The Atacama Cosmology Telescope was designed to measure small-scale anisotropies in the Cosmic Microwave Background and detect galaxy clusters through the Sunyaev-Zel'dovich effect. The instrument is located on Cerro Toco in the Atacama Desert, at an altitude of 5190 meters. A six-meter off-axis Gregorian telescope feeds a new type of cryogenic receiver, the Millimeter Bolometer Array Camera. The receiver features three 1000-element arrays of transition-edge sensor bolometers for observations at 148 GHz, 218 GHz, and 277 GHz. Each detector array is fed by free space mm-wave optics. Each frequency band has a field of view of approximately 22' x 26'. The telescope was commissioned in 2007 and has completed its third year of operations. We discuss the major components of the telescope, camera, and related systems, and summarize the instrument performance.

Figures (10)

• Figure 1: Picture (left) and mechanical rendering (right) of ACT and its ground screens. The telescope has a low profile; the full height is 12 m. The entire upper structure ("Azimuth structure" and above) rotates as a unit. The surrounding outer ground screen shields the telescope from ground emission. The screen also acts as a wind shield. An inner ground screen mounted on the telescope connects the sides of the secondary and primary. The primary reflector is 6 m and is surrounded by a 0.5 m guard ring (Figures courtesy of AMEC Dynamic Structures).
• Figure 2: Ray trace of ACT's primary and secondary reflectors. The telescope is an off-axis numerically optimized Gregorian. The rays are traced into the MBAC cryostat, mounted on the far right of the receiver cabin. The service position (position where the receiver-cabin floor is level) is shown, corresponding to a viewing elevation of 60°. The nominal observing elevation is 50$.\!\!^{\circ}$5. The rays are traced for the highest (blue), central (green), and lowest (red) fields in both the 277 GHz camera (higher in the cryostat) and the 218 GHz camera (lower in the cryostat). The Figure also shows the dimensions and location of the receiver cabin and MBAC mounting structure.
• Figure 3: Primary reflector layout and final reflector alignment. The primary is composed of 71 approximately rectangular panels arranged in eight rows. The panels are attached to the telescope BUS and are aligned using four adjustment screws on the panel back-side. Panel positions are measured using a laser tracker. The residuals of a fit to the reflector's equation give the necessary adjustments. The figure shows the residual after the final adjustments for the 2008 observing season were made. The reflector was aligned to better than 30 $\micro$m rms.
• Figure 4: Three dimensional model of the cold reimaging optics for MBAC. The optical elements for each array are separated into individual optics tubes. Each array has a similar set of optical elements. The 277 GHz elements and temperatures are labeled. The lenses are labeled Lens 1 to 3, with Lens 1 one being closest to the 300 K window. The low-pass capacitive-mesh filters are labeled LP and the band-pass filter as BP. Infrared blocking filters are labeled IR. The temperature of the components decreases moving toward the arrays to reduce the loading, with the band-pass filter, the third lens, and arrays all held at 0.3 K.
• Figure 5: The idealized arrangement of the ACT detectors on the sky. This arrangement on the sky is the reverse of the view when looking into MBAC (Fig. \ref{['fig:optics']}).