The Atacama Cosmology Telescope (ACT) is designed to measure temperature anisotropies of the cosmic microwave background (CMB) at arcminute resolution. It is the first CMB experiment to employ a 32×32 close-packed array of free-space-coupled transition-edge superconducting bolometers. We describe the organization of the telescope systems and software for autonomous, scheduled operations. When paired with real-time data streaming and display, we are able to operate the telescope at the remote site in the Chilean Altiplano via the Internet from North America. The telescope had a data rate of 70 GB/day in the 2007 season, and the 2008 upgrade to three arrays will bring this to 210 GB/day.
The Atacama Cosmology Telescope is a six meter, off-axis Gregorian telescope for measuring the cosmic microwave background at arcminute resolutions. The Millimeter Bolometer Array Camera (MBAC) is its current science instrument. Erected in the Atacama Desert of Chile in early 2007, it saw first light with the MBAC on 22 October 2007. In this paper we review its performance after one month of observing, focusing in particular on issues surrounding the alignment of the optical system that impact the sensitivity of the experiment. We discuss the telescope motion, pointing, and susceptibility to thermal distortions. We describe the mirror alignment procedure, which has yielded surface deviations of 31 μm rms on the primary and 10 μm rms on the secondary. Observations of planets show that the optical performance is consistent with the telescope design parameters. Preliminary analysis measures a solid angle of about 215 nanosteradians with a full width at half maximum of 1.44 arcminutes at 145 GHz.
The Atacama Cosmology Telescope (ACT) aims to measure the Cosmic Microwave Background (CMB) temperature
anisotropies on arcminute scales. The primary receiver for ACT is the Millimeter Bolometer Array
Camera (MBAC). The MBAC is comprised of three 32×32 transition edge sensor (TES) bolometer arrays, each
observing the sky with an independent set of band-defining filters. The MBAC arrays will be the largest pop-up
detector arrays fielded, and among the largest TES arrays built. Prior to its assembly into an array and installation
into the MBAC, a column of 32 bolometers is tested at ~ 0.4 K in a quick-turn-around dip probe. In
this paper we describe the properties of the ACT bolometers as revealed by data from those tests, emphasizing
a characterization that accounts for both the complex impedance and the noise as a function of frequency.
The 6-meter Atacama Cosmology Telescope will map the cosmic microwave background at millimeter wavelengths.
The commissioning instrument for the telescope, the Millimeter Bolometer Array Camera, is based on a
refractive optical system which simultaneously images three separate fields of view at three different frequencies:
145, 220, and 280 GHz. Each frequency band contains around twelve individual optical elements at five different
temperature stages ranging from 300 K to 300 mK and a 32 x 32 array of Transition Edge Sensor bolometers at
300 mK. We discuss the design of the close-packed on-axis optical design of the three frequencies. The thermal
design and performance of the system are presented in the context of the scientific requirements and observing
schedule. A major part of the design was the incorporation of multiple layers of magnetic shielding. We discuss
the performance of the 145 GHz optical system in 2007 and the implementation of the additional two frequency
channels in 2008.
The Atacama Cosmology Telescope observes the Cosmic Microwave Background with arcminute resolution
from the Atacama desert in Chile. For the first observing season one array of 32 x 32 Transition Edge
Sensor (TES) bolometers was installed in the primary ACT receiver, the Millimeter Bolometer Array Camera
(MBAC). In the next season, three independent arrays working at 145, 220 and 280 GHz will be installed in
MBAC. The three bolometer arrays are each coupled to a time-domain multiplexer developed at the National
Institute of Standard and Technology, Boulder, which comprises three stages of superconducting quantum
interference devices (SQUIDs). The arrays and multiplexers are read-out and controlled by the Multi Channel
Electronics (MCE) developed at the University of British Columbia, Vancouver.
A number of experiments plan to use the MCE as read-out electronics and thus the procedure for tuning the
three stage SQUID system is of general interest. Here we describe the automated array tuning procedures and
algorithms we have developed. During array tuning, the SQUIDs are biased near their critical currents. SQUID
feedback currents and lock points are selected to maximize linearity, dynamic range, and gain of the SQUID
response curves. Our automatic array characterization optimizes the tuning of all three stages of SQUIDs by
selecting over 1100 parameters per array during the first observing season and over 2100 parameters during the
second observing season. We discuss the timing, performance, and reliability of this array tuning procedure
as well as planned and recently implemented improvements.
The Millimeter Bolometer Array Camera (MBAC) was commissioned in the fall of 2007 on the new 6-meter
Atacama Cosmology Telescope (ACT). The MBAC on the ACT will map the temperature anisotropies of the
Cosmic Microwave Background (CMB) with arc-minute resolution. For this first observing season, the MBAC
contained a diffraction-limited, 32 by 32 element, focal plane array of Transition Edge Sensor (TES) bolometers
for observations at 145 GHz. This array was coupled to the telescope with a series of cold, refractive, reimaging
optics. To meet the performance specifications, the MBAC employs four stages of cooling using closed-cycle
3He/4He sorption fridge systems in combination with pulse tube coolers. In this paper we present the design of
the instrument and discuss its performance during the first observing season. Finally, we report on the status
of the MBAC for the 2008 observing season, when the instrument will be upgraded to a total of three separate
1024-element arrays at 145 GHz, 220 GHz and 280 GHz.
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