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Cosmic Background Imager |
The CBI must be on a high, dry site to ensure that atmospheric brightness fluctuations do not limit the sensitivity of the instrument, and, in addition, the horizon must be low to minimize ground pickup. The site we have adopted is at longitude 67° 46' W, latitude 23° 02' S. This is at an altitude of 5080 m about 40 km east of San Pedro de Atacama in Chile. The site is near the center of the 10-km plateau of Llano de Chajnantor and the horizon is below 6° except for one peak at 8°. This is close to the site proposed for the Atacama Large Millimeter Array.
The Llano de Chajnantor in winter 1996, looking down from Cerro Toco.
The CBI arriving on the Llano de Chajnantor (August 1999).
The CBI is housed in a retractable dome to provide some protection from rain, snow and wind. On most days, the wind at Chajnantor peaks late in the afternoon at about 15 m/s (30 mph). Under these conditions wind chill is a severe problem, but the dome provides a sheltered work space where we can repair and maintain the CBI. The dome is a hemispherical steel frame covered with polyester cloth, sitting atop a 2-m high wall 10.5 m in diameter. Each half of the dome has four segments which nest in a compact ring round the wall when the dome is retracted. The dome can be opened and closed manually or using an electric winch. The wall has a 2-m wide walkway round the top to facilitate access to the telescope.
The CBI dome and support facilities (October 1999).
The CBI and its dome require concrete support pads, but this is the only site construction work. All the other facilities are in standard ISO shipping containers placed directly on the ground. This modest approach to site infrastructure reduces costs, reduces our maintenance load, has a fairly low impact on the environment, and makes the instrument somewhat portable.
The site facilities include a control room, laboratory, machine shop, power plant, two workrooms/bedrooms, and a bathroom - all within the shipping containers. All the equipment required for repairs and modifications is at the site. Additional containers used for storage are placed around the dome to provide a wind break. To counteract the effects of high altitude, the air in the work and living areas is oxygen-enriched (using molecular sieves to separate oxygen from the air), and people working outside can wear portable oxygen tanks with demand regulators when necessary to improve efficiency and safety. The power plant and fuel tanks are located about 100 m east of the CBI; the average power consumption for the site is about 100 kW and the power plant has a pair of diesel generators rated for 150 kW at 5000 m.
The base facilities are located in the historic oasis town of San Pedro de Atacama at an elevation of 2500 m, and include bedrooms, kitchen, conference room, and computer room; the facilities are provided by the Hosteria Casa de Don Tomás. Most of the data reduction is done in San Pedro.
More information about the site:
CBI home page