CBI   Cosmic Background Imager  

Mount

The CBI mount is an elevation-azimuth design with an additional axis to allow rotation of the array about the source vector. The elevation range of the mount is restricted to above 40░ to reduce ground pickup and the size and cost of the instrument. At 40░ elevation, the ground contribution in a CBI image is ~ 3 ÁK. This could be reduced by adding a ground shield, but the required structure is very large and wind loading is a serious problem. Alternatively, the ground can be ``imaged'' by observing many different fields at the same declination but over the same range of hour angle. The ground contribution can then be subtracted from all the field images. This approach restricts the choice of fields but avoids an expensive ground shield.

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Left: a face-on view of the CBI antenna platform showing one possible configuration of the 13 antennas.
Right: the CBI at elevation 50░.

The antenna platform is an irregular hexagon ~ 6.5 m in diameter and 0.6 m thick. It is a space-frame structure with 52 1-m triangular cells, each with four antenna locations. This allows many different array configurations, including baselines that are not simply integer multiples of 1 m. The pointing and stiffness requirements for the CBI antenna platform are driven by mosaicing. A visibility phase error of 1░ at wavelength 1 cm corresponds to a 1'' pointing error on a 6-m baseline or a displacement of 28 Ám in the surface of the antenna platform. The azimuth and platform rotation bearings in the CBI have ~ 2'' axial errors and the worst-case gravitational deformation in the platform is ~ 50 Ám. Thus, the worst-case visibility phase error on any CBI baseline will be ~ 5░.

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The CBI mount on its concrete foundation in Chile, prior to installation of the antennas, electronics, and dome (August 1999).


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