CBI   Cosmic Background Imager  


Observing frequency: 26-36 GHz (wavelength ~ 1 cm)
Number of channels: 10 channels, each 1 GHz wide
Number of antennas: 13
Number of baselines: 78
Receivers: HEMT amplifiers, cooled to 6 K
Correlator: 780 analog complex correlators
Antenna: Cassegrain, 0.90 m diameter
Primary beam: Gaussian FWHM 44 arcmin
Minimum baseline: 1.00 m (l ~ 630), or 0.58 m with smaller antennas
Maximum baseline: 5.51 m (l ~ 3500)
Synthesized beamwidth: FWHM 4.5 - 8.0 arcmin, depending on configuration
System temperature: 20 K
Noise in visibility measurements: 3.3 Jy s-1/2 rms in each 1-GHz channel
Noise in image from a single pointing: 4 mJy/beam rms in 900 s, 10 channels, at center of primary beam
Noise in mosaic image (multiple 900-s pointings separated by 20 arcmin): 2.4 mJy/beam rms, ~ uniform across image
Brightness sensitivity in mosaic image: 13 - 41 ľK rms, depending on configuration
Polarization: Circular (RCP), giving images of Stokes I. Images of Q and U can be made (more slowly) by switching some antennas to LCP and cross-correlating L and R

The CBI is an array of 13 antennas, each 90 cm in diameter, mounted on a single altaz platform, giving interferometer baselines that can be adjusted in length from 1 to 5.5 m. An interferometer of baseline d is sensitive to cosmic microwave background radiation structure with multipole l = 2pid/lambda, where lambda is the wavelength (about 1 cm). The orientation of the baselines can be changed by rotating the platform. The CBI measures 10 channels for each of 78 baselines simultaneously. The instantaneous field of view is set by the primary beam of the antennas. Larger fields can be imaged by assembling data from multiple pointings (called ``mosaicing''). From a mosaic observation of a 2° × 2° field of 9 h duration (36 × 15 min) we can use standard synthesis imaging techniques to make an image with rms noise over the whole field of 2.4 mJy/beam.

* CBI home page