|Cosmic Background Imager|
The CBI antennas can be close-packed for maximum sensitivity. However, in a close-packed array, false signals due to coupling between the antennas can be a serious problem. Noise emitted from the input of a receiver can scatter into an adjacent antenna and cause a response at the correlator output. With 15 K receivers and -120 dB coupling between antennas, the false signal can be as large as 15 ľK. The scattered noise is not completely correlated with the receiver noise and it also arrives at the correlator with a delay error. Both these effects reduce the false signals, but the residual may still exceed the noise in a CBI image. Further rejection of the false signals is achieved by configuring the CBI as a planar, rather than a tracking, array. In a planar array, all the antennas are attached to a single tracking platform and since the individual antennas do not move relative to each other, the coupling is constant. Rotating the planar array about its axis allows us to discriminate between signals from the sky and artifacts of the instrument, including the false signals caused by inter-antenna coupling. The additional rotation is a significant complication of the instrument mount, but it provides a powerful test of the origin of the features in a CBI image.
Minimizing the inter-antenna coupling requires an antenna design with very low scattering. A clear aperture such as a corrugated feed-horn is ideal, but the length of a 1-m diameter horn is prohibitive and adding a lens to reduce the length of the horn severely degrades the system noise. An offset reflector is also attractive but it is difficult to close-pack antennas of this type. As a compromise between the requirements of close-packing, low scattering and high sensitivity, the CBI uses a shielded Cassegrain design. This has classical Cassegrain optics but the cylindrical shield guides radiation scattered from the secondary blockage and from the hole in the primary onto the sky. The top edge of the shield is rolled with a radius of a few wavelengths to reduce scattering from the shield itself. The secondary support structure is a significant source of scattering in most Cassegrain designs, but the CBI antennas have a lightweight carbon-fiber reinforced epoxy secondary supported by expanded polystyrene foam feed-legs which are essentially transparent at ~ 1 cm. With the shields in place, the average inter-antenna coupling in a 1-GHz wide band is -110 to -120 dB. This gives false signals in the range ~ 0.5 to 1.5 ľK, assuming 15 K receivers with completely correlated outgoing and receiver noises and 9 ns delay error for the coupled noise.
CBI antenna with shield can removed, showing the subreflector and its supports.
Coupling between adjacent CBI antennnas when separated by 1 m. (a) Without shield cans; (b, c) with shield cans in place for two different orientations of the E field. The shield can reduces coupling between adjacent elements by ~ 30 dB.