Instrument Description

Telescope and Site

Anisotropy observations were made with the BIMA array in Hat Creek California. The BIMA antennae are 6.1 m in diameter and produce a 6.6' primary beam at 28.5 GHz.

In order to maximize the sensitivity of the BIMA array to objects on the sky as large as a cluster of galaxies, the telescopes are placed in a compact configuration (left). The compact configuration is sensitive to structure on the sky that is 2 arcminutes in diameter and smaller. In this configuration, the telescopes are close enough to touch.

HEMT Receivers

A typical receiver used in anisotropy observations is shown to the right and to the left. The first component in the receiver is the corrugated feed horn. The incoming signal passes through the corrugated field horn into the first active component in the receiver, the HEMT amplifier. The HEMT amplifier is an integrated circuit of transistors and other discrete components that is designed to operate at cm wavelengths. This amplifier is sensitive to radiation at 1 cm, or 26-36 GHz, and amplifies the signal by a factor of 10,000. This step effectively eliminates any instrumental noise from the receiver components that lie behind the HEMT. The signal from the HEMT amplifier is then down converted to the GHz range with a local oscillator and mixer. The down conversion stage is neccessary in order to carry the signal from the telescopes to the correlator room with minimal attenuation from the cables. The system temperature of the receiver is approximately 12-18 K, while the additional loading from the sky and telescope is expected to be 30-40 K, depending on weather conditions and telescope elevation.

The lower figure shows Marshall Joy sitting inside the cabin of one of the BIMA telescopes. The receiver is mounted inside the cabin and directly behind the primary dish of the telescope.

BIMA Correlator

The correlator is the computer that retrieves the signal from the telescopes and computes the visibilities for each pair of telescopes. The resulting visibilities are a representation of the data in the fourier plane as opposed to the image plane.

The signal from the telescopes arrives at the correlator with a bandwidth of 800 MHz, centered at a frequency of 500 MHz. There are several modes available with the BIMA correlator that determine the number and the resolution of spectral channels. Since the frequency spectrum of the SZ effect is relatively flat over this frequency range, we choose the mode that maximizes bandwidth at the expense of spectral resolution. This mode utilizes the full 800 MHz of bandwidth distributed over 8 wideband channels.

The correlator multiplies the signal from every pair of telescopes and performs a two bit digitization. The signal is time averaged over an interval of 50 seconds and then written to disk for data analysis. The data can be imaged by performing a two dimensional fourier transform on the visibilities with the analysis package, MIRIAD, provided by the staff from BIMA. However, the visibility data itself carries all the essential information for the anisotropy analysis. All estimates of CMB anisotropy are derived directly from the visibility plane.