Telescope and Site
SZ observations were made with the BIMA array in Hat Creek
California and the OVRO array in Big Pine,
CA. The BIMA antennae(right) are 6.1 m in diameter and produce a
6.6' primary beam at 28.5 GHz. The OVRO antennae (below) are
10.4 m in diameter and produce a 4' 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, most
telescopes are placed in a compact configuration. The compact
configuration is sensitive to structure on the sky that is 2
arcminutes in diameter and smaller. The remaining telescopes
are moved away from the compact array to provide longer
baselines which are only sensitive to smaller structure on the
sky. This type of hybrid array acts as a filter for
discriminating clusters of galaxies from radio point sources
which act as contaminants in the observations.
A typical receiver used in SZ 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.
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 BIMA 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. With the OVRO telescope, a the IF signal has
a bandwidth of 2 GHz, all of which is used for observations.
The correlator multiplies the signal from every pair of
telescopes and performs a two bit digitization for the BIMA
array. The OVRO correlators manage the data stream as an analog
signal. The signal is time averaged over an interval of 50
seconds and then written to disk for data analysis. The data is
reduced and imaged by performing a two dimensional fourier
transform on the visibilities with the analysis package, MIRIAD, provided by
the staff from BIMA, and DIFMAP,
written at Cal Tech. Data from the OVRO telescopes is reduced
with the package mma.
However, the visibility data itself carries all the essential
information for the SZ analysis. All models for the galaxy
clusters are derived directly from the visibility plane.