The Hat Creek millimeter-wave interferometer

A fixed-baseline millimeter-wave interferometer, operating initially at 13.5 mm, has been put into operation at the Hat Creek Observatory as the first step in the development of a two-element aperture synthesis telescope. The first system consists of a 3-m antenna and a 6-m antenna spaced 265 m apart. Large receiver bandwidths may be used at high frequencies, and this system employs an intermediate frequency bandwidth of about 400 MHz. It also has automatic gain control and a phase stabilized local-oscillator reference cable. Observations may be made either in the continuum or with a 128-channel spectrometer. The baseline vector has been obtained from observations of about 7 QSO's. The instrument has been used to derive accurate absolute positions of interstellar water vapor sources, to study Mercury, Venus, and Mars, and to make crude maps of a few complex continuum sources. Measurements of the interferometer phase fluctuations due to the atmosphere indicate that interferometer is possible under average weather conditions at Hat Creek at wavelengths as short as 2 mm. The synthesis telescope, the next stage, has two 6-m antennas which can be located at various stations on a T-shaped track. The east-west leg is 300 m and the north-south leg is 200 m, permitting full synthesis for sources on the equator and at declinations as low as -30° as well as at high declinations. Operation at wavelengths down to 2 mm will be possible with resolution of 1"-2".