A new generation of low-noise, dual-polarization heterodyne receivers covering all atmospheric windows between 68 and 300 GHz has been developed for the 12-m telescope on Kitt Peak, AZ, operated by the National Radio Astronomy Observatory (NRAO). A modular approach to the receiver construction has been used. Each receiver channel is constructed as a self-contained insert which is mounted in a dewar equipped with a closed-cycle 4 K refrigerator capable of cooling up to eight inserts, thereby enabling several frequency bands to be contained within one dewar. The telescope and receiver optics are so arranged that switching between frequency bands may be accomplished rapidly by rotating the optical elements placed in the signal path. Single-sideband noise temperatures ranging from 50 to 400 K are obtained across the frequency range.>

The significance of radio astronomy at millimeter and submillimeter wavelengths is described, and some of the more important results are given. The limitations imposed by the atmosphere are discussed, and the observation techniques used to overcome these limitations are outlined. The major new millimeter and submillimeter telescopes are described, and the techniques used to construct large-diameter, high-precision antennas are discussed. Methods of measuring the surface of a high-precision antenna are described with particular emphasis on microwave holography. The current state of the high-sensitivity receiving systems for these wavelengths is outlined with emphasis on recent work in superconducting devices and multibeam receivers. A brief description of astronomical millimeter-wave interferometers is given, and the recent progress made in the application of very-long-baseline interferometry techniques at millimeter wavelengths is described.>