The Atacama Large Millimeter Array

The Atacama Large Millimeter Array (ALMA) is a large international telescope project which is being built in northern Chile on a site at 5 km elevation. The site was chosen to provide excellent atmospheric transmission in the millimeter and sub-millimeter wavelength region. Functionally, the project consists of two parts. First, the 12 m interferometric array, composed of 50 (present status) to 64 (project goal) antennas that can be placed on 175 different stations. The second part is the Atacama Compact Array or ACA. This is used to measure source total power and extended structure. It consists of four 12-m antennas for measuring source total power and twelve 7-m antennas in compact configurations. The ACA will provide data for extended sources, as well as source total flux density. The RMS surface accuracy of the antennas is 25 microns or better. ALMA will provide high sensitivity measurements between wavelengths from lambda =3.5 to 0.3 mm, or frequencies from 84 GHz to 950 GHz. At the longest baseline, 14.5 km, ALMA will reach an angular resolution of 0.005 arc seconds. The receivers will be superconducting mixers cooled to 4 K to obtain the lowest possible receiver noise.