The ALMA Front End Optics—System Aspects and European Measurement Results

The Atacama large millimeter/submillimeter array (ALMA) is a radio telescope array with 50 Cassegrain style antennas that is currently being built in the Atacama desert in Chile. The telescopes are designed for a frequency range from 30 to 960 GHz with two orthogonal linear polarizations. Due to the extremely wide range, ten receivers are needed to cover the band, the divisions coinciding with the atmospheric windows. All receivers are built into a common cryostat on separate cartridges. The front end optics include the optical elements between the mixers and the secondary reflector. There are three different generic optical layouts. The two lower frequency bands use lenses that double as vacuum breaks. The next two frequency bands use ambient-temperature mirror systema and the upper six frequency bands have their complete optics cooled to cryogenic temperatures. In addition to the very demanding performance requirements, the optics design needs to take into account the constructional and operational aspects. As a large number of front ends will be built, the design must be robust with respect to mechanical and alignment tolerances. The exchange of receivers must be easy and should require as few alignments as possible. During operation, no mechanical adjustments are allowed. Measurement results are shown for the ALMA baseline bands. These measurements were made for single cartridges with the IRAM near-field measurement system and at SRON. The results of these measurements validate the design approach.