The ALMA Front End system is the first element in a complex chain of signal receiving, conversion, processing and recording. The Front End is designed to receive signals of ten different frequency bands.
The ALMA Front End is far superior to any existing systems. Indeed, spin offs of the ALMA prototypes are leading to improved sensitivities in existing millimeter and submillimeter observatories around the world. The Front End units are comprised of numerous elements, produced at different locations in Europe, North America, East Asia and Chile.
The ALMA Cryostats
The largest single element of the Front End system is the cryostat (vacuum vessel) with the cryo-cooler attached. The cryostats will house the receivers, which are assembled in cartridges and can relatively easily be installed or replaced. The corresponding warm optics, windows and infrared filters were delivered by the Institut de Radio Astronomie Millimétrique (IRAM, France). The operating temperature of the cryostats will be as low as 4 K (equivalent to 269 C below 0).
The ALMA Receiver Bands
In the initial phase of operations, the antennas will be equipped with at least four receiver bands: Band 3 (3mm), Band 6 (1mm), Band 7 (0.85mm), Band 9 (0.45mm). It is planned to equip the antennas with the missing bands at a later stage of ALMA operations (see below: 10 Frequency bands of the ALMA antennas).
The Technical Specifications of the various receivers is quite demanding, and, at the time of definition, at the state of the art level or in some cases even beyond.
The development programmes were successful, as the requirements could be met – and sometimes the performance is even better than defined in the specifications.
The 10 Frequency Bands of the ALMA antennas
The ALMA Front End Integration Centers
A construction project like ALMA, involving several partners in four different continents, requires consensus on several organisational and managerial decisions concerning the actual execution of certain construction activities. Several different scenarios for assembling and integrating the Front End components were extensively studied. This study revealed that the best solution was a “parallel approach”, installing half of the Front End in Europe and the other half in North America with identical and parallel procedures. This scenario was preferred in view of logistics, organization and programme risks. Mainly based on considerations of risk mitigation, the parallel FEIC (Front End Integration Centers) was selected. The European FEIC is located at Rutherford Appleton Laboratory and the North American FEIC at NRAO. A third FEIC is installed in Taiwan to carry out the integration of Front End assemblies required for the antennas supplied by NAOJ.
The Water Vapor Radiometers
Water Vapor Radiometers (WVRs) are needed to provide a correction of the atmospheric water vapor fluctuations. The development of two different prototype WVRs at Cambridge University and Onsala Space Observatory (OSO, Sweden) has been completed and both prototypes have undergone intensive tests at the Sub-Millimeter Array (SMA) on Mauna Kea (Hawaii). Key performance of both prototypes is well in agreement with the requirements. The WVRs will be manufactured by qualified industry and ESO has contracted for the production of 53 units.