The ALMA Front End system is the first element in a complex chain of signal reception, conversion, processing and recording. It is designed to capture signals from ten different frequency bands. This system is far superior to any other in existence. In fact, products derived from ALMA prototypes are improving the sensitivities of other radio telescopes around the world. The Front End units are composed of numerous elements that are produced as far away as Europe, North America, Eastern Asia and Chile.
Cryostats: The largest individual element in the Front End system is the cryostat and its adjoining cryo-refrigerator, and it keeps the receivers in extremely cold temperatures.
Cryostats contain receivers or bands that are mounted on cartridges and can be installed or replaced with relative ease. The “warm” optics, the part located in the ‘warmer’ area of the cryostat, the windows and the corresponding infrared filters were provided by the Institut de Radioastronomie Millimétrique (IRAM). The cryostats’ operating temperature is extremely low, reaching four degrees Kelvin (equivalent to ‑ 269 ºC).
Cryostats contain receivers or bands that are mounted on cartridges and can be installed or replaced with relative ease. The “warm” optics, the windows and the corresponding infrared filters were provided by the Institut de Radioastronomie Millimétrique (IRAM). The cryostats’ operating temperature is extremely low, reaching four degrees Kelvin (equivalent to ‑ 269 ºC).
Frequency bands of receivers: When it has all 10 bands incorporated, ALMA will measure signals from 8.6 mm to 0.32 mm. the following table shows the specifications of the bands astronomers will be able to use for their observations with ALMA (available from Cycle 5, in construction, and under development):
|1||8,6 – 6||35 – 50|
|2||4,6 – 3,3||65 – 90|
|3||3,6 – 2,6||84 – 116|
|4||2,4 – 1,8||125 – 163|
|5||1,8 – 1,4||163 – 211|
|6||1,4 – 1,1||211 – 275|
|7||1,1 – 0,8||275 – 373|
|8||0,8 – 0,6||385 – 500|
|9||0,5 – 0,4||602 – 720|
|10||0,4 – 0,3||787 – 950|
During Cycle O and Cycle 1, the antennas were outfitted with four bands: Band 3, Band 6, Band 7, and Band 9. Two more were added in Cycle 2: Band 4 and Band 8. Band 10 was added in Cycle 3 and 4.
The technical specifications of the different receivers are very demanding and at the time they were defined, they were cutting edge and beyond. In some cases, the performance of these receivers has gone beyond expectations.
The Front End integration centers: The construction of ALMA required consensus of the partners on several organizational and management decisions. This required several in-depth studies of different scenarios for assembling and integrating Front end components, and identified that the best solution was a “parallel approach,” installing one Front End in Europe, another in North America and another in Taiwan, all with identical procedures. This was the preferred scenario due to logistical, organizational and programmatic considerations.
The European Front End Integration Center (FEIC) is located in the Rutherford Appleton Laboratory, the North American one in NRAO and the Asian one in Taiwan, all used for integration of the Front End assemblies required for the antennas.
Water vapor radiometers: One of the primary factors that limits the resolution (and to a lesser degree, sensitivity) of the millimetric and submillimetric wavelength radio telescopes is the atmospheric fluctuation, specifically water vapor. This is due to a combination of two factors: its uneven dispersion into the atmosphere and its high refractory index. The Water Vapor Radiometers (WVRs) allow for correction of the atmospheric water vapor fluctuations. Two different WVR prototypes were developed at the University of Cambridge (UK) and the Onsala Space Observatory (OSO, Sweden), and underwent intensive testing in the Submillimeter Array (SMA) in Mauna Kea, Hawaii.
ALMA’s Back End systems convert analog signals -received by Front End units installed on each antenna- to digital signals and sends these to the Correlator installed in the Array Operations Site (AOS) Technical Building. This diagram shows signal processing and data transfer.
The analog data, produced by Front End electronics, are processed and digitized before being entered into the data coder, followed by optic fiber transmitter units and multiplexers. These elements are all installed in each antenna’s receiver booth. The optic signals are then transmitted by fiber to the AOS Technical Building where they are demultiplexed and de-formatted before entering the Correlator.