The ALMA Front End

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. In the initial phase of operations the antennas will be equipped with six bands. These are Band 3, Band 4, Band 6, Band 7, Band 8 and Band 9. It is planned to equip the antennas with the missing bands at a later stage of ALMA operations.

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 millimetre and submillimetre observatories around the world. The Front End units are comprised of numerous elements, produced at different locations in Europe, North America and East Asia.

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 be relatively easily installed or replaced. The corresponding warm optics, windows and IR filters were delivered by the Institut de Radio Astronomie Millimetrique (IRAM, France). The operating temperature of the cryostats will be as low as 4 K.

The ALMA Receiver Bands

In the initial phase of operations the radio telescopes of the ALMA Observatory will be equipped with the six receiver bands mentioned above. The technical specifications of the various receivers are 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 very successful, meeting and in some cases surpassing the specifications.

ALMA Band Frequency Range (GHz) Receiver Noise over 80% of the RF Band (K) Temperature (K) at any RF Frequency To be produced by Receiver Configuration
3 84-116 37 60 HIA 2SB
4 125-163 51 82 NAOJ 2SB
5 163-211 65 105 OSO 2SB
6 211-275 83 136 NRAO 2SB
7 275-373 147 219 IRAM 2SB
8 385-500 196 292 NAOJ 2SB
9 602-720 175 261 NOVA DSB
10 787-950 230 344 NAOJ DSB

2SB = sideband separating: the receiver outputs each sideband separately,
DSB = double sideband: the receiver outputs the sum of the two sidebands.
A 2SB design now exists for Band 9 and may form part of a future upgrade.

In the framework of the European FP6 programme, ESO is leading a group of European institutes to develop and build six single-polarisation Band 5 receiver cartridges. This project was approved by the European Commission at the end of 2005. In parallel, NAOJ is leading activities related to the development of Band 10 receivers.

The ALMA Front End Integration Centres

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, organisation and programme risks. Mainly based on considerations of risk mitigation the parallel Front End Integration Centre (FEIC) was selected. The European FEIC will be located at Rutherford Appleton Laboratory (RAL); the North American FEIC will be installed at NRAO. A third FEIC is installed in Taiwan to carry out the integration of Front End assemblies required for the antennas to be 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 Submillimeter Array (SMA) on Mauna Kea (Hawaii). Key performance of both prototypes is well in agreement with the requirements and the tests made at the SMA demonstrated that a correction for the varying water vapour content in the atmosphere can be made using this technique. Based on the postive outcome of these experiments ESO has decided to contract the final design and production of these WVRs to industry. Following a competive call for tender in all ESO member states, ESO has contracted for this work package to Omnisys Instruments AB based in Gothenburg, Sweden. The contract foresees the design and delivery of 58 units to the ALMA Observatory between 2009 and 2010. A first unit was successfully accepted by the ALMA Observatory and installed in an antenna in April 2009.