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Rabbit Helps Radio Telescopes See the Stars

Rabbit has found its way into another large scientific undertaking, the Atacama Large Millimeter/Submillimeter Array (ALMA) project. The Atacama Larger Millimeter/Submillimeter Array is an international astronomy project which consists of an astronomical interferometer formed by an array of radio telescopes at about 16,000 feet in the Chilean Andes. Slated to revolutionize modern astronomy, this massive scientific undertaking aims to provide a premier tool for studying stars, planetary objects, and galaxies both near and far. These telescopes can detect millimeter and sub millimeter wave lengths emitted by objects in outer space. To increase the sensitivity or resolution of an astronomical interferometer, the size of a satellite dish must be increased to a considerable size. Satellites of a larger size will incur much higher costs, not only because of their construction but also by their maintenance. Alternatives to building a rather large and expensive piece of equipment, several smaller radio telescopes are tied together to achieve the same result. With the array configuration, a higher level of sensitivity can be achieved while keeping the telescope at a manageable size.

Jason Castro, a member of the photonics group of the ALMA project talks about the RabbitCore® RCM4100 important role in the operation ALMA telescope array. The backend photonics group of the ALMA project is responsible for the main timing signal to each antenna in order for entire radio interferometer to work.

"To create the timing signal, two lasers beams are ‘beaten’ together," says Castro.

The two laser beams consists of equal wave lengths. When the two beams are interfered with each other, the interference produces a highly accurate measurement. The measurement helps with the timing signal which allows the 64 satellite array act as one unit.

The main communications protocol employed by the radio interferometer is a modified CAN bus interface. The interface is called ALMA Monitor and Control Bus Standard Interface 2 (AMBSI2), which is a dump CAN to SPI converter with an SPI clock running at 4 Mhz. The RCM4100 interfaces to the CAN bus via the available SPI, and perform all of the monitoring and control functions. The functions include reading temperature and voltages, switching optical switches, driving polarization controllers, and running optimization loops. Furthermore the Rabbit firmware can be updated through the CAN bus interface.

"We use the RCM4100 in a total of 3 different modules, the Photonic Distribution monitor and Control Module (FDMC), the Sub Array Switch (SAS) and the Line Length Corrector (LLC)," explains Castro.

The FDMC is a CAN to RS-232 bridge, which talks to commercial optical fiber amplifiers. In this application the CAN is converted to SPI, which communicates to the Rabbit, which then communicates to the amplifiers via RS-232. The SAS is a custom module that assigns an antenna to one of the 4 timing signals. The SAS also performs optimization algorithms to maximize the light through the module by measuring and adjusting the polarization. LLC module is designed to compensate for variations in the fiber length due to mechanical perturbation or temperature changes.

Castro explains "The RabbitCore® RCM4100 is a fairly simple solution that got us up and running with minimal time and effort."

Rabbit has also allowed the photonics group of the ALMA project to develop modules that are 10 times smaller than their previous solution.

"This allowed us to make our modules much smaller and reduced the number of equipment racks needed by about half," says Castro.

The RCM4100 offers the right feature set such as SPI functionality to enable the desired communication method for the ALMA array. Furthermore features such as 60 MHz clock speed, 40 GPIO, 512K Flash, up to 512K Data SRAM and analog makes the RCM4100 a venerable monitoring and control solution. With Dynamic C® development environment, the ALMA photonics group quickly developed a solution pivotal to the operation of the telescope. Clearly Rabbit has provided a solution that truly reduces cost, without sacrificing the performance and reliability required for projects of this nature.

For more information on the ALMA project, visit: http://www.almaobservatory.org/

“The RabbitCore® RCM4100 is a fairly simple solution that got us up and running with minimal time and effort.”
- Jason Castro, ALMA Photonics group


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