Beamformer for the Atacama Large Millimeter/submillimeter Array
VLBI has traditionally been done at cm wavelengths. In recent years MPIfR, MIT Haystack Observatory and other institutes have pioneered VLBI also for wavelengths down to 1 mm. Telescopes which can observe at mm wavelengths are smaller than the traditional cm-wave radio telescopes, the receivers are less sensitive, and instabilities in the atmosphere have a stronger impact at shorter wavelengths so that the sensitivity of VLBI observations has been the most challenging aspect for VLBI. A way to overcome the limited size of mm-telescopes is to coherently add the signals of the antennas of local interferometers working at mm wavelengths and use this summed signal for VLBI observations.
In Chile the Atacama Large Millimeter/submillimeter Array (ALMA) has been built and has started to observed in an interferometer array mode. A global consortium lead by MIT Haystack Observatory has started a project to coherently add the signals of up to all 66 antennas of ALMA and record the summed signal for later VLBI correlation with other antennas or phased arrays.
MPIfR's task is to implement the so-called phasing algorithm which is a calibration scheme to determine how the signal phases of the individual antennas have to be adjusted to add in a coherent way. In addition MPIfR is leading the task to convert the linearly polarized signals of ALMA to circularly polarized ones which is the standard in VLBI, recorded at all other antennas worldwide.
Digital Baseband Converters
Since about 2005 the VLBI technology group has been the major partner in the development of so-called digital baseband converters (DBBC) for VLBI networks. Those converters are digital backends and have replaced the old analogue converters in the VLBI data acquisition systems at many telescopes. As they are fully backwards compatible to the analogue backends they have also been used to equip new telescopes for geodetic and astronomical VLBI.
The DBBC project was extended in 2012 with EU support to develop in a first step a new backend with 4 GHz bandwidth and up to 32 Gbps output data rate (project code DIVA). In a second step the system will probably be upgraded to handle up to 14 GHz This will allow to develop digital receivers without any downconversion in the range up to 14 GHz.
he project is a collaboration between INAF - Istituto di Radioastronomia, Noto, Italy and the MPIfR VLBI technology group. MPIfR contributes by developing boards for sampling and processing, as well as final system checks. DBBCs are now produced by the INAF spin-off company HAT-Lab.
VLBI at APEX
The Atacama Pathfinder EXperiment APEX is a telecopes of the ALMA type modified for single dish operation. The VLBI technical division has upgraded this telescope for VLBI observing at 1 mm wavelength - a challenging project at 5000 m altitude.
Water Vapour Radiometer
A first version of a Water Vapour Radiometer was developed together with the receiver labs of the MPIfR.
As a result of the findings a second version with improved characteristics has been developed and is in the implementation phase.