MRI: Development of an ALMA Beamformer for Ultra High Resolution VLBI and High Frequency Phased Array Science

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The ability of astronomers to detect fine structure details in distant objects depends upon the angular resolution obtainable from their telescopes and instruments. Angular resolution is related to the wavelength at which the observations are made (the smaller the wavelength, the higher the resolution that can be achieved) and also to the diameter of the observing objective (the larger the objective, the higher the resolution). In most cases the observing objective is the size of the telescope's primary mirror. But when multiple telescopes are used simultaneously and their beams are combined the objective size becomes the baseline distance between the telescopes used. For several decades, radio astronomers have taken advantage of large baselines by coordinating observations between radio telescopes on different continents in a technique termed Very Long Baseline Interferometry (VLBI). This technique has enabled radio astronomers (who work at quite long wavelengths compared to that of visual light) to achieve angular resolutions that rival, and in some cases surpass, the kinds of resolutions available to optical astronomers. Dr. Sheperd Doeleman of the MIT Haystack Observatory has been successfully increasing VLBI resolution by using shorter wavelengths and he now wants to add VLBI capability to the new Atacama Large Millimeter Array (ALMA) that is now becoming operational in northern Chile. By adding ALMA to the VLBI network the angular resolution will be improved by a factor of two and the sensitivity of the ensemble of telescopes will increase by a factor of ten. Technically, this will involve combining the beams from up to 66 telescopes in ALMA into one phased and coherent signal that can then be used as one VLBI station. This new capability will enable scientists to use VLBI to study the environment around and very close to Black Holes like the one at the center of our own Milky Way Galaxy and another in a nearby very massive galaxy. The results of these investigations are likely to lead to new understanding of the physics in strong gravitational fields and will provide new tests for the theory of General Relativity. This work will be made possible through an award from the National Science Foundation's Major Research Instrumentation program.