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

天文学家探测遥远天体的精细结构细节的能力取决于他们的望远镜和仪器所能获得的角分辨率。角分辨率与进行观测的波长有关（波长越小，可以实现的分辨率越高），也与观测物镜的直径有关（物镜越大，分辨率越高）。在大多数情况下，观测物镜是望远镜主镜的大小。但是当同时使用多个望远镜并且它们的光束被组合时，物镜的大小就变成了所使用的望远镜之间的基线距离。几十年来，射电天文学家一直利用大基线，通过一种称为甚长基线干涉测量（VLBI）的技术协调不同大陆射电望远镜之间的观测。这种技术使射电天文学家（他们的工作波长比可见光的波长长得多）能够获得与光学天文学家相媲美的角分辨率，在某些情况下甚至超过光学天文学家的分辨率。麻省理工学院Haystack天文台的Sheperd Doeleman博士通过使用较短波长成功地提高了VLBI的分辨率，他现在希望将VLBI能力添加到新的阿塔卡马大型毫米波阵列（阿尔马）中，该阵列现已在智利北方投入使用。 通过将阿尔马加入VLBI网，角分辨率将提高两倍，望远镜集合的灵敏度将提高十倍。 从技术上讲，这将涉及将来自阿尔马多达66个望远镜的光束合并成一个相位和相干信号，然后可以用作一个VLBI站。这种新的能力将使科学家能够使用VLBI来研究黑洞周围和非常接近的环境，比如我们银河系中心的黑洞和附近一个非常巨大的星系。 这些研究的结果可能会导致对强引力场物理的新理解，并将为广义相对论提供新的检验。 这项工作将通过国家科学基金会的主要研究仪器计划的奖励成为可能。

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.