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度分辨率。角分辨率与观测的波长有关（波长越小，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，当多个望远镜同时使用并且它们的光束组合在一起时，物镜尺寸就成为所使用的望远镜之间的基线距离。几十年来，射电天文学家利用一种称为甚长基线干涉测量（VLBI）的技术，在不同大陆的射电望远镜之间协调观测，从而利用了大基线。这项技术使射电天文学家（与可见光相比，射电天文学家工作的波长相当长）能够获得与光学天文学家所能获得的分辨率相媲美的角分辨率，在某些情况下甚至超过了这种分辨率。麻省理工学院干草堆天文台的shepherd Doeleman博士已经通过使用更短的波长成功地提高了VLBI的分辨率，他现在想把VLBI的能力添加到新的阿塔卡马大型毫米波阵列（ALMA）上，该望远镜现在正在智利北部投入使用。通过将ALMA加入VLBI网络，角度分辨率将提高两倍，望远镜集合的灵敏度将提高十倍。从技术上讲，这将涉及到将ALMA中多达66个望远镜的光束组合成一个相控相干信号，然后作为一个VLBI站使用。这种新能力将使科学家们能够使用VLBI来研究黑洞周围和非常接近黑洞的环境，比如我们银河系中心的黑洞和附近一个非常大的星系中的黑洞。这些研究结果可能会导致对强引力场物理学的新认识，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项目的奖励而成为可能。

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.