The Introduction of FAST (为2018全俄科学节准备的解说词。由于观众基本不说英语，所以没有用上，全靠志愿者帮忙解说了。) Dear all, this is the model of FAST. FAST stands for Five-hundred-meter Aperture Spherical radio Telescope, with a diameter of 500 meter at the crown opening. It is currently the world largest single dish radio telescope. FAST situated in a Karst depression in southwest China. This Karst depression is close to a sphere, which minimize the excavation. It is with a higher altitude than the Karst depressions around, which makes a good shielding of radio frequency interference (RFI). There are several subsystems of FAST. This is the active reflector, which will form a parabola in observing. The feed cabin is suspended upon the focus sphere. There are some instruments to measure and control the position and orientation of the feed cabin. During the observation, a source will go across the sky, the reflector will deform and so the feed cabin will move accordingly. Why bother to deform the reflector? Because the parallel light rays will be focused to a line by a spherical reflector, to a point by a parabola. During the observation, a parabola of 300-meter aperture will be formed in real time, in order to track a source. Why do we hang the feed cabin with cables? This is a design to minimize the weight of the feed support system. Compared with the thousand-ton feed support system of Arecibo telescope, the feed cabin of FAST is only 30 tons. This small feed cabin also makes least obscuration. The device to receive the signal is call a receiver. The receivers are suspended under the feed cabin. FAST have several science goals, including Galactic HI mapping, HI galaxy search, pulsar search, interstellar molecules. HI is the hydrogen atoms in the ground state. HI will radiate line emission with wavelength 21 cm. There are HI in many galaxies, including our Milky Way. We can thus observe this 21 cm line to trace HI in the Milky Way and other galaxies. Pulsars are rotating neutron stars. The emit radio wave from the poles. When the radiation beam passes our Earth, we can see a pulse. There are also molecules in the interstellar space, which can be traced by molecular lines. Now we have found the interstellar molecules of sugar. Maybe we can find amino acid in the space in the future! 20181015补充，志愿者 даша帮忙翻译成了俄语 Описание FAST Дорогие посетители, это модель FAST. FAST - это телескоп, у которого диаметр сферического отражателя 500 метров. В настоящее время это крупнейший в мире радиотелескоп с одним отражателем. FAST расположен в карстовой впадине на юго-западе Китая. Эта карстовая впадина по форме близка к сфере, что позволило уменьшить затраты времени на искусственное формирование впадины для размещения отражателя. Эта карстовая впадина находится на более высокой высоте, чем остальные впадины рядом, что обеспечивает хорошую защиту от радиочастотных помех. FAST состоит из нескольких элементов. Одним из них является активный отражатель, который образует параболу при наблюдении. Облучатель подвешен над отражателем в фокусе параболоида. Существуют некоторые инструменты для измерения и контроля положения и ориентации облучателя. Во время наблюдения источник будет перемещаться по небу, отражатель будет деформироваться, и поэтому облучатель будет двигаться вслед за сигналом от источника. Почему необходимо деформировать отражатель? Затем, что параллельные световые лучи не могут быть сфокусированы облучателем, деформируясь, отражатель позволяет сфокусировать сигнал в облучатель. Во время наблюдения парабола с 300-метровым диаметром будет формироваться в реальном времени, чтобы следить за источником. Почему мы вешаем облучатель с помощью кабелей? Эта конструкция позволяет минимизировать вес системы облучателя. По сравнению с многотоновой системой облучателя телескопа Аресибо, облучатель FAST весит всего 30 тонн. Небольшие размеры кабины облучателя позволяют ему отбрасывать не очень большую тень. Устройство для приема сигнала называется приемник. Приемники подвешены под кабиной облучателя. FAST преследует несколько научных целей, включая галактический HI маппинг, HI поиск галактик, поиск пульсаров, межзвездных молекул. HI - атомы водорода в основном состоянии. Йодид водорода излучает линии с длиной волны 21 см. Во многих галактиках присутствует HI , включая наш Млечный Путь. Таким образом, мы можем наблюдать эту линию, длиной 21 см, для отслеживания наличия HI в Млечном Пути и других галактиках. Пульсары – это вращающиеся нейтронные звезды. Они излучают радиоволны с полюсов. Когда излученный пучок достигает Земли, мы можем наблюдать импульс. В межзвездном пространстве также есть молекулы, которые можно проследить по молекулярным линиям. Так, мы нашли межзвездные молекулы сахара. Возможно, в будущем мы сможем найти аминокислоту в пространстве!
FAST Pulsar Exploration （2018年9月7日在大窝凼的讲稿） FAST stands for Five-hundred-meter Aperture Spherical radio Telescope, with a diameter of 500 meter at the crown opening. It is currently the world largest single dish telescope. FAST situated in a Karst depression in southwest China. This Karst depression is close to a sphere, which minimize the excavation. It is higher than the Karst depressions around, which makes a good shielding of radio interference. There are several subsystems of FAST. This is the active reflector, which will form a parabola in observing. The feed cabin is suspended up in on the focus sphere. There are some instruments to measure and control the position and orientation of the feed cabin. During the observation, a source will cross the sky, so the feed cabin will move accordingly, the reflector will deform accordingly. Why bother to deform the reflector? Here you can see that the parallel light rays will be focused to a line by a spherical reflector. In order to focus to a point, or an Airy dot, a parabola is needed. Let’s watch a video to have a direct impression. The reflector is a parabola. You can see the balls all focus to the focal point. You can imagine the bell is the receiver. This is the real receiver. On the left, it is the ultra-wideband receiver. On the right is the 19-beam receiver. FAST have several science goals, including Galactic HI mapping, HI galaxy search, pulsar search, interstellar molecules. Now let’s have a look at pulsar studies. The title of this talk is ‘cosmic lighthouse’. Let’s have a look at lighthouse. Lighthouses are once critical to navigation. Look closely, you can see a rotating light beam. That’s why a lighthouse is pulsating. When the weather is cloudy all the time, and you do not have a GPS system, the most efficient way is building lighthouses. That’s the USSR have done in the 1960s. When you know the position of two lighthouses, by measuring the angle, you can get a rough estimate of the distance. Now we have GPS on a car, a ship and even in a cell phone. The principle is similar. But here the key point is an accurate clock. What is a clock? Something that is periodic can be a clock. In principle, a periodic pulsating star can be used as a clock for navigation. The only thing is that most pulsating stars are not accurate enough. But we have accurate clocks in the universe. They are pulsars. Looks like a lighthouse, right? Many people think that pulsars are neutron stars. Some people don’t agree. A pulsar is a small object. It is no larger then a big city. It is however weighted to a solar mass. Typically, it rotates several rounds per second. You can imagine the material in a pulsar is extremely dense. What happens? In normal matter, there are a lot of space in the atom. But for the matter in a pulsar (neutron star), the atoms have been squeezed together. This is possible in a collapse at the death of stars. Usually, stars rotate slowly. Have a look at our sun. How can slowly rotating stars becomes rapidly rotating pulsars? The key point is compression. The angular momentum is conserved, lower the moment of inertia, you get a higher rotating speed. Try this experiment. The first pulsar is discovered by Jocelyn Bell. She was just awarded the Breakthrough Prize. Light will disperse in a lens. The magnetic wave will also have dispersion in the interstellar medium. High frequency wave will arrive earlier. This is the signal of the first pulsar discovered by Jocelyn Bell. Where is the high frequency signal? Since we know how the wave is dispersed. We can extract the de-dispersed signal. Then we added up the de-dispersed signal, and got a pulse. That’s how we search for pulars. Now we have discovered several pulsars. Have a look at this website if you are interested. (http://crafts.bao.ac.cn/pulsar/fast_all_pulsar_list/) What to do next? We may use pulsar for navigation and search for gravitational waves.
FAST and SETI (COSPAR 2018报告讲稿) FAST stands for Five-hundred-meter Aperture Spherical radio Telescope. It is currently the world largest single dish telescope. FAST situated in a Karst depression in southwest China. This Karst depression is close to a sphere, which minimize the excavation. It is higher than the Karst depressions around, which makes a good shielding of radio interference. The control room is well behind that hill. FAST has several science goals. We are interested in the HI both insider and outside of Milky way. Pulsars are also the key science targets. FAST may also contribute a lot in the studies of interstellar molecules. FAST will also join VLBI network and work on fast radio burst (FRB), SETI etc.. I would like to mention, not like FRB, SETI is the science goal of FAST from the beginning. We plan to achieve these science goals simultaneously in a survey called commensal radio astronomy FAST survey (CRAFTS). The reflector of FAST is active, forming a 300-meter aperture parabola in real time. The feed support system is light-weight, weight only 30 tons. Since there is no solid connection between the feed cabin and the reflector, the measurement and control system play a crucial role. The feed will illuminate an area with 300-meter aperture at one time. When FAST is tracking a source, the feed and the illuminated area will move accordingly. In the initial design, there are 9 sets of receivers, covering 70 MHz to 3 GHz. Now it has been reduced to 7 receivers, since now we have a ultrawide band receiver, covering 270 MHz to 1.62 GHz. Now a new ultrawide band receiver is planned, covering about 560 MHz to 3.3 GHz. These two receivers are the ultrawide band receiver on the left and multibeam receiver with 19 beams on the right. The ultrawide band receiver worked from 2016 to April this year. Now the multibeam receiver has been installed. It is now under testing. Let’s have a close look at the 19-beam receiver. They are arranged in a hexagon pattern. The beams are numbered in this way. The receiver can rotate, so a uniform scan observation is possible. If the 19-beam receiver is rotated by 23.413 (about 23.4) degrees, we can make a uniform scan with 2 passes. This would be the scan strategy of the commensal survey. The backends of FAST are based on ROACH 2 or CRANE. In the future, the backends will be mainly based on ROACH2. The FRB and SETI backends are from Dan’s group. Since the 19-beam receiver has just be installed for about 2 months, the results we got comes mainly from the ultrawide band receiver. FAST has discovered more than 40 pulsars. Most of these pulsar are discovered during drift scan. There is a millisecond pulsar discovered by tracking a gamma ray point source discovered by the Fermi satellite. The period is found with FAST observation. Then the Fermi data are reprocessed to confirm this discovery. The list of new pulsars discovered by FAST can be found here on the CRAFTS website. Half of the FAST pulsars are discovered by single pulse search. Look at the one single pulse, the signal from pulsar, FRB are similar, maybe the SETI signal will be also similar. At least the data looks similar. For SETI observation, there is an observing window from 1 to 10 GHz. FAST covers part of this window. To have an idea of SETI signal, maybe we can have a look at the RFI on our Earth. This is the RFI signal on the FAST site. We can see they appear on some frequencies. This is because there is a committee to allocate the frequencies. Specific activities use specific frequencies. Let’s compare the pulsar, FRB and SETI signals. In general, the pulsar signal is wide band and periodic, with typical dispersion measure up to several hundred. The FRB signal is wide band and aperiodic, with larger DM up to several thousand. About the SETI signal, we are less certain about their properties. But they may have small dispersion measure, which make it difficult to distinguish them from the interference. Typically, there are 1 M channels in the SETI backend, with channel width of 5 Hz. It would be hard to cover the whole band of FAST. We have to either increase the channel number of the SETI backend or narrow down the frequency range to be searched. In the future, we plan to build some small dishes about FAST, expanding FAST to an array called FAST A+. This array is initially intended to localize FRB, but it may also help distinguish and better localize SETI signal. Although SETI is a science goal of FAST and we will have the SETI backend soon, we have to learn more about the properties of the SETI signal in order to dig it out of the pile of RFI signals.
慎思之，明辨之-再谈FAST射电望远镜 “ 满招损，谦受益—谈谈FAST射电望远镜 ”一文在观察者网和科学网发表后，收到不少批评和质疑。问题集中在三个方面：1)有些读者认为单口径大型射电望远镜的灵敏度远高于综合孔径射电望远镜，对二者作比较时不能只强调角分辨率而忽视灵敏度指标。2)单口径和综合孔径射电望远镜就像苹果和桔子，根本不应放在一起比较。3)FAST射电望远镜将来也可加入VLBA，起到综合孔径射电望远镜的作用。 有批评和争论是好事情。《四书》十九章云：“博学之，审问之，慎思之，明辨之，笃行之。”说的就是为学的几个递进的阶段。读者阅览博客即是“博学之”，留言和评论就是“审问之”。“博学”乃为学的第一阶段，跳越此阶段，为学则成无根之木、无源之水。“审问”为第二阶段，有所不明必追问到底，对所学加以怀疑是必须的。但是问后应有深入的思考，把分析和研究上升到理论层面，是为“慎思”，否则所学所问徒有虚表，难有实质之收获。“明辩”为第四阶段。学是越辩越明，不辩，鱼龙混杂，何分良莠。本文希望为读者“慎思”和“明辩”提供力所能及的指导和帮助。 何谓射电望远镜的灵敏度？简单地说，就是射电望远镜能够测量到最弱的电磁辐射的能力。具有大面积反射镜的单口径射电望远镜在单位时间里可以捕获相对更多的电磁辐射波，因而有较高的灵敏度，这一点没有错，但是认为它的灵敏度远胜于综合孔径射电望远镜，则是有点想当然了。 综合孔径射电望远镜成像是汇总了众多接收天线捕捉到的电磁辐射信号，因而它的成像灵敏度不仅取决于每个接收天线的几何尺寸，而且与整个系统中天线的总量有关。同时必须指出，综合孔径射电望远镜成像是由系统内所有天线中接收的信号分别作两两正交乘积得到的，由于接收机增益和热噪声的变化是非相干的，所以经过一段时间累积平均后这些干涉因素在正交乘积处理中可以抵消 。因而原则上，综合孔径射电望远镜成像的灵敏度是可以做得非常好的，尽管它的每个反射天线尺寸小于单口径射电望远镜，但只要系统中有足够多的反射天线，并且对观测目标作较长时间“曝光”，就可大幅提升灵敏度 。 我们再以综合孔径射电望远镜ALMA为例，它的成像灵敏度是非常高的，它可以探索130亿光年外新形成的河外星系团，而宇宙的边界大约是137亿光年，所以ALMA的成像灵敏度对于研究宇宙的边界和起源是完全合格的 。从这个层面上再来讨论角分辨率，可能更有说服力。 经简单计算可知，在130亿光年的距离上，ALMA可以分辨的光斑直径大约为10 3 光年，而中国的FAST大约为10 7 光年。我们又知道一般的星系团的直径大约在10 5 光年，如果把该星系团区划为100X100的方格，ALMA可以看到并辨别出其中每一个小格，因而基本上可以这样认为，ALMA能得到宇宙边缘上星系团的精细结构。但FAST不仅无法看清星系团内部的结构，它只能把该处的星系团和它邻近的星系团都看成了一个光斑而已。 如果把视线从宇宙的边缘收回来，去探索我们自己的银河系及其周围的星系团，ALMA的高清分辨率足可探测分析星系团里面各个恒星系的结构，研究行星的形成过程，发现更多的系外行星，这已经为ALML发表的最新研究成果所证实 。FAST对这些任务大多是无能为力的，它最多也只能看到整个银河系的大致结构而已。由此可知，综合孔径比单口径的射电望远镜的功能至少要高出一个层次。 综合孔径望远镜ALMA不仅望得远，而且看得清，它在多项重要性能指标上均大幅领先。ALMA的每只反射天线尺寸比较小，它可以放在活动支架上，因而可以有更宽广可变的仰望夹角。FAST的500米反射天线是固定的，只能靠吊在空中的馈源移动来改变观测的仰角，为此牺牲了反射天线的有效孔径，实则上FAST有效工作直径只有300米，有效面积丢掉了一半以上，才换来天顶角40度的天空覆盖（即入射光线于与地面法线间的夹角）。综合孔径望远镜系统中的反射天线还可以不断增加，系统的可扩展性和结构的可变性使得它可以适应多种科学探索任务，这更是单口径射电望远镜望尘莫及的。 还有些读者在评论中强调综合孔径射电望远镜与单口径射电望远镜作用不一样，是不同的系统，不能放在一起比较。这种观点也不正确。综合孔径射电望远镜和单口径射电望远镜都是射电望远镜，都是为射电天文科学研究服务的，把它们的性能作比较，可以更好地了解射电望远镜技术的发展趋势。实际上综合孔径射电望远镜是很容易转换成一台单口径射电望远镜工作模式的，硬件结构不变，只要调用系统的中央处理器中特定的软件系统就可以办到。换言之，综合孔径射电望远镜兼有两者的优势，单口径射电望远镜可以做的，它都能做，反之则不然，一台单口径射电望远镜是无法改成综合口径射电望远镜的。当综合孔径射电望远镜工作在单口径模式时，其反射天线等效面积可接近系统内所有天线面积之和，经过天线之间两两相交的信号处理，系统灵敏度远好于大尺寸单口径射电望远镜 。读到这里，一味地强调单口径射电望远镜的重要性和特殊性还有什么意义吗？ 下面通过定量分析，给出综合孔径射电望远镜工作在单口径模式时的灵敏度计算公式。 从上篇的附件“radio interferometry.pdf”的公式(3)可推出： 考虑到噪声的存在，天线在 和 处接收到的电场强度分别为 和 ， 由此得到的相干函数为 其中长时间噪声平均值 此结果指出相干函数 与噪声无关。 由此可得出清晰的射电源的强度分布函数 为 ： 这里 表示为离散值，L是uv平面上天线阵中任取两个天线的组合的个数， 假定有N个天线，则有: 综合孔径射电望远镜也可以重组为单口径射电望远镜来使用， 这时候的成像在其中心位置，图像为: 其中 是一个平均的相干函数 , 且与噪声无关，恰如前面指出的那样， 而接收到的射电源的强度与N的平方成正比。 这里还提供一个附件 ， 里面有综合孔径射电望远镜作为单口径射电望远镜来使用时两者的灵敏度的实际数据分析比较。 从更高的层次来看，提高单口径望远镜的性能必须建造越来越大的反射天线，而这势必在土木工程、金属材料和机械加工诸领域面临许多难以克服的障碍，而综合孔径射电望远镜技术则是另辟蹊径，使用现代先进的电子技术和信息处理技术突破这些瓶颈，使得系统的性能更优越、造价更经济合理。而这种做法实乃大势所趋，它发生在我们身边的方方面面，机械手表被电子手表代替，六分仪被GPS代替，航母上的蒸汽弹射会被电磁弹射取代。这已经成为一种主流趋势，浩浩荡荡无法阻挡，综合孔径射电望远镜必定是射电天文学技术今后发展的主流，这才是太空探索真正的利器。 以上所有分析只有一个目的，就是澄清综合孔径与单口径射电望远镜之间的区别和联系，但是这些分析和比较都是相对的，和一般意义上的，具体问题又必须具体分析。我没有丝毫贬损大型单口径射电望远镜的意思，相反，我对中国在建设500米大型射电望远镜中展露出来的非凡才智深感敬佩，我为中国的科技进步无比自豪。中国制造的500米射电望远镜是世界上最大的单口径射电望远镜，而且它不是简单的山寨放大版，它采用了多项最新技术。 中国的FAST是球面形射电望远镜，其主反射镜的支撑面做成球形，馈源被六根柔性悬索吊挂在离开主反射镜近百米的高空，馈源在同心球面移动时与主反射镜基准面保持等距，其定位精度达毫米级。而主反射镜做成分布可调单元确保对馈源的每个瞬间形成抛物面，使有效反射面上的所有入射电磁波以点方式聚集于馈源上。FAST是世界上最大最好的单口径射电望远镜，这一点毫无争议。 在我前文的读者留言和评论中有一条写得很好：“不用争，一切是以出成果的事实来说话，其他都是浮云！”我们必须明白，不管是哪一类射电望远镜，它们都仅是工具而已，它们都是为各种天文物理研究服务的。争论工具的好坏事实上没有太大意义，而认为只要有了最先进的设备就能在科研中领先世界，这样的观点更是错误的，我们千万不要掉入唯武器论的思维模式。能否作出一流科研成果的关键首先在人，而不是设备的好坏。即使搬一套ALMA到青藏高原，几年之内中国也未必能出多少惊人的成果，反之，认真用好FAST，中国倒有可能在天体物理探索中作出一些贡献。 还有一条评论写得更精彩：“不管如何，FAST是我国自己最大的射电望远镜，自己的孩子自己喜欢！”中国是一个正在走向世界超强的大国，建设一座世界最大的射电望远镜是完全应该的，从性价比来看也是非常值得。我觉得建设FAST更应该被看成是追赶而不是超越，说实话，中国在天文研究领域严重落后，有许多的空白需要填补，有不少的领域必须补课。中国必须脚踏实地，一步一步向前迈进，即使FAST并非顶尖天文观测设施，但它可能就是向前发展绕不开的坎。今天这个坎被填平，在此基础上，认认真真做研究，实实在在培养人，造就一支顶级的天文科研队伍才是正道。 从某种意义上看，中国贵州的FAST有点像中国海军的辽宁号航母，虽然辽宁号并非世界上最先进的航母，但中国的远洋海军必须从拥有辽宁号起步，它是中国海军自己的航母，中国一批又一批的海军舰载机飞行员将从这里生长起来并飞向远方，假以时日，辽宁号也会具有不可小觑的战斗力。同样道理，中国贵州平塘定会引来成群的金凤凰，它将成为中国天文研究人才的摇篮，中国天文物理的明日之星很可能将在那里冉冉升起。 说明：因篇幅关系，有关FAST射电望远镜加入VLBA的有关分析只能放到续篇之二，敬请继续关注批评。 https://www.mpifr-bonn.mpg.de/948285/Possenti_Why_Single_Dish.pdf （页面13/42） http://www.phys.unm.edu/~gbtaylor/astr423/s98book.pdf （请阅读该书的第九章，尽管书中的理论分析的模型与工程实际系统并不完全一致，但结论大致上是正确的。特别注意公式 9-19 ） http://alma.mtk.nao.ac.jp/e/faq/faq02/ Astronomers found a sign of a growing planet around TW Hydra, a nearby young star, using the Atacama Large Millimeter/submillimeter Array (ALMA). Based on the distance from the central star and distribution of tiny dust grains, the baby planet is estimated to be an icy giant, similar to Uranus and Neptune in our Solar System. This result is another step for understanding the origins of various types of planets. 附件由好友王博士提供，里面有综合孔径射电望远镜作为单口径射电望远镜来使用时两者的灵敏度的实际数据分析比较。 灵敏度数据分析.pdf
满招损，谦受益—谈谈FAST射电望远镜 中国成功建造世界最大射电望远镜的新闻令海内外华人莫不欢欣鼓舞之至。数月前，当500米直径的主反射镜拼装完工时，我在中国科学网发表了博文： 那是一只望穿深空的天眼 ，我对中国工程技术飞速进步的由衷敬佩和对从事该项工程全体工作人员的深深敬意，全都倾注在了这篇短文之中。 但是，在这几天的一片欢呼声中，有一种倾向值得引起注意。有些媒体报导 ：中国建最牛“天眼”领先国际20年；外国科学家参观后也感到很震撼，“他们都很期待我们这个全世界最先进的天文设备能成为人类观测外太空的利器，有新的科研成果出来。”；人们开始幻想，它能否听到“天外来客”的声音？我认为这样的报道违背了实事求是的原则，这种宣传手法有些像拙劣的整容手术，结果往往恰得其反，使原本光辉的印象蒙尘。我觉得有必要拂尘明镜，把事实真相还给大众。 射电望远镜和所有的望远镜一样，把远处相邻物体区分开来的能力是它最重要的性能指标。这种分辨本领一般用成像系统对两个可辨目标之间的最小张角来表示，亦称角分辨率。角分辨率不变，观察物越远，望远镜的最小分辨间距变大，所以当观察研究的天文对象越遥远，我们就必须使用角分辨率更小的射电望远镜，否则就无法得到研究对象的精细结构。通常情况下，望远镜的角分辨率基本上决定了其“望远”的本领。 物理学告诉我们光波就是波长较短的电磁波，因而射电望远镜和光学望远镜实际上就是同一类工具，它们检测的只是不同波长的电磁波。这有点像体温计和气温表，它们的差别就是测量温度的不同的区域而已，在下面的讨论中，除非特别注明，我们把射电和光学望远镜都统称为望远镜。望远镜的角分辨率是由电磁波在其aperture（或antenna dish）上的衍射特性决定的，它是可以根据瑞利公式计算出来的（参见图1）。望远镜的角分辨率与望远镜的主反射镜的直径成反比，而与工作的波长成正比，角分辨率越小越好。 P1)由于光的衍射特性的存在，导致点光源形成艾里斑，重叠以后就难以分辨。 贵州平塘的FAST，其硕大的主反射镜有效孔径为300米，工作波长在0.3m附近，而一般的光学望远镜工作波长在可见光波段，最长不会超过800nm (即0.0000008m），那么它与直径为多少的光学望远镜的角分辨率相当呢？这是一道小学生算术题， （300/0.3）*0.0000008=0.0008m，答案是小于1毫米。这个结果令人十分伤心，市场上的大众化商业望远镜的孔径至少也有十几毫米吧。换言之，贵州平塘的FAST射电望远镜的角分辨率远不及业余爱好者的望远镜，想依靠它作出惊人的科学发现可能有点不切实际。 事实上FAST射电望远镜的角分辨率还不及我们人的肉眼，一般人眼的瞳孔直径为3—9毫米，人眼的角分辨率比FAST也要强几倍。顺便提一下，千万别小看了人眼的望远能力。著名天文学史家席泽宗先生指出:中国的天文学家甘德在公元前四世纪中叶凭肉眼可能就观测到了木星的卫星木卫二。甘德的发现早了伽利略近两千年，这可能也是一个奇迹吧 。 一般来说，射电望远镜的角分辨率都比较差，因为它的工作波长太长，即使把主反射镜做到几百米之巨，其角分辨率仍难以与光学望远镜媲美，但是我们知道反射镜表面的加工精度一般与工作波长同一量级，相比光学望远镜而言，射电望远镜的反射镜的制作要容易做得多，所以直径也容易做得很大。“有无相生,难易相成,长短相较,高下相倾”这世上实在也没有捷径可走的，总体而言，平衡和公平是天道的主旋律。 射电望远镜把工作波长设在厘米、毫米和亚毫米波段首先是为了天文研究的需要，许多温度不高的天体的电磁辐射就在这个波段。这个波段的电磁波的波长至少是可见光的数万倍，现代好一点的光学望远镜的主反射镜的直径都在数米以上，如果要让射电望远镜赶上这个角分辨率水准，其主反射镜的直径至少要数万米，即几十公里之巨。在工程上制造和控制这样的主反射镜已经成了不可能完成的任务，说得难听点，FAST这样的超大型单口径射电望远镜有些像侏罗纪的恐龙，它们未来的发展前途十分有限。 除了增大主反射镜的直径，改善射电望远镜的角分辨率是否还有其它途径？“山穷水尽疑无路，柳暗花明又一村。”七十年代由英国开始研制的综合孔径射电望远镜可以大幅提高系统的角分辨率和接收灵敏度。总的思路是“众人拾柴火焰高”，使用分散的多台射电望远镜同时接收射电信号，然后把信号汇总交计算机分析比较，产生高分辨率的天体射电幅射图象。这种发动群众、依靠群众的思维方式和处事原则才具有真正的普世价值。 ALMA（阿塔卡马大型毫米波/亚毫米波阵列望远镜）是世界上功能最强大、技术最先进的综合孔径射电望远镜系统。这个由欧洲、美国、加拿大、日本、智利等国家和地区合作建设的射电望远镜阵列共有54台12米直径和12台7米直径的射电望远镜组成，散布在南美智利的5千米的高原平台上。整个系统的建设化了十多年，总投资14亿美金，被誉为二十一世纪的金字塔工程。整个ALMA系统的角分辨率比哈勃太空光学望远镜还要强五倍！它的角分辨率比中国的FAST大概强数千倍。 图2这张ALMA的全景照十分震撼人心，这几十台既庞大又精密的射电望远镜散布在高原荒野之中，最大间距约十六公里。它们各自的位置可以根据研究项目作相应的调整。把ALMA放在南半球这块远离文明的5千米高原上是经过深思熟虑的。ALMA的工作波长在0.3—9毫米波段，从图3中可看出大气层对这个波段的电磁波的吸收和干涉十分严重。建在5千米高原的ALMA超越了大气层最紧密的底层，把大气的不利影响減至最少。智利查南托高原的阿塔卡马沙漠是世界上最干燥的地方，这样的地理位置意味着每个夜晚都是较好的观测天气。据统计，在1570至1971年间，这里没有明显的降雨过程，对望远镜的维护极为有利。 P2) P3)横轴是电磁波的波长，纵轴是大气层对电磁辐射的吸收率，曲线显示不同电磁辐射穿透大气能力与波长的关系。从图中可看出，中国FAST的工作波段处在大气层的电磁辐射窗口，因而可建在较低的海拔高度上，而工作在毫米和亚毫米波段的ALMA必须建在高海拔地区。 ALMA选址在远离文明的智利高原上也可避开人为的电磁辐射污染。建在南半球的天文站有得天独厚的视角优势，它更利于对银河系核心区域的观察研究。当然，相对稳定的政治环境，也是ALMA选择智利的一个重要原因。智利对ALMA项目最为关注起劲，大概也是整个计划最大的蠃家。 在接近西藏的唐古拉山口高度的那块荒原上，在南半球离天最近的地方，建设一座规模如此巨大的射电望远镜阵列必定面临一系列工程挑战。整个项目的管理和技术中心设在海拔2900米，所有的设备先汇集于此，经装配调试后再运送至5千米的高原平台（见图4）。直径12米的射电望远镜连支架和附属设备每台重一百吨！为此由德国的专业设备公司设计了特种的高原重载车辆，这些车辆不仅用以输送射电望远镜到5千米的高原平台，而且也负责在整个阵列系统中调整和变换每个射电望远镜的精确位置（见图5）。图片6显示的就是特种运输车辆驮载着百吨重的射电望远镜在亘古的旷野上艰难地往上挺进，看着这张照片，我为人类探索大自然的不屈不挠的精神深深地感动。 P4) P5) P6) 由天线阵列的信息理论可知，如果分布在高原上的ALMA系统中所有射电望远镜都对准同一片星空，每台射电望远镜收到磁辐射信号后先作预处理，讯号经数字化后由光纤送至海拔2900米处的技术中心。各路讯号汇总后送大型计算中心处理（图片7），那么经傅里叶变换后即可直接得到天体的电磁辐射图象 。对综合孔径和单口径射电望远镜都看得见的天体而言，在每个单口径射电望远镜所观测到的光斑内，综合孔径射电望远镜都能给出－幅精细結构图像，这是单口径射电望远镜完全无能为力的。 P7) P8) ALMA投入运行两年多，已经产生了一批重大成果，它必将会对天文物理的研究产生难以估量的贡献 。这里是ALMA在今年九月发布的最新图片（图片8），在非常年轻的恒星（TW Hydrae）的四周的星尘环中间有明显的间隙，在靠近中心的轨道中很可能有一颗海王星大小的系外行星，而且很可能是与海王星一样的巨大的冰球体，这个发现对认识行星生成机制将会有深远的影响 。 作个小结： 1）综合孔径射电望远镜是射电天文观测技术发展的主流，它们才是天文物理研究的利器。 2）“尺有所短寸有所长”，综合孔径射电望远镜技术也并非完美无缺，单口径射电望远镜也不是一无长处，也许不久的将来它们可以联网合作，优势互补。 3）中国的500米单口径射电望远镜具有极高的灵敏度，由于采用多波束扫描，巡天速度效率高，它工作在厘米波段，受大气干涉影响小，它可能会在一些特定的天文研究领域中发挥积极作用。 4）近日看到有这样的评论：FAST的高灵敏度使其能测到一个人在月球上打手机的信号。我们不仿就此作些估算。 假定手机信号frequency是2GHz，它的波长约是15cm，相应的FAST的角分辨率约为0.15／300＝0.0005 弧度，以此乘以地月之间的距离38万公里，不难得出0.0005 ＊380000=190公里 。 这結果指出如果有两个以上的人在190公里的范围内从月球上打手机，FAST测到的信号是无法区分到底是一个还是多个人在打手机的。 5）事实上，中国参与并主导的国际SKA项目就是要建设世界最大的综合孔径射电望远镜系统，这方面中国也有长足的进步，只是少为人知而已。综合孔径射电望远镜的基础是电动力学和信号处理技术，中国不缺这方面的人才和经验，我对SKA项目的未来充满信心。 由于篇幅关系，对以上几点结论的详细介绍和分析将放在本文的续篇中。也希望有兴趣和有一定理工基础的读者能辛苦一下，读一读本文的附件 ，它是续篇所有分析和讨论的基础。这个附件是我的好友王博士特地为本文精心制作的。为了更好更全面地理解综合孔径射电望远镜，我曾向王博士多次讨教，在学习和讨论中产生了几十页之多的草稿，最后几经修改，遂成此附件。该文从电动力学最基本的概念出发，导出了综合孔径射电望远镜的工作原理，文章推理严谨、文字简明扼要，综观各种教科书和网上文献无出其右了。谨此对好友王博士表示深深的感谢。 本文是应【观察者网】约稿而作，首发于观察者网9月25日首页。 http://news.sciencenet.cn/htmlnews/2016/8/353470.shtm Zezong, Xi, The Discovery of Jupiter's Satellite Made by Gan De 2000 years Before Galileo, Chinese Physics 2 (3) (1982): 664–67. 感谢史𣇈雷博主和 Spherical 的评论和批评。 添加一篇重要文献：中科院自然科学史研究所刘金沂在1981年第7期《自然杂志》发表了《木卫的肉眼观测》。 文中提到的木卫二更可能是木卫三。 插入附件 radio interferometry.pdf http://www.almaobservatory.org/en/press-room/press-releases Astronomers found a sign of a growing planet around TW Hydra, a nearby young star, using the Atacama Large Millimeter/submillimeter Array (ALMA). Based on the distance from the central star and distribution of tiny dust grains, the baby planet is estimated to be an icy giant, similar to Uranus and Neptune in Solar System. This result is another step for understanding the origins of various types of planets.
function Y = runmean(X, m, dim, modestr) ; % RUNMEAN - Very fast running mean (aka moving average) filter % For vectors, Y = RUNMEAN(X,M) computes a running mean (also known as % moving average) on the elements of the vector X. It uses a window of % 2*M+1 datapoints. M an positive integer defining (half) the size of the % window. In pseudo code: % Y(i) = sum(X(j)) / (2*M+1), for j = (i-M):(i+M), and i=1:length(X) % % For matrices, Y = RUNMEAN(X,M) or RUNMEAN(X,M, ,1) % % - 1.33 2 3 4 4.67 % runmean( ,1,'mean') % % - 2 2 3 4 4 % runmean( ,1,1) % dimension 1 is larger than 2*(M=1)+1 ... % % - 2 4 6 8 10 % runmean(ones(10,7),3,2,'zero') ; % along columns, using mode 'zero' % runmean(repmat( ,5,1),2,2) ; % % - all NaN result % A = rand(10,10) ; A(2,7) = NaN ; % runmean(A,3,2) ; % % - column 7 is all NaN % runmean(1:2:10,100) % mean % % - 5 5 5 5 5 % % This is an incredibly fast implementation of a running mean, since % execution time does not depend on the size of the window. % % See also MEAN, FILTER % for Matlab R13 % version 3.0 (sep 2006) % Jos van der Geest % email: jos@jasen.nl % History: % 1.0 (2003) created, after a snippet from Peter Acklam (?) % 1.1 (feb 2006) made suitable for the File Exchange (extended help and % documentation) % 1.2 (feb 2006) added a warning when the window size is too big % 1.3 (feb 2006) improved help section % 2.0 (sep 2006) working across a dimension of a matrix. % 3.0 (sep 2006) several treatments of the edges. % Acknowledgements: (sep 2006) Thanks to Markus Hahn for the idea of % working in multi-dimensions and the way to treat edges. error(nargchk(2,4,nargin)) ; if ~isnumeric(m) || (numel(m) ~= 1) || (m 0) || fix(m) ~= m, error('The window size (M) should be a positive integer') ; end if nargin == 2, dim = ; else modestr = 'edge' ; end end modestr = lower(modestr) ; % check mode specifier if ~ismember(modestr,{'edge','zero','mean'}), error('Unknown mode') ; end szX = size(X) ; if isempty(dim), dim = min(find(szX1)) ; end if m == 0 || dim ndims(X), % easy Y = X ; else mm = 2*m+1 ; if mm = szX(dim), % if the window is larger than X, average all sz2 = ones(size(szX)) ; sz2(dim) = szX(dim) ; Y = repmat(mean(X,dim),sz2) ; else % here starts the real stuff % shift dimensions so that the desired dimensions comes first = shiftdim(X, dim-1); szX = size(X) ; % make the rest of the dimensions columns, so we have a 2D matrix % (suggested of Markus Hahn) X = reshape(X,szX(1), ; % the cumsum trick (by Peter Acklam ?) Y = cumsum(Y,1) ; Y = (Y(mm+1:end,:)-Y(1:end-mm,:)) ./ mm ; % reshape into original size Y = reshape(Y,szX) ; % and re-shift the dimensions Y = shiftdim(Y,ndims(Y)-nshifts) ; end end % ===================== % CODE OF VERSION 1.3 % ===================== % function Y = runmean(X,m) ; % % RUNMEAN - Very fast running mean filter for vectors % % Y = RUNMEAN(X,M) computes a running mean on vector X using a window of % % 2*M+1 datapoints. X is a vector, and M an positive integer defining % % (half) the size of the window. In pseudo code: % % Y(i) = sum(X(j)) / (2*M+1), for j = (i-M):(i+M), and i=1:length(X) % % % % If the total window size (2M+1) is larger than the length of the vector, the overall % % average is returned. % % % % Example: % % runmean(1:10,1) % - % % % % % % This is an incredibly fast implementation of a running average, since % % execution time does not depend on the size of the window. % % % % X should not contains NaNs (a NaN will result in a all NaN result) % % At both ends the values of Y can be inaccurate, as the first and last % % values of X are used multiple times. % % % % See also MEAN % % % for Matlab R13 % % version 1.3 (feb 2006) % % Jos van der Geest % % email: jos@jasen.nl % % % History: % % 1.0 (2003) created, after a snippet from Peter Acklam (?) % % 1.1 (feb 2006) made suitable for the File Exchange (extended help and % % documentation) % % 1.2 (feb 2006) added a warning when the window size is too big % % 1.3 (feb 2006) improved help section % % error(nargchk(2,2,nargin)) ; % % sz = size(X) ; % % if numel(sz) ~= 2 || (min(sz) ~= 1), % error('X should be a vector') ; % end % % if any(isnan(X)), % error('NaNs cannot be dealt with') ; % end % % if ~isnumeric(m) || (numel(m) ~= 1) || (m 0) || fix(m) ~= m, % error('The window size (M) should be a positive integer') ; % elseif m == 0, % Y = X ; % return ; % end % % mm = 2*m+1 ; % % if mm = prod(sz), % % if the window is larger than X, average all % warning('Window size is larger than the length of the vector.') % Y = repmat(mean(X),sz) ; % else % % the cumsum trick ... % Y = ; % Y = ; % Y = (Y(mm+1:end)-Y(1:end-mm)) / mm ; % Y = reshape(Y,sz) ; % end
I was not so keen on taking the fast train from Hangzhou to Beijing (6.5 hrs), but my brother preferred train. Considering taking subway Line #4 between Beijing South and Beida, I gave in. The train ride was fine, except for the food: RMB30 for a set, no other choices. I didn't want to try it (and ended up wasting half). Getting on Subway Line #4 was easy, as long as you have two 1-yuan coins. A young man had RMB100, and had to beg someone to get him a ticket. I think a manned station to exchange bills would be necessary (since labors in China are not expensive), or the ticket machine should take credit cards. I thought the 2-yuan subway ride was way too cheap. In Guangzhou, the cost is determined by distance. Not sure what kind of systems other cities use. The visibility got worse and worse as the train traveled north. But, I could see light blue sky over Beida, and the moon. I feel lucky.
That's how I feel about today's life in China. Six friends met for dinner on Friday, and three of them needed to go to work on Saturday! I am not talking about scientists who work every day. These three people are doctor, law-enforcement personal, and senior engineer. Stress can cause real harm, if it's not dealt with. I am sure you all know this. Everywhere I visited, my host was on the move. In Guangzhou, my host had to leave for a conference in the US the very next day after our conference ended. In Nanjing, my host stayed for my one-day visit, and left for Beijing the very next morning. I lectured at the NUIST on Friday; my host there was called away in a short notice and called me that morning on his way to the airport, to make sure I was on my way for my lectures. I was going to visit the Shenzhen Campus of Tsinghua University on the 13th, and just received an e-mail to move my visit to the 12th because my host has a meeting on the 13th, again in a short notice. How do you guys (gals) handle life like this? Don't you want some peace and time for yourself?
It took three hours to travel from Nanjing to Hangzhou by a fast train (for RMB218). (I heard the travel time will be less than two hours when Gao Tie is finished.) However, it took 40 mins to get a taxi at Hanghzhou Station, where people mountain people sea was more than being accurate. Thanks to bad traffic on a Saturday afternoon around 4pm (why?), I was freezing to death in a taxi filled with stinky cigarette odor.
If you are a member of the AGU (American Geophysical Union), you probably have heard about or published in GRL (Geophysical Research Letters) . Recently, I heard complaints from colleagues about how their manuscripts are returned without being reviewed . I was surprised: Really? Now, I learned the truth. Here is why, and more about GRL (with my notes): New GRL policies ps. I suppose that there are things Chinese letter journals can learn from GRL, but hopefully not everything.
请见 http://sciencewatch.com/dr/fmf/2010/10mayfmf/ May 2010 From the database of Essential Science Indicators SM , this list of Fast Moving Fronts has been generated by a comparison of the data sets for the current period of January 2004-December 31, 2009, and the previous period of November 2003-October 31, 2009 (sliding 6-year period). Research Front Maps may be selected from the current Research Front set that are relevant to the core papers of the chosen field. View them at the site-wide listing of all Research Front Maps (sorted by field. This symbol indicates that the author(s) of the comments below have also sent along accompanying images/descriptions of their work.
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