液体镜望远镜将成为现实

Yan — Mon, 02 Jun 2008 12:31:57 -0700

以前提过液体镜望远镜方面的新闻，今天《新科学家》上有篇文章说，世界上第一个专用液体镜望远镜的天文台预期于 2009 年上岗。以前的液体镜望远镜都是实验性质的，这一台完成，将运行使用。

这台缩写为 ILMT（International Liquid Mirror Telescope）的望远镜有一块四米宽的液体主镜，由加拿大和比利时资助，将落成于印度北部某处山顶。

下面摘抄一段文中关于液体镜望远镜的 FAQ，懒得翻译成中文了。

What is a liquid-mirror telescope and how does it work?

The concept is simple: spin a pool of liquid and its surface curves into a parabolic shape – by coincidence, exactly the shape needed for a telescope mirror to focus light. Using a reflective liquid like mercury, a telescope mirror can be made quickly and relatively inexpensively.

What are the advantages of a liquid-mirror telescope?

Liquid mirrors are lighter, simpler and much less time-consuming to construct. They can be built for just a tenth of the cost of a conventional glass telescope mirror.

So why haven't liquid-mirror telescopes already replaced regular telescopes?

The major limitation of liquid-mirror telescopes is that they can only look straight up, so they can only observe what passes directly overhead.

What kind of observations are liquid-mirror telescopes suited for?

They are ideal for sky surveys. For example, an 8-metre telescope at the right location could observe 1 billion galaxies, just by staring at the strip of sky that passes overhead. It would also detect large numbers of supernovae, which would allow astronomers to better determine the properties of dark energy – the baffling effect that makes the expansion of the universe accelerate.

What liquid-mirror telescopes are being planned?

In addition to the ILMT, Arlin Crotts of Columbia University in New York, US, is leading design studies for a proposed Earth-based 8-metre telescope called ALPACA (Advanced Liquid-mirror Probe of Astrophysics, Cosmology and Asteroids). The ultimate liquid-mirror telescope could one day be constructed on the Moon, with a diameter of 20 to 100 metres, which would offer insight into the formation of galaxies in the very early universe. A Canadian team is studying the possibility of a smaller 2-metre liquid-mirror telescope for the Moon as a step towards this.

What does the future hold for liquid-mirror telescopes?

Ermanno Borra of Laval University in Quebec City, Canada, and his colleagues are developing reflective liquids with high viscosity that could allow liquid mirrors to tilt up to 30 degrees from vertical, opening up much more of the sky to their gaze.

As astronomers push for larger and larger telescopes, their costs are increasing accordingly, making liquid mirrors more and more attractive, Borra says.

"If you go back to the beginning of the 20th century, it was mostly the refracting [lens-based] telescopes that were used and it took some time for glass mirrors to take over," he told New Scientist. "My feeling is that we're going to see the same thing with liquid mirrors."

天文爱好者

用液体镜做望远镜

Yan — Thu, 21 Jun 2007 09:33:00 -0700

新一期《Nature》上有一篇文章（Nature 447, 979-981 (21 June 2007) ），介绍了在液体上镀金属膜的工作，他们说目的是做月球望远镜。原来作者之一 Ermanno F. Borra 在 1991 年曾经提出过使用液体镜为主镜来做月球望远镜的想法。

在月球上建望远镜的好处在于“空气稀薄”。大气保护了地球上的生命，也阻碍地球上的生命了解外界。一方面，由于大气的吸收，部分谱段的光到达不了地球表面；另一方面，由于空气的扰动，影响了地球上天文望远镜的分辨率。因此人们发射哈勃之类太空望远镜来避开这些困难。自然而然地，人们也会想到在月球上建天文台。：）

液体镜望远镜的最大优势，根据 WikiPedia，是便宜，只有常规望远镜的 1%。不清楚为啥液体镜对月球望远镜这么有吸引力，也许是运输、安装方便方面的因素吧。

液体镜的原理挺简单，就是把液体放容器里，然后旋转，液体表面自然地会成为弧面。如果是理想的抛物面，那就可以用来做理想的凹面镜了。不过技术上实现起来不容易。这个想法居然可以上溯到牛顿，不过到 1872 年，才有人（Skey）真正做了些尝试。1909 年的时候 Robert Wood 成功地制作了一台 51 厘米的液体镜望远镜。但后来他放弃了这个工作，因为这样的液体镜只能面向重力的反方向，因此做成的天文望远镜只能看天顶，不可以调角度。

虽然有这样的局限性，但有些天文研究（比如：long-term sky surveys，supernova searches，根据 WikiPedia）并不受其影响。而且，天文学家们也发展了一些技术来弥补缺陷；可倾斜的液体镜也在研究中。

Ermanno F. Borra 所在的 UBC 已经建了一台 6m 直径的液体主镜望远镜。关于在月亮上建大型液体主镜望远镜的想法似乎也没停止过，Wired 上有一篇相关报道。

以上图片来自 UBC LMT 网站。