The effectiveness of a telescope depends on its size; a larger telescope can gather more signals than a smaller one and therefore provide more information about the object being observed.
Since 1963 the 305-metre Arecibo Observatory has been the world’s largest and (therefore) most sensitive full-dish radio telescope.
But in March of this year China began construction of FAST, the Five Hundred Metre Aperture Spherical Telescope, in a natural depression in southwest China.
Costing $100m and due to be completed by the end of 2013, FAST will be unlike Arecibo in that the surface will be “adaptive”, able to change its shape to create 300-metre sub-dishes than can point up to 40° away from the vertical. Arecibo is only able to point vertically upwards and relies on the rotation of the Earth to sweep it across the sky.
As previously mentioned, size matters when it comes to telescopes: the bigger the mirror, the better the telescope (i.e. the greater the resolution and light-gathering capability). The world’s largest single telescope mirrors are the 8.4-metre telescopes used by the Large Binocular Telescope.
Telescope mirrors must be perfectly polished and accurate in shape and size to within a billionth of a metre. This means that making telescope mirrors is extremely difficult and therefore time consuming and very expensive, costing millions of dollars.
But there is an alternative to polished metal and silvered glass: mercury. Simply filling a pan with liquid mercury will create a perfectly flat* reflective surface; the surface can then be made curved by spinning the pan. No expensive manufacturing is required and this keeps costs low: a liquid mirror costs about 1% of the cost of a similar-sized conventional mirror. (They do have the disadvantage that they can only point upwards, however.)
The University of British Columbia’s six-metre Large Zenith Telescope in Vancouver is the world’s largest liquid mirror telescope.
The simplicity of constructing a liquid mirror telescope has even led to suggestions that one should be built on the Moon.
* To give you some idea of how flat a liquid mirror is: if you built a mirror the size of the Earth, the largest bump would be less than a millimetre in height.
I love the way that scientific institutions name telescopes.
It started with the “large” telescopes like the Large Binocular Telescope (LBT), an 11.4-metre telescope (made up of two 8.4-metre mirrors) in the Pinaleno Mountains of Arizona; the Very Large Telescope (VLT), an array of four 8.2-metre telescopes in the Atacama Desert in Chile; and the Large Synoptic Survey Telescope (LSST), an 8.4-metre telescope also planned for the Chilean desert.
After the “large” telescopes came the “giant” telescopes: the Giant Magellan Telescope (GMT) is 21.4-metre telescope under construction at the Las Campanas Observatory in Chile and the Giant Segmented Mirror Telescope (GSMT) is a planned 20-30-metre telescope.
Some of the larger telescopes, like the Thirty Metre Telescope (TMT) have disappointingly simple names; it seems that after using up “giant” they went back to “large”, but with adverbs. There’s the 42-metre European Extremely Large Telescope (EELT) and my personal favourite: the 60-metre Overwhelmingly Large Telescope (OWL).