16-06-2007 The Modern Telescope (Article 90)

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Simon Kenny
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16-06-2007 The Modern Telescope (Article 90)

Post by Simon Kenny » Wed Feb 04, 2009 11:02 am

The Modern Telescope (Article 90)

A basic fact about any telescope is the bigger, the better. With a bigger scope, you can see more, more clearly and further away. A telescope’s ‘bigness’ is determined by its light-gathering ability and this is determined by the size of its objective lens or its mirror, depending on the kind you use. The 400 year history of the telescope is partly about the struggle to make ever larger scopes to coax ever fainter and subtler detail from the panoply of luminescent objects inhabiting the night sky. This relentless quest for size has led from Galileo’s little ten-centimetre telescope to the impressive ten-metre behemoths built on top of Mauna Kea in Hawaii today, with a massive 42-metre scheduled for near future.

The massive telescopes at the cutting edge of astronomy today are very difficult to build, so a casual observer could be forgiven for asking: why, if they are so difficult to build, do they compound the difficulty by putting them on the tops of high mountains? The answer lies in the sharpness of the image. When someone tries to use high magnification with any telescope at sea level, the image is constantly shifting, like the ‘heat-waves’ over a road in the summer heat. This ‘boiling’ of the image increases with the magnification, to the point where further magnification is pointless. The cause of this problem is the dense 10 kilometre layer of atmosphere just above our heads, which is seldom steady enough for viewing at high power. The answer to the problem is to get above as much of the atmosphere as possible, at least above the denser lower atmosphere, which also has most of the cloud and weather systems, another problem for good viewing! Using telescopes above the atmosphere has an added benefit: parts of the electromagnetic spectrum blocked by the lower atmosphere could also be studied: the infra-red and ultra-violet spectra of stars. That is why the largest telescopes ever made are also built in some of the highest and remote places on earth. One of the most famous examples of these is Mauna Kea, which is 4,200 metres high and has some of the world’s biggest telescopes. Apart form the two Kecks, it has the Japanese 8.3 metre Subaru Telescope and one of the pair of 8.1 metre Gemini observatories: Gemini North. Its twin, Gemini South, is on Cerro Tololo, over 2 kilometres up in the Peruvian Andes. The Indian Astronomical Observatory holds the current record for the highest observatory, at 4,500 metres, in the Western Himalayas. However, increasing altitude brings more than better telescopic views: it increases accessibility problems and high-altitude health problems for the builders, scientists and engineers as well as the astronomers involved in the project. These inherent limitations on the altitude of ground based telescopes have led to the development of a type of telescope considered impossible until the relatively recent developments of rocket science and electronics: the space telescope. This will be discussed in a later article. Clear skies!

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