Size matters

With telescopes, the diameter matters. Bigger is better in two ways. First of all, the wider the telescope, the more photons of light (or radio waves/X-rays/whatever) it can collect and the more photons the better the quality of the images or measurements. That is called the sensitivity and can be thought of as the number of grey-scale levels in your image. Secondly, due to the physics of optics, the larger the diameter of your collecting bucket, the better the resolution; the image looks less blurry. For instance, a 10 inch (25cm) diameter optical telescope can theoretically tell apart two stars that are spaced by only half an arc second (a 6500th of a degree). In reality, our atmosphere smears everything out so you don't actually get images as good as that. Pity!

For optical telescopes, the atmospheric smearing (or 'seeing') meant that improvements in resolution were limited until the Hubble Space Telescope (HST) in the 1990s when, for the first time, a telescope could get a perfectly clear view (that is one of the reasons Hubble has done so well). However, throwing enough clever people at the problem of 'seeing' has meant that in the last few years fancy techniques like adaptive optics are starting to give images from the ground that are nearly as good (or better) as those from space. The next steps in optical astronomy should see the launch of the next-generation HST (named the James Webb Space Telescope) and the construction of 30-100 metre diameter optical telescopes. That would be a sight to see!

But astronomy isn't just about optical telescopes. Over the past 60-70 years radio astronomers have been steadily improving the quality of their observations of the Universe too. Nowadays, radio telescopes are often linked together to make an instrument the size of our planet (VLBI), so the resolution has got about as good as it can get unless we build radio telescopes elsewhere in the solar system and that is a tad expensive just now. So, the next step in making radio telescopes better is to improve the sensitivity.

The first purpose-build radio telescope was built in 1937 in the back garden of Grote Reber who lived in Chicago. It was nine metres in diameter which is pretty big for a back garden. After those initial observations, no major new instruments were built until the 1950s with the construction of Dwingeloo in the Netherlands and Jodrell Bank in the UK. With much larger areas, these instruments were 10s to 100s of times more sensitive than Grote's first dish so could do better measurements. Progress has continued at a tremendous pace over the 1960s (Parkes, Arecibo and Effelsberg), 1970/80s (VLA, ATCA), and up to the present day (GMRT, GBT) each time, increasing the sensitivity. Today, the most sensitive radio telescopes are about a few hundred thousand times more sensitive than the state of the art in 1937. This rate of improvement is pretty impressive and in the next few years eMERLIN and EVLA will push that up to a million times more sensitive than we were in 1937 (see plot below and be amazed). Although bigger and better usually means more money and more difficult, plans for the next-next generation are already under way. More on that in my next post.

The sensitivity of radio telescopes since 1937 CREDIT: Square Kilometre Array website

Posted in astro blog by Stuart on Friday 29th Sep 2006 (13:06 BST) | Permalink
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