Size Does Matter!
Unfeasibly Large?
Dr Ruben Sanchez-Janssen provided a fascinating exposition of the scale of the Extremely Large Telescope (ELT) project currently under construction in the Atacama Desert in Chile. A true behemoth with a 39m primary made up of around 800 segments, with a telescope weight of nearly 4000 tonnes. First light is scheduled for 2028, when it will begin its task of studying supermassive black holes, early galaxies and protoplanetary discs. Though an even bigger ‘Overwhelmingly Large Telescope’ has been considered, perhaps this should be the ‘Unfeasibly Large Telescope’ - because of practical construction considerations, Ruben believes that the ELT will never be surpassed in size!
Don’t forget the lens cap
With a 30-year life expectancy and a cost approaching £1.5 billion, this translates into an hourly viewing cost of over £10,000. Understandably, there is a lot of competition for telescope time! The ELT’s location provides perfect viewing conditions, but of course, there is still significant atmospheric disturbance. In the reflector’s optical train is a deformable mirror, much smaller than the primary, but with a key role - to unravel the optical wavefront disturbances caused by the atmosphere using adaptive optics. Reflected light from artificial ‘atmospheric stars’ created by six lasers is analysed in real time, and 5000 actuators on the deformable mirror are separately adjusted until these ‘stars’ are focused to a point. Sounds simple? Not really - this has to be done a thousand times a second! Finally, the corrected light can be analysed. There are five key imaging and spectrograph instruments that will undertake the science, with significant UK involvement in all of them.
Move over James Webb Space Telescope (JWST)?
Despite the construction challenges, it’s still easier to build a terrestrial telescope than launching one into space, so is JWST going to be redundant? No, these scopes are very complementary. Space-based systems avoid atmospheric absorption in the infrared, which is important for looking at the faintest objects in the early universe. Also, with a larger field of view, the JWST will always produce the most stunning images. On the other hand, by collecting 40 times more light, having a six times better angular resolution, and with a focus on sophisticated and upgradeable spectrographic instruments, the ELT will have an advantage in studying exoplanets and undertaking higher-resolution studies of distant galaxies.
A recording of the lecture, including a member talk by Chris Sutcliffe on dark skies, will be made available to members via the link in the weekly newsletter.
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