Thursday, April 29, 2010
On the subject of gigantic telescopes again, Wikipedia has a list here of the largest telescopes currently in operation. The largest one so far is the Large Binocular Telescope with a pair of 8.4-metre mirrors giving a surface area equivalent to a single 11.8-metre mirror, and the smallest on the list have a diameter of 3 metres. The average cost for an impressive observatory of this type usually ends up being in the tens to hundreds of millions of dollars, and as mentioned in the post earlier the Extremely Large Telescope to begin operation in 2018 is about $1 billion, so this is the upper end. The Overwhelmingly Large Telescope that the ESO chose not to construct due to cost and technical difficulties would have been twice that, $2 billion.
Near the top of the list though, we find one interesting telescope of 6 metres in diameter called the Large Zenith Telescope. A telescope of this size is usually about $100 million to construct, but this one only cost $1 million. Why? Because it's a liquid telescope, using a flat area filled with mercury that spins at a certain rate making it into a parabola, which then becomes just like any other telescope except for one disadvantage: because it's a liquid it can only observe whatever happens to be directly above it. Liquid telescopes have been proposed for construction on the Moon as well, but in the meantime they can be constructed here as well for 1 - 5% of the cost of a normal telescope requiring a cast and polished mirror.
The Large Zenith Telescope seems to have been built more as a demonstration of the concept, because it's located in an area quite far from ideal: very close to Vancouver. It was constructed on a mountain in a relatively dark area, but the mountain is still only about 395 metres in height and the climate is still quite cloudy. See a picture of the observatory here and here.
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For just $1 million to demonstrate the concept there's nothing wrong with the location, but any other telescopes of this type will need to be constructed in a better location than that.
So...if $2 billion can't be scrounged up for the construction of the Overwhelmingly Large Telescope in Chile, why not construct a liquid telescope of the same size?
If liquid telescopes are that much cheaper than conventional ones then one this size could be constructed for just $20 to $100 million, and there would also be two reasons not to be so concerned about the disadvantages a liquid telescope would have:
1) Even perfectly stationary, it would have a collecting area some 70 times larger than our largest telescope today, and five times that of the largest Extremely Large Telescope set to begin in 2018,
2) Research has been underway for a while on developing a liquid mirror that can withstand being tilted a bit - some say ten, others say up to thirty degrees. But even ten degrees would be sufficient for a telescope of such impressive size.
So if a massive liquid telescope were to be built, it would simply be constructed with the assumption that it would remain facing the zenith the entire time, but also done in such a way that the telescope can be improved without having to rebuild it if the technology is developed that allows it to tilt.
New Scientist has an article here written in 2007 by one of the people that constructed the Large Zenith Telescope, where he describes the process and the pitfalls involved in making it, plus (on page 4) other liquid telescopes under development. There are two of them, both in Chile, and he does mention that the site in Vancouver was chosen out of necessity as it was close to his house, so liquid telescopes still have yet to be given the opportunity to impress.
So how are these two projects doing now almost three years later? The first one (the International Liquid Mirror Telescope, 4 metres in diameter) had a successful spin cast in September (videos here). The other one (Advanced Liquid-mirror Probe for Astrophysics, Cosmology and Asteroids) - not so sure, as its page hasn't been updated since 2004.
Final thought: perhaps the problem is that the idea hasn't been promoted as actively as it could. If so, maybe liquid mirror telescopes of a much less ambitious size (1 metre or so) could be developed and mass produced in a single design to be sold to smaller but still professional observatories, enabling them to make observations with these telescopes at a much lower price than they otherwise could using a mirror of the same size.