ELTs phone home

In my first NAM post I mentioned Extremely Large Telescopes (ELTs). These have been proposed by several nations and even groups of nations such as Europe. They were previously known as OWLs (Over-Whelmingly Large Telescopes) although that name seems to have fallen out of favour and the proposed sizes are no longer quite so over-whelming as the original 100 m (328 ft) diameter. So, I decided to go to the National Astronomy Meeting session on ELTs to find out what the current status of the European effort is.

The European design and technology program is now aiming at a 30-60m diameter telescope with a baseline of 42 m (the answer to life, the universe and everything crops up everywhere!) and a "large" field of view of about 5 to 10 arc minutes (one sixth to a third the angular size of the Moon).

So why do we want huge telescopes? Firstly, the increased collecting area gives the ability to look at fainter objects and, secondly, the increased diameter gives better resolution. Significantly improving these two aspects opens up some new areas of study. For instance, it will be possible to directly image faint exo-planets from their companion star and it should also be able to detect planets with masses as low as the Earth with the radial velocity method.

New study won't just be limited to established planets though as an ELT will be able to image proto-planetary discs where solar systems are forming. On larger scales, and ELT will be used to explore the physics of how galaxies form. Other interesting things that it is hoped to study include dark energy by observing the spectra of Type 1a supernova to huge distances (z~4), variation in fundamental physical parameters in the universe and the physics of black holes.

There are at least two reasons why such large optical telescopes have not been built so far. The most practical problem is the sheer amount of engineering require to build a structure so vast and yet so precise. It will need to point to anywhere in the sky and also withstand earthquakes and wind which will make the whole thing sway. The current European design will oscillate at a rate of 2.1 Hz (with a wind speed of 10 m/s) which one of the speakers - Guy Monnet - pointed out will lead to a wobble of the image of around 1 arc second. This will need to be corrected for.

The other major limiting factor is the Earth's atmosphere. It sits in front of every ground-based telescope wobbling away and distorting the images. In recent years this limitation has started to be corrected for on large, professional telescopes using adaptive optics. However, that doesn't mean that the problem is solved because it becomes a much harder task as you make your telescope larger.

So where might it be? An ELT site needs clear skies (lacking in clouds, dust, contrails etc), the atmosphere has to be suitable for adaptive optics, and it needs to be suitable from a geographical, economical and political point of view (astronomers need to be able to get in and out of the country to be able to observe). A decision on the site may happen by the end of 2008.

So what does the future hold? Well, there is a lot of work being put into this but we aren't going to see anything for at least 10 or 15 years.

I'll finish with a great quote from Guy Monnet who said (in reference to ELTs):

"It has gone from being enormous to being half enormous"

Posted in astro blog by Stuart on Tuesday 17th Apr 2007 (10:07 BST) | Permalink
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