European Extremely Large Telescope

In astronomy we are very limited by the instruments we use. Telescopes are very fickle pieces of equipment that are limited by quite a few factors. You have to correct for outside light sources, atmospheric perturbations, how detailed you can accurately resolve details for your image, and a number of other things. In order to solve most of these issues astronomers have taken to putting telescopes such as the Hubble Space Telescope into, you guessed it, space. This solves a lot of problems for correcting atmospheric disturbances and other earth based issues. The negative side to this is that sending something into space requires quite a bit of time, money and energy. You are limited on how large you can make the telescope when you put it in space. That's why with more technological advancements being made it makes much more sense to build massive telescopes here on Earth.

There are many plans in the making for some very large telescopes. As a side note astronomers are not very creative and they all have names like  "Very Large Telescope," which is among one of the precursors to my topic of discussion today: The European Extremely Large Telescope or E-ELT for short.

An artists rendering of the E-ELT
http://www.eso.org/sci/facilities/eelt/telescope/dome/

This monster of a telescope will be built high in the mountains(3064m) in the Cerro Armazones in northern Chile. This area is apparently cherished in astronomy because it has a cloudless nights about 89% of the year due to the high altitudes.

Originally the telescope was supposed to feature a massive 42m aperture for its main mirror, but this was reduced to 39.3 after careful financial considerations. It features a 4.2m secondary mirror to allow for a whopping collecting area of 978 square meters of night sky. To put this in perspective, the largest telescope in operation today is the 10.4m Gran Telescopio Canarias in the Canary Islands. This telescope is able to collect light over an area of only 74 square meters. This means that the new E-ELT will be able to observe over 13 times more of the sky at one time than any telescope currently operating. You may think well that is nice, but how does it help? Based on current designs it be able to resolve images about 15 times better than the Hubble Space Telescope.

A Hubble image of the Carina Nebula

http://www.theatlantic.com/infocus/2012/11/2012-hubble-space-telescope-advent-calendar/100415/

The above image is a Hubble image taken in high resolution. Notice the minute details that are resolved in the structure of the gas and imagine being able to see it almost 15 time BETTER, from Earth. That is truly an admirable goal. 

Now it should also be mentioned that the E-ELT is not meant to be a standalone research telescope. It is meant to work in conjunction with functioning space telescopes since time on a space telescope is much sought after and hard to come by. 

The purpose of building the E-ELT is not for any single reason. Pretty much any question posed today in astronomy today can be cited as a reason for building the telescope. So a general answer would simply be for curiosities sake. If that's not good enough for you then the answer will be to look for extrasolar planets. A hot topic in astronomy right now is being able to identify and classify Earth-like planets simply from an observational basis. However this is extremely difficult given the fact that planets generally don't comparatively radiate in an easily observable spectrum. To make a long story short we are looking for planets that may support life.

Other possible things it will be looking for are evidences of the early universe such as what happened during the era of reionization, evidence for dark matter, and evidence for how galaxies and black holes are formed and operate. A another major topic for study is direct measurement of the expansion of the universe which would have huge implications if possible.

The telescope will use a large array of instruments to view objects in the visible and infrared spectrum. The telescope will feature a number of supplementary instruments such as wide field lenses, spectroscopes in visible and infrared and multiple cameras. The telescope will also feature cutting edge adaptive optics(used to correct for atmospheric perturbation) to produce stable and crystal clear images.

The primary mirror on the telescope will consist of 798 hexagonal mirrors each 1.45 in diameter but amazingly only 50mm thick. Each of these little mirrors will be supported by a self correcting robotic arm of sorts that will adjust the mirrors position thousands of times per second. The optical path and mount for the telescope is called a Nasmyth mount which is a form of a Cassegrain telescope where instead of light being focused through hole in the primary mirror it is directed out via a third mirror to an eyepiece.

Nasmyth optical design

http://en.wikipedia.org/wiki/File:Nasmyth-Telescope.svg

As of right now there are no large speed bumps in the project. The telescope is still in the R&D phase of its life so it will be quite a while before it is operational. As of right now its completion is planned for the early 2020's