Just how far can we see?

Just how far can we see?

Well, on a clear night with just the naked eye you can see for 23,651,826,181,450km (2.5 million light years) to the Andromeda galaxy. With modern telescopes and advances in science and technology we can see many times further than this, and the further we look into space, the further back in the history of the universe we see. Not only do we want to look deeper into space, we want to be able to observe it in greater detail. Seeing further and in more detail requires larger and larger mirrors in telescopes. However there is a simple mechanical limit to how large a mirror can be made. Too large a mirror will deform and bend under its own weight, warping the light that falls on it.
For almost three quarters of a century the largest, most powerful telescope in the world was the Leviathan which was built by the third Earl of Rosse in Birr, Co. Offaly in 1845. This telescope’s mirror is 72 inches in diameter and with this, the third Earl of Rosse was the first to observe the spiral structure of many nebulae and also observe individual stars within them. Following on from this work, Edwin Hubble realised that these nebulae were in fact galaxies completely separate to, and often much larger than our own.
The largest ground based telescope today is the Keck telescope located atop Mauna Kea in Hawaii. The Keck is revolutionary in its design in that it consists of 36 hexagonal mirrors all working together in an array so that the telescope has an effective diameter of 10m. Each mirror is small enough to be supported by itself and can be moved independently to a precision of four nanometres (250,000 times thinner than a human hair) to give the optimal shape of an overall much larger mirror. With the Keck we have been able to see further than ever before from our planet. And yet we are still limited in our view of the universe. To get into really fine detail and to see further still we need to go above and beyond the atmosphere of the earth. Perhaps the most well known telescope in space is the Hubble Space Observatory, a telescope with a mirror 2.4m in diameter orbiting 589km above the earth’s surface. Hubble has produced some of the most breathtaking images in astronomy, including the “Hubble Ultra Deep Field” – an image which looks deeper into space than any other image ever taken. Hubble and the Keck have also observed the single most distant object seen to date – a faint galaxy 13 billion light years away. They are seeing this galaxy as it was 13 billion years ago, when the first galaxies were being formed.
Yet we are still pushing to see further back in time and in greater detail. How far will we be able to push this? Right back to when the very first stars “switched on” and began emitting light? One telescope which hopes to do just this is NASA’s James Webb Space Telescope which is due for launch in 2013. It will be similar in design to the Keck, being composed of 18 hexagonal mirrors which will give an overall mirror diameter of 6.6m, significantly larger than Hubble and without the atmospheric distortions that affect the Keck. Construction of the 18 mirrors was completed last week and they are currently being ground and polished, before being assembled in NASA's Goddard Space Flight Center. This telescope hopes even to be able to shed light on the origins of life on other planets.
So just how far can we see? 13 billion light years so far, but we are constantly pushing this boundary further back. The James Webb telescope will look further than ever before and who knows what light it will be able to shed on the origins of our universe, but it will no doubt be an exciting time for astronomers and scientists everywhere.

Sarah Kennelly is the President of the Physics and Astronomy Society, Astro2 and is currently in her final year of Physics with Astrophysics.