Luke Ramsay, of King's High School, asks :-

How far can astronomers see into space?

Frank Andrews, an astronomer at Wellington's Carter Observatory, responded.

That depends on how large a mirror the telescope has, how much air is above the telescope and how sensitive the light detector is.

Modern electronic light detectors, such as a charge coupled device (CCD), are very efficient and register almost every photon of light that strikes them. Today a 40cm diameter telescope fitted with a CCD camera can detect objects as faint as the limit which the Mount Palomar telescope, whose mirror is 5 metres in diameter, had in 1949.

Light is collected by the telescope mirror so the bigger the mirror the more effective it is in gathering photons and hence weak images. At present the largest diameter mirror on an optical telescope is the 10 metre diameter mirror on the Keck telescopes.

To reduce the degredation of the image due to scattering in dirty and turbulent air the Keck telescopes were placed on the summit of a dormant volcano, four kilometres high, in Hawaii. The Hubble Space Telescope is only 2.4 metres in diameter but because it is in orbit above the Earth's atmosphere it can see objects which are at least as faint as those seen by the Keck telescopes. If you go outside on a dark, moonless, night and look at the faintest star you can see then the Hubble Space Telescope can see objects 10 billion times fainter.

Even larger earth and space based telescopes are being planned for the near future. Currently we can see objects for which the light left them about 10 billion years ago. Since light travels at 300 thousand kilometres every second and there are 365x24x60x60 = 31,536,000 seconds per year these objects are about 100 thousand billion billion km away.