How does a telescope work?

Credit: ESO/B. Tafreshi (twanight.org)

A larger telescope mirror (or lens) collects more light and reveals fainter objects. It also produces a sharper image, showing more detail. Magnification – determined by the focal length of the eyepiece – is less important.

Catching the light: With their 8.2-metre mirrors, the four Unit Telescopes of ESO’s Very Large Telescope in Chile collect huge amounts of starlight.

Credit: ESO/G. Lombardi (glphoto.it)

Eye in the sky: At 2.4 metres across, the main mirror of NASA/ESA’s Hubble Space Telescope is relatively small, but operating in space gives it many important advantages.

Credit: ESO/(M. Kornmesser

For telescopes, bigger is better. A larger telescope aperture means a brighter and sharper image: the bigger the primary lens or mirror of a telescope is, the fainter the stars it can detect and the finer the detail it reveals. In astronomy, magnification is not so important: by using an eyepiece with a long focal length, a telescope’s magnification can be increased significantly, but the resulting image is fainter and the resolution – the amount of detail – does not get any better. Seen through a telescope, planets or galaxies look bigger than with the naked eye: the image is spread out over a larger part of your retina or the camera detector.

Ground truth: The Apollo 12 Moon lander has been imaged by NASA’s Lunar Reconnaissance Orbiter. It’s too small to be seen from Earth, even with the world’s largest telescopes.

Credit: NASA

Have you ever looked through a telescope? What you see depends on the telescope’s objective and eyepiece. You can try it here. Choose your target and explore!