Telescopes

A telescope is an instrument that collects and focuses light to make distant objects appear brighter, larger and more detailed than they do to the unaided eye. Its main value comes from its aperture, the diameter of its main lens or mirror: a larger aperture gathers more light, allowing fainter stars, galaxies and nebulae to be seen and revealing finer detail. This light-gathering ability, more than mere magnification, determines how much a telescope can show.

There are two basic optical designs. A refracting telescope uses a lens at the front to bend and focus incoming light, while a reflecting telescope uses a curved mirror, an approach associated with Isaac Newton. Most large modern telescopes are reflectors, because large mirrors are easier to build and support than large lenses. Telescopes are mounted so they can track objects across the sky as Earth rotates, and many are paired with cameras and detectors rather than used only for direct viewing.

Ground-based telescopes are limited by Earth's atmosphere, which blurs images and absorbs some wavelengths of radiation. Observatories are therefore built on high, dry mountain sites to reduce these effects. Placing a telescope in orbit removes the atmosphere altogether: the Hubble Space Telescope, operating in space, has produced exceptionally clear images of remote objects. Telescopes are also built to detect radiation beyond visible light, such as radio waves, infrared and X-rays, broadening the view of the universe far past what the eye can perceive.

Frequently asked questions

What is the difference between a refracting and a reflecting telescope?

A refractor focuses light with a lens at the front, while a reflector uses a curved mirror. Most large telescopes are reflectors because big mirrors are easier to make than big lenses.

Why does a telescope's size matter?

A larger main lens or mirror gathers more light, so it can reveal fainter objects and finer detail. Light-gathering ability matters more than magnification alone.

Why put a telescope in space?

Above the atmosphere there is no air to blur the image or block certain wavelengths, so an orbiting telescope such as Hubble can produce much sharper views.