The main purpose of a telescope is to collect and focus light. Reflecting telescopes achieve this by the use of systems of mirrors. Typically, a large curved mirror collects the light. The curved surface redirects the light rays into focus. A smaller secondary mirror routes the light through an eyepiece.

Refracting telescopes collect and focus light using systems of lenses. A large lens collects light and bends the rays to focus at a point. The light then passes through a smaller lens in the eyepiece to the observer.

This telescope at the Yerkes Observatory at the University of Chicago is the largest refracting telescope in the world. The primary lens of the telescope is forty inches in diameter, and the main tube is sixty feet long. Built in 1897, the Yerkes Observatory was the first observatory to primarily function as an astrophysics research facility.

Light bends, or refracts, when it passes through different substances. Here, we see light bending as it passes from air through the glass of a prism, and back through air again. Different wavelengths of light bend to different degrees. Blue light tends to bend more than red light.

Light can be made to converge and focus at a point by passing through the curved surface of a converging lens.

The fact that the amount of refraction depends on the wavelength of light causes different colors to have different focal points. This effect is called "chromatic aberration" and is a problem for refracting telescopes.

Another issue that refracting telescopes have is that, as the light passes through the glass, some light is absorbed by the glass.

Convex lenses have two surfaces. The two surfaces must match in curvature very well for the focus to be accurate. Also, large lenses can be quite thick. The lenses can become heavy enough to sag under their own weight.

The Giant Magellan Telescope, slated for completion in 2024, will be the largest reflecting telescope in the world. Its seven mirrors will measure 25 meters across. It will be located in the dry climate of Chile's Atacama Desert, far from any population center, where the night sky will not be compromised by city lights. This one billion dollar telescope will produce images ten times sharper than those captured by the Hubble Space telescope. The image above is an artist's rendition of the telescope.

A mirror also causes light rays to focus. The light does not pass through glass, but just reflects off of the surface. Since the light does not pass through the glass, there is no difference in focal points for different wavelengths of light. Chromatic aberration is not an issue for reflecting telescopes. Also, much less light is absorbed by the glass.

The angular resolution of a telescope refers to its ability to discern small details of an object. For example, a telescope with good angular resolution would be able to distinguish two stars that were very close together, whereas a telescope with lower angular resolution would see the objects smeared together into one.

The larger the telescope, the more light that can be gathered, and the more detail can be seen in the images. Larger telescopes have advantages over small er telescopes:

• More light gathering power
  • Greater collecting area
  • See faint objects better
  • Observed brightness proportional to area
    • 5m telescope image 25x brighter than 1m telescope
• Angular resolution
  • Ability to form distinct images of objects that are close  to each other
• Less diffraction
  • Bending of light around edges

Light pollution is an issue that faces all ground-based telescopes. Ambient light from nearby sources like city lights is diffused through moisture in the air and interferes with image-taking.

With the growth and expansion of urban areas, it is increasingly hard to find suitable locations for ground-based telescopes, as indicated in this image of the United States at night, from space.

To counteract the effects of distortion caused by the atmosphere, many supports beneath a flat mirror can be adjusted to change the shape of the surface of the mirror, correcting distortions. This method is called active optics, and has been developed and used extensively at the Keck Observatory in Hawaii.