Telescopes

• Telescopes collect light.

  • Their ability to magnify is secondary.

• A telescope's light collecting ability is proportional to the collecting area of the aperture.

  

• The diameter of the dark-adapted pupil of the human eye is just under 1 centimeter.

• The diameter of the largest optical telescope is 1000 centimeters (10 meters), so the largest telescope has one million times the collecting area of the human eye.

  

• There are two basic types of telescope: refractors (based on lenses) and reflectors (based on mirrors).

• Most large telescopes are reflectors because mirrors don't have to be transparent and can thus be supported from behind.

Resolving Power vs. Telescope Diameter

• In addition to collecting light from faint objects, the astronomer's goal is also to see fine detail in images of objects.

  • The degree to which detail can be discerned is called resolution.

    • Fuzzy images = poor resolution
    • Sharp images = good resolution

• Because of the wave nature of light and a phenomena called diffraction the ultimate resolving power of a telescope is limited by the diameter of its mirror.

  

• Radio telescopes (long wavelengths) must be very large in order to provide good resolution.

  • Sometimes the input of many telescopes is combined to synthesize the resolution of a telescope as big as the separation between telescopes. This technique is known as interferometry.

• Working at visible wavelengths from the ground astronomers rarely if ever can achieve the ultimate resolving power of a telescope due to the blurring effects of the atmosphere (seeing).

  • This is the same effect, caised by the turbulent atmosphere, that causes stars to twinkle.

  • In order to avoid this atmospheric blurring astronomers

    • Launch telescopes into space (e.g. Hubble).
    • Place telescopes on high mountains with stable atmospheric conditions and compensate for and remove atmospheric blurring (adaptive optics).

Updated March 14, 2001