Optical and Radio Telescopes

Optical and Radio Telescopes

Optical Telescopes

General Points

  • An optical telescope is a telescope designed to collect and magnify visible light from astronomical objects to produce images.
  • They come in two types: refracting telescopes (use lenses) and reflecting telescopes (use mirrors).

Key Aspects

  • Refracting telescopes employ a large objective lens which focuses light, forming an image at the eyepiece. However, a problem known as chromatic aberration, where colours get spread out like in a prism, can occur.
  • Reflecting telescopes, on the other hand, use a large objective mirror. They’re notably free from chromatic aberration because mirrors reflect all colours equally.

Limitations

  • Optical telescopes on Earth are limited by atmospheric turbulence (which causes stars to twinkle) and the day-night cycle.
  • Absorption and distortion of light by the atmosphere can limit the effectiveness of ground-based optical telescopes. This is known as atmospheric absorption.

Radio Telescopes

General Points

  • A radio telescope captures radio waves emitted by celestial objects. Size can range from metres to many kilometres.
  • Unlike optical telescopes, radio telescopes can operate day and night, in all weather conditions.

Key Aspects

  • Most radio telescopes are parabolic dishes which focus radio waves at the dish’s focus, where the aerial (or receiver) is located.
  • Arrays of multiple radio telescopes, called interferometric arrays, can improve resolution by combining their data - a technique called aperture synthesis.

Limitations

  • Earth’s atmosphere is transparent to radio waves, so ground-based radio telescopes can observe the universe unimpeded by atmospheric absorption.
  • However, due to the longer wavelength of radio waves, radio telescopes generally have a lower resolution than their optical counterparts.

Comparing Optical and Radio Telescopes

Wavelengths and Resolution

  • Optical and radio telescopes are designed to detect different wavelengths. This impacts their resolving power, as shorter wavelengths provide better resolution.
  • Despite the lower resolution, radio telescopes can observe large structures in the universe that are not detectable in visible light.

Impact of Atmosphere

  • Visibility for optical telescopes from Earth can be affected by the atmosphere, light pollution, and weather conditions. Radio telescopes are far less influenced by these conditions.
  • However, terrestrial radio interference (from broadcast stations, mobile phones etc.) can cause problems for radio observations.

Operational Factors

  • Due to their ability to operate day and night in all weather conditions, radio telescopes often have a higher utilisation than optical telescopes.