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.