Radio and optical telescope comparison

Radio and optical telescopes collect radiation from astronomical sources and focus it so that a receiver can record a signal for study. The main difference is the radiation they collect: radio telescopes collect radio waves, while optical telescopes collect visible light.

How the telescopes focus radiation

A single-dish radio telescope has a large, curved conducting reflector. Incoming radio waves reflect from the dish and are brought together at a receiver.

A reflecting optical telescope works on a similar principle: a curved mirror reflects incoming visible light and brings it to a focus. A refracting optical telescope instead uses a lens to bend the light and bring it to a focus. A receiver records the focused signal for study.

Ground-based observing conditions

Both radio and optical telescopes can be placed on Earth, but their sites are chosen to reduce different kinds of interference. Radio telescopes are often placed far from human-made radio transmissions, which can interfere with signals from astronomical sources. Optical telescopes benefit from dark, clear skies, so they are often placed at high altitudes and away from light pollution.

Radio telescopes can generally observe through cloud and during daylight. Ground-based optical telescopes cannot observe sources through cloud, and their observations are usually made at night, when the sky is dark.

Angular resolution of radio and optical telescopes

A telescope has good angular resolution if it can distinguish two sources that appear very close together in the sky. Its resolution can be described by the smallest angular separation at which the sources can be seen as separate. A smaller minimum angular separation means better resolution.

Light or radio waves passing through a telescope’s aperture spread because of diffraction. This limits how clearly the telescope can separate the images of nearby sources. For a diffraction-limited telescope, the minimum resolvable angular separation is:

Here, $$\theta$$ is the minimum angular separation, $$\lambda$$ is the wavelength of the radiation, and $$D$$ is the aperture diameter. A longer wavelength increases $$\theta$$, giving poorer resolution. A larger aperture decreases $$\theta$$, giving better resolution.

Comparing radio and optical telescopes

Radio waves have much longer wavelengths than visible light. If a radio telescope and an optical telescope have the same aperture diameter, the radio telescope therefore has poorer diffraction-limited angular resolution.

To achieve comparable resolution, a radio telescope needs a much larger aperture. Increasing the aperture diameter improves the diffraction-limited resolution of either type of telescope.

Collecting power of radio and optical telescopes

A telescope gathers incoming radiation over an area and directs it to a receiver. Its collecting power describes how much radiation it can gather. For the same incident energy flux, a larger collecting area receives more energy each second.

Energy flux is the energy arriving per unit area per unit time. If the flux is $$F$$ and the telescope’s effective collecting area is $$A_{\text{eff}}$$, the received power is:

Here, $$P$$ is the energy received per unit time. For a given flux, doubling the effective collecting area doubles the received power.

Collecting area and diameter

For an unobstructed circular aperture of diameter $$D$$, the geometric collecting area is:

The area is therefore proportional to $$D^2$$. Doubling the diameter gives four times the area, so it collects four times as much radiation per unit time if the incident flux and collecting efficiency are unchanged.

Comparing radio and optical telescopes

A radio dish gathers radio waves over its area and directs them to a receiver. An optical telescope gathers visible light through its aperture and directs it to a detector. Their collecting powers can be compared using their effective collecting areas: for the same incident flux, the telescope with the larger effective area receives more power. Effective area takes account of how much of the incoming radiation is actually collected, so it need not equal the geometric area.

Collecting more radiation helps a telescope detect weaker signals. Collecting power is not the same as angular resolution, which describes how well a telescope can distinguish objects that appear close together in the sky.