Similarities and differences of radio telescopes compared to optical…

The idea

Radio and optical telescopes both collect electromagnetic radiation with a curved reflecting surface and focus it onto a detector, so both are limited by diffraction and by the collecting area of the dish or mirror. A single-dish radio telescope uses a large parabolic metal dish that reflects radio waves to a receiver at the focus; an optical telescope uses a parabolic or spherical mirror that reflects visible light to a camera or eyepiece. Radio wavelengths are about 10⁵ times longer than visible wavelengths, so a radio dish must be far wider than an optical mirror to achieve the same angular resolution, although its large area can still collect weak signals. Radio dishes are also mounted to track sources for long integration times and can operate in cloud and daylight, whereas optical telescopes need clear, dark skies.

What earns the marks

• A single-dish radio telescope uses a large parabolic metal dish and a receiver at the focus; an optical telescope uses a mirror and a camera or eyepiece.
• Radio wavelengths are much longer than visible wavelengths, so a radio dish needs a much larger diameter than an optical mirror to match the same resolving power.
• Radio dishes can observe through cloud and in daylight and can integrate a signal for long periods, while optical telescopes require clear, dark conditions.
• The collecting power of either telescope increases with the area of the dish or mirror, so a larger aperture gathers more radiation per second.

Common mistakes and how to put them right

• Saying that radio telescopes cannot resolve detail at all.

  Put it right by explaining that they are diffraction-limited and that resolution improves with a larger dish diameter or a shorter wavelength.

• Claiming that radio dishes work because radio waves are sound waves.

  Put it right by stating that radio waves are electromagnetic radiation and are detected electronically, not heard.

• Assuming that a radio dish and an optical mirror of equal diameter have equal resolving power.

  Put it right by noting that the much longer radio wavelength gives poorer resolution for the same diameter.

Exam tips

• Structure the answer as similarities first, then differences, using the headings collecting power, resolving power, structure and observing conditions.
• Use the diffraction relationship qualitatively: state that resolution improves as aperture increases and as wavelength decreases, and apply it to the radio-versus-optical comparison.
• Refer to a named example such as a single dish like the Lovell Telescope or Parkes to support the description of structure and use.