A radio telescope is a reflecting telescope for radio waves. A large parabolic dish reflects radio waves onto an antenna (the receiver) at its focus. The signal is amplified and processed by a computer to build up an image, one point at a time, as the dish is steered to scan the sky.
Similarities with optical reflecting telescopes: both use a parabolic reflector to bring waves to a focus, and both have collecting power proportional to D2 and resolution given by θ ≈ λ/D.Differences:
• radio wavelengths (cm to m) are about a million times longer than visible light, so for the same diameter the resolving power is much poorer. Dishes must be very large (tens to hundreds of metres), and even then resolution is worse than a small optical telescope.
• the dish surface need not be as precise: a wire mesh works, as long as the holes and surface errors are smaller than about λ/20. This makes large dishes lighter and cheaper.
• radio waves pass through cloud and are not swamped by sunlight, so radio telescopes can be used in the day and through cloud, from the ground.Worked example: the Lovell telescope has a 76 m dish. At λ = 0.21 m (hydrogen emission):
θ ≈ 0.21 / 76 = 2.8 × 10-3 rad, thousands of times worse than a 2.4 m optical telescope. To get the same resolution at λ = 0.42 m the dish would need to be twice as wide, 152 m.Signals from several separated dishes can be combined to act like one much larger dish, greatly improving resolution.
Similarities with optical reflecting telescopes: both use a parabolic reflector to bring waves to a focus, and both have collecting power proportional to D2 and resolution given by θ ≈ λ/D.Differences:• radio wavelengths (cm to m) are about a million times longer than visible light, so for the same diameter the resolving power is much poorer. Dishes must be very large (tens to hundreds of metres), and even then resolution is worse than a small optical telescope.
• the dish surface need not be as precise: a wire mesh works, as long as the holes and surface errors are smaller than about λ/20. This makes large dishes lighter and cheaper.
• radio waves pass through cloud and are not swamped by sunlight, so radio telescopes can be used in the day and through cloud, from the ground.Worked example: the Lovell telescope has a 76 m dish. At λ = 0.21 m (hydrogen emission):
θ ≈ 0.21 / 76 = 2.8 × 10-3 rad, thousands of times worse than a 2.4 m optical telescope. To get the same resolution at λ = 0.42 m the dish would need to be twice as wide, 152 m.Signals from several separated dishes can be combined to act like one much larger dish, greatly improving resolution.