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Non-Optical Telescopes. Astrophysics Lesson 6. Learning Objectives. Single dish radio telescopes, I-R, U-V and X-ray telescopes Similarities and differences compared to optical telescopes including:- Structure Positioning and use
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Non-Optical Telescopes Astrophysics Lesson 6
Learning Objectives Single dish radio telescopes, I-R, U-V and X-ray telescopes • Similarities and differences compared to optical telescopes including:- • Structure • Positioning and use • Compare the resolving and collecting power of telescopes operating at different wavelengths.
Homework • Marking last homework. For this Friday:- • Describe and explain the structure, positioning, and uses of I-R, U-V and X-ray telescopes. • Reference correctly.
Astronomy across the Spectrum Same patch of sky viewed at different wavelengths:-
Single dish radio telescopes In Britain, the first work to be done with Radio Telescopes was carried out by Professor Sir Bernard Lovell and a team from the University of Manchester. This massive instrument at Jodrell Bank is 75 m across. started work in the late 1950s.
Arecibo Observatory • The largest radio telescope in the world is in Puerto Rico. It is built between some small hills that had a roughly parabolic valley. It is 300m across. • The valley floor is paved in metal sheeting to act as the mirror.
Radio Telescope • The dish is parabolic, reflecting radio waves onto an antenna at the principal focus. The radio waves are very weak, and the focusing by the reflector makes them much more intense. • The receiver has to be tuned in, just like any other radio set. • The signal is passed down to very high quality amplifiers, and the signals are analysed by a computer.
Gathering Power • The energy/photons collected per second. • The gathering power is proportional to the area:- • So a bigger dish collects more photons and energy from an object in a given time can detect fainter objects.
Questions • Calculate the resolution of the Lovell (75m) and Arecibo (300m) telescope observing at a wavelength of 21cm in radians and degrees. • Compare their light gathering power. • What disadvantage does the Puerto Rico instrument have over the one at Jodrell Bank?
Answers • θ = 0.21/75 = 0.0028 rad (= 0.16º) θ = 0.21/300 = 0.0007 rad (= 0.04º) • The diameter of the Puerto Rico instrument is four times that of Jodrell Bank. Therefore its gathering power is 16 times larger. • The Puerto Rico instrument is static, so it cannot track objects. It sweeps across the sky as a result of the Earth's rotation.
Similarities with Optical • Parabolic dish objective mirror in telescopes. • Antenna used as detector at principal focus instead of eye or camera • Tracking of Source telescope moves with the source’s position in the sky as the Earth rotates.
Differences from Optical • Resolving power is worse because of large λ. ^ linking lots of telescopes together. • Need to scan across the radio source to build up the image. • Precision of about λ/20 needed in shape of dish required to avoid spherical aberration. • Fine wire mesh will do, since radio waves will not pass through a gap less than one wavelength. ^ last two points mean cheaper and easier to build.
Revealed New Radio Sources Quasars Pulsars
Radio Radio waves can penetrate dust, so we can look at the centre of our galaxy. However radio waves of wavelength less than about 1 cm are blocked out by carbon dioxide and water. Radio waves of wavelength 20 m and above are absorbed by the atmosphere. Also radio signals from Earth can cause interference, just like light pollution for light telescopes. Passing satellites can also obscure the field of view.
