Radio Telescopes Jansky's Telescope
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Radio Telescopes Jansky’s Telescope • Karl Jansky built a radio antenna in 1931. – Polarized array – Study lightning noise • Detected noise that shifted 4 minutes each day. – Direction of Sagitarrius – Consistent with galactic source Reber Telescope • In 1937, Grote Reber built a 32foot-diameter parabolic dish antenna in his backyard in Wheaton, Illinois to seek cosmic radio emissions. Reber’s Map In the spring of 1939, he was able to detect cosmic radio emissions with his equipment. In 1941, he made the first survey of the sky at radio wavelengths (160MHz). Radio Spectrum • The radio spectrum is divided into three bands. frequency wavelength Radio 100MHz - 100GHz 3m-3mm Millimeter 100GHz-300GHz 3mm-1mm Submillimeter 300GHz-1000GHz 1mm-0.3mm Atmospheric Window • The atmosphere is transparent from 50 m to 0.5 mm – Long wavelengths reflected by ionosphere – Short wavelength absorbed by O2, H2O; less at altitude Radio Emission 2n 2 Fn 2 kT c Fn n 0.2 1.2 • Radio sources are measured in Janskys (Jy). – 1 Jy = 10-26 W m-2 Hz-1 – Differential flux Fn • Thermal emission follows the Rayleigh – Jeans law. • Synchrotron radiation comes from magnetic fields. Antenna • For MHz radio waves a telescope is a half-wave dipole antenna. – Consists of two conductors – Short separation – Quarter wavelength each • Cables at the center connect to the receiver. Jocelyn Bell and 81.5 MHz radio telescope (1967) Radio Horns • Horn antennas are used to collect waves at GHz and higher. – Factor of 8 in bandwidth – May be dielectric filled – Waveguides to detector ~1 meter ~2cm VLBA telescope Sensors • Radio sensors are matched to the desired wavelengths. • High frequency sensors are usually SIS. – Superconductorinsulatorsuperconductor – NiO – AlO – Photon assisted tunneling VLBA telescope Noise • Small radio signals can be lost in noise. – Minimum detectable brightness Bmin – Integration time t – Frequency bandwidth n – Receiver-based constant K • Noise temperature based on multiamplifier stages with gain Gn. Bmin 2 Kn 2 2 kTs c tn Tn T2 Ts T1 G1 G1G2 Gn 1 Cryogenics • One way to lower noise is to operate the electronics at low temperature. – Reduces thermal noise Vs 4kTs R • Cooling to less than 20K is accomplished with high pressure helium gas. – Compressor and pump – Thermally isolated cryostat Helium pump Cryostat Components Horn Purpose of the horn is to collect the radiation directed to it from the antenna. Amplifier Increases the amplitude of the signal Mixer Used to change the frequency to a more easily used frequency Heterodyne • The simplest receiver is a heterodyne receiver similar to a consumer radio. – Preamp gain 10-1000 • A local oscillator mixes with the input signal. – Beat or intermediate frequency • Further amplification may be by factors of 106 to 109. • Heterodyne receivers have high system temperatures. – 10 K at radio – 10 M at millimeter • Phase sensitive techniques reduce noise. n sig n osc n IF Receivers Radio Horn Preamp Mixer Convert to desired form to record/analyze Store data Millimeter and Submillimeter Convert to desired form to record/analyze Store data Radio Receiver Receivers are inside the dome Aricibo Millimeter Wave Receiver Horns ~0.5 m Amplifiers Millimeter Wave Oscillator Local oscillator Millimeter Wave Electronics Intermediate frequency (IF) plate Submillimeter receiver CSO, Mauna Kea Submillimeter receiver 230 GHz mixer block CSO, Mauna Kea Antenna Dish • A radio telescope is often noted for the large dish. • This is for the same optics as an optical reflecting telescope. – Rayleigh criterion applies – Beam width at first nulls – Dipole length x 1.22 BWFN 2 x D Steering gear Receiver Antenna mirror Gain • The maximum gain for a radio telescope depends on the effective area. – Typically 1.6 for half-wave dipole • The effective area compares the output power to the incoming flux. – Must be correctly polarized 4n 2 g 2 Aeff c Pn Aeff Fn Surface Errors • The surface error of a dish must be controlled. – Less than 1/20 wavelength – Losses to less than 30% 1/20 =0.15mm VLBA (3mm-3m) Radio Mauna Kea Very Smooth • The surface error requirements are much stricter for submillimeter telescopes than for radio telescopes. 1/20 =0.05mm Millimeter NRAO 12 meter (1mm3mm) Kitt Peak 1/20 =0.015mm Sub-millimeter JCMT (0.3mm-2.0mm) Mauna Kea Holographic Test • This image was taken during the SMT reflector's holographic testing showing that the deviations of the reflector. • Deviations are nearing the targeted 15 microns (about the thickness of a human hair).
