Advanced Topic in Astrophysics, Lecture 1 Radio Astronomy
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Advanced Topic in Astrophysics, Lecture 1 Radio Astronomy - Antennas Phil Diamond, University of Manchester Course Structure •Modules – Module 1, lectures 1-6 (Phil Diamond, Maria Rioja) • Mon March 1, 8, 15: 3 double lectures • Radio astronomy: antennas, interferometry and VLBI – Module 2, lectures 7-12 (J-P Macquart) • Radiation Mechanisms I: 3 double lectures, dates TBD – Module 3, lectures 13-18 (J-P Macquart) • Radiation Mechanisms II: 3 double lectures, dates TBD – Module 4, lectures 19-24 (Ken Freeman) • Dynamics of the Milky Way • Dates TBD Resources •ATNF Synthesis Imaging workshops 2003, 2006 – http://www.atnf.csiro.au/whats_on/workshops/synthesis2006/prog.h tml •Texts –Radio Astronomy: Kraus –Tools of Radio Astronomy: Rohlfs & Wilson –Interferometry and Synthesis in Radio Astronomy, Thompson, Moran & Swenson. –Very Long Baseline Interferometry and the VLBA: Napier, Diamond & Zensus., ASP Conf series, Vol 82, 1995. Outline •Imaging –Resolution –Direct and indirect imaging •Telescopes –Parabolic and hyperbolic •The Antenna –Horns and receivers •Radio astronomy fundamentals and tools of the trade –Brightness temperature –Flux density –Radiometer equation Direct imaging onto a focal plane Resolution Δθ 2’’ 2” 1’ 10’ Diffraction limits Δθ=1.22 λ/D Δθ=1” λ D Optical 500 nm 125 mm Radio 20 cm 50 km Direct and indirect imaging •Direct imaging –Normal imaging method whereby an image is projected onto a detector. –Examples: camera, telescope, single-dish radio telescope •Indirect imaging –Used where we cannot form a direct map of the object on the focal plane. –We infer the properties of the object from certain characteristics of the received electromagnetic field. –Examples: interferometry, NMR, ultrasound, PET, speckle. Direct Imaging: Single dish radio telescope Parabolic reflector: A parabolic reflector adds all the fields from a surface (aperture) at a single focal point. Direct Imaging - Angular Resolution •Angular Resolution (radians) D λ θ = D •Wavelength, λ=0.21m, D=64m –⇒θ=0.003 rad, or 11 arcmin •Such a radio telescope only matches the resolution of the human eye at its shortest wavelength 1-2cm. A radio telescope at Parkes, NSW (prime focus) Hyperbolic reflector . A B Light from the point A reflecting off the hyperbola appears to come from point B Parabolic reflector Prime Focus Hyperbolic reflector Parabolic reflector Classical Cassegrain AT antenna: •Cassegrain configuration •22-m diameter primary main reflector •2.75-m secondary reflector Reflector types Prime Focus Cassegrain Focus Offset Cassegrain Naysmith Beam Waveguide Dual offset Reflector types Prime Focus Cassegrain Focus e.g. GMRT e.g. Mopra (AT), e-MERLIN Offset Cassegrain Naysmith e.g. VLA and e.g. OVRO ALMA Beam Waveguide Dual offset e.g. NRO e.g. ATA Signal path: Signal path: The antenna collects the E-field over the aperture at the focus The focus is a fixed spot at all frequencies. The reflector antenna is achromatic. EM-wave electric field oscillations induce voltage oscillations at the antenna focus, in a device called a feed. Feeds are ‘compact’ and ‘corrugated’ horns The inner profile is curved The inner surface has grooves in order to increase the surface impedance, so that the wave does not set up voltages in the surface material, but is channelled into a dipole at the end of the horn. Cross-section of a horn Brightness Temperature Radio photons are pretty wimpy: Radio photons are too wimpy to do very much - we cannot detect individual photons e.g. optical photons of 600 nanometre => 2 eV or 20000 Kelvin (hv/kT) e.g. radio photons of 1 metre => 0.000001 eV or 0.012 Kelvin Photon counting in the radio is not an option, must think classically in terms of electric fields, i.e. the best we can do is for the incoming EM-wave electric field oscillations to induce voltage oscillations in a conductor. Brightness Temperature Solid angle of source, Ω Radio photons are pretty wimpy: Rayleigh-Jeans Law: 2 Iν α ν 1 Jansky (Jy) = 10-26 W m-2 Hz-1 •In radio astronomy T is known as the brightness temperature. •Radiation mechanisms in radio astronomy are often non-thermal, but this does not stop astronomers talking about the “brightness temperature” of a source: i.e. the equivalent or effective temperature that a blackbody would need to have in order to be that bright! Brightness Temperature Examples of brightness temperatures: “Blank” sky ~ 2.73 K (big bang BB radiation) Sun at 300 MHz, 500000 K Orion Nebula at 300 GHz ~ 10-100 K (“warm” molecular clouds) Quasars at 5 GHz ~ 1012 K (synchrotron) Brightness temperature 18 Orion H2O super-maser: Tb > 10 K • However, the giant pulses observed from the Crab 34 Pulsar have Tb ~ 10 K Antenna Performance The antenna aperture efficiency = Power collected by feed Power incident on antenna There are many different potential loss factors: Surface Aperture Feed Feed Misc. - other ~ 0.4 <== efficiency blockage spillover illumination minor losses ~ 0.8 efficiency efficiency efficiency ~ e.g feed mis- ~ 0.8 ~ 0.8 0.8 match. Spillover past the sub-reflector (ray A) Feed Blockage of antenna surface by sub-reflector Spillover past main reflector (ray C) Feed does not illuminate all of antenna surface equally Antenna Temperature and Gain Solid angle of antenna pattern ΩA • Antenna temperature is same as brightness temperature only if the source fills the antenna beam, e.g. Ta ~ Tb x beam filling factor •Examples – Parkes 64-m antenna (η=60%): G=0.7 K Jy-1 – SKA 1km2: G=362 K Jy-1 Antenna surface efficiency According to the Ruze (1966) formula, the surface efficiency of a paraboloid is well described by: where sigma is the r.m.s. error in the surface of the antenna. Or re-arranging: e.g. For a surface efficiency of 0.7 (typical target value), the required surface error (r.m.s.) is ~ lambda/20. ==> at 7 mm (43 GHz) the surface accuracy must be ~ 350 micron. ==> many different forces acting on an antenna and its surface... Appreciating the scale of large radio telescopes.... How can we possibly achieve a 350 µm accuracy (the thickness of three human hairs) – over a 100 metre diameter surface – an area equal to 2 football fields! ==> “active surface”. GBT Surface has 2004 panels average panel rms: 68µm. 2209 precision actuators are located under each set of surface panel corners Actuator Control Room (left): 26000 control and supply wires terminate in this room! How big can parabolic telescopes be? As the size (diameter) of a radio telescope increases, the gravitational and wind loads on the structure become difficult to manage. The worst problem is the problem of surviving a gale-force wind. The degree of wind distortion between paraboloids of different diameters (D) scales as D3. The cost of antennas also scales roughly as D3. Telescopes like the Jodrell Bank Mark V (right) with a diameter of ~ 305 metres (1970), will probably always remain in model form! ‘System’ Temperature, Tsys •System Temperature is the equivalent noise power produced in the antenna receiver from sources other than the object being observed. •It is the same noise that would be produced by a perfect resistor in a heat bath of temperature Tsys. • Tsys=Trx+Tspill+Tatm+Tcmb+Tgal+… –Trx, receiver: internal noise in electronics (4-20K at low frequency) –Tspill, spillover: reflected ground emission (few % of 300K) –Tatm, atmosphere: frequency dependent emission (2-300K) –Tcmb, cosmic microwave background (3K) -2.7 –Tgal, Galactic background (10K at 1 GHz, Tgal ∝ν ) Temperature sensitivity •Radiometer equation T ΔT = τΔν ΔT = Temperature sensitivity (K); T = System temperature (K); τ= Integration time (s); Δν = Bandwidth (Hz) •T=20K, τ=7200 sec, Δν=200 MHz ⇒ΔT=50μK ! •System temperature describes the signal a telescope ‘sees’ if looking at a blackbody of temperature T which fills the field of view. NOT physical temperature. p.32 Other parts of telescope system Radio Telescope Block Diagram Radio Source Antenna Receiver Frequency Heterodyne system with LNAs, mixers etc Conversion Signal Processing Computer Signal Post-detection Detection Bolometers, e.g. SCUBA-2 Processing Other Antenna types Dipoles Yagi Log-periodic Helix Some less conventional (weird!)reflector types The Jodrell Bank Mark 2 telescope (1964). Was considered to be a prototype of the then planned giant 300-metre MkIV. The aperture is elliptical - the idea was that a 300-metre would require an elliptical surface in order to reduce the height of the structure off the ground. The off-axis cylinder radio telescope at Ooty. India (1970) The big-ear antenna built by Kraus: Other similar examples: Ratan 600 - Russia Nancay (France): Cross antennas Instead of building an entire parabola, Cross Antennas employ only a narrow section of a parabola, we get a beam narrow in the antenna’s wide direction, and broad in the other direction: . By observing a source with two orthogonal beams, we can get a 2-d image of the sky. The cross antenna response is similar to the crossing point of the two beams (but with very high side-lobes): The first Cross telescope - was built by Bernie Mills in Australia. The Mills cross: Molonglo cross Northern cross (Bologna) The 305-m Arecibo telescope is fixed in the ground. A spherical reflector is therefore employed: Northern cross (Bologna) While a parabola has a single focus point, a spherical reflector focus the incoming radio waves on a line: Northern cross (Bologna) By having a moving secondary a spherical reflector can be pointed in different (but still somewhat limited) directions on the sky. Note that only part of the total surface area is useable for any given direction. Part of the primary: Northern cross (Bologna) Looking on to the surface: quite a lot of litter! Northern cross (Bologna) While a parabola has a single focus point, a spherical reflector focus the incoming radio waves on a line: Northern cross (Bologna) By having a moving secondary a spherical reflector can be pointed in different (but still somewhat limited) directions on the sky.