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Radio Astronomy

Radio Astronomy. ASTR 3010 Lecture 25. Intro to Radio Astronomy Concepts - Amplifiers - Mixers (down-conversion) - Principles of Radar - Radio Astronomy basics: System temperature, Receiver temperature Brightness temperature,

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Radio Astronomy

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  1. Radio Astronomy ASTR 3010 Lecture 25

  2. Intro to Radio Astronomy • Concepts • - Amplifiers • - Mixers (down-conversion) • - Principles of Radar • - Radio Astronomy basics: • System temperature, Receiver temperature • Brightness temperature, • The beam (q = l / D) [ its usually BIG] • Interferometry (c.f. the Very Large Array – VLA) • Aperture synthesis

  3. History of Radio Astronomy • (the second window on the Universe) • 1929 - Karl Jansky (Bell Telephone Labs) • 1030s - Grote Reber • 1940s - WWII, radar • - 21 cm (Jan Oort etc.) • 1950s - Early single dish & interferometry • - `radio stars’, first map of Milky Way • - Cambridge surveys (3C etc) • 1960s - quasars, pulsars, CMB, radar, VLBI • aperture synthesis, molecules, masers (cm) • 1970s - CO, molecular clouds, astro-chemistry (mm) • 1980s /90s – CMB anisotropy, (sub-mm)

  4. NRAO/AUI/NSF

  5. Optical and Radio can be done from the ground! NRAO/AUI/NSF

  6. Radio Telescope Optical Telescope Nowadays, there are more similarities between optical and radio telescopes than ever before. NRAO/AUI/NSF

  7. Outline • A Simple Heterodyne Receiver System • mixers and amplification • Observing in the Radio • resolution • brightness temperature • Radio Interferometry • Aperture synthesis

  8. Mixing: Adding waves together Freflect = f trans + / - Df f trans Df = 1850 Hz

  9. Mixers A mixer takes two inputs: the signal and a local oscillator (LO). The mixer outputs the sum and difference frequencies. In radio astronomy, we usually filter out the high frequency (sum) component. local oscillator signal in LO w1 w2 signal out w1+w2 and w1-w2

  10. mixed signal Mixers LO signal original signal 0 Hz frequency

  11. mixed signal Mixers The negative frequencies in the difference appear the same as a positive frequency. To avoid this, we can use “Single Sideband Mixers” (SSBs) which eliminate the negative frequency components. LO signal original signal 0 Hz frequency

  12. W-band (94 GHz, 4 mm) amplifier

  13. filter A Simple Heterodyne Receiver signal @ 1420 MHz 1570 MHz receiver horn LO low noise amplifier 1420 MHz 150 MHz + + tunable filter Analog-to-Digital Converter Computer outputs a power spectrum tunable LO ~150 MHz

  14. Observing in the Radio I • We get frequency and phase information, but not position on the sky • 2D detector • A CCD is also a 2D detector (we get x & y position)

  15. Observing in the Radio II:Typical Beamsize (Resolution) • i.e. The BURAO 21 cm horn (D ~ 1 m)

  16. Observing in the Radio II • i.e. The NRAO GBT (D ~ 100 m) at 21cm = 1.420 GHz at 0.3 cm = 100 GHz

  17. Observing in the Radio II • i.e. The Arecibo Telescope (D ~ 300 m) at 21cm = 1.420 GHz at 0.3 cm = 100 GHz

  18. Observing in the Radio III:Brightness Temperature Flux: erg s-1 sr-1 cm-2 Hz-1 (1023 Jy) Bu(T): erg s-1 sr-1 cm-2 Hz-1 (1023 Jy) We can use temperature as a proxy for flux (Jy) Conveniently, most radio signals have hu/kT << 1, so we can use the Raleigh-Jeans approximation Bu(T) = 2kT/l2 Thus, flux is linear with temperature

  19. Antenna Temperature TB = Ful2/2k • Brightness temperature (TB) gives the surface temperature of the source (if it’s a thermal spectrum) • Antenna Temperature (TA): if the antenna beam is larger than the source, it will see the source and some sky background, in which case TA is less than TB • Noise in the system is characterized by the system temperature (Tsys) • i.e. you want your system temperature (especially in the first amp) to be low TA ~ TBWs/Wb

  20. Radio Interferometry to source positional phase delay Q Q + East

  21. Two Dish Interferometry • The fringe pattern as a function of time gives the East-West (RA) position of the object • Also think of the interferometer as painting a fringe pattern on the sky • the source moves through this pattern, changing the amplitude as it goes

  22. Aperture Synthesis • A two dish interferometer only gives information on the E-W (RA) structure of a source • To get 2D information, we want to use several dishes spread out over two dimensions on the ground

  23. Radio Telescope Arrays The VLA: An array of 27 antennas with 25 meter apertures maximum baseline: 36 km 75 Mhz to 43 GHz

  24. Very Large Array radio telescope (near Socorro NM)

  25. VLBA

  26. Radio Telescope Arrays ALMA: An array of 64 antennas with 12 meter apertures maximum baseline: 10 km 35 GHz to 850 GHz

  27. The U-V Plane • Think of an array as a partially filled aperture • the point source function (PSF) will have complicated structure (not an airy disk) • the U-V plane shows what part of the aperture is filled by a telescope • this changes with time as the object rises and sets • a long exposure will have a better PSF because there is better U-V plane coverage (closer to a filled aperture)

  28. The U-V plane a snapshot of the U-V plane (VLBA) U-V coverage in a horizon to horizon exposure

  29. Point Spread Function The dirty beam : the diffraction pattern of the array

  30. Examples of weighting Dirty Beams: A snapshot (few min) Full 10 hrs VLA+VLBA+GBT

  31. Image Deconvolution • Interferometers have nasty PSFs • To get a good image we “deconvolve” the image with the PSF • we know the PSF from the UV plane coverage • computer programs take a PSF pattern in the image and replace it with a point • the image becomes a collection of point sources

  32. UV Plane Coverage and PSF images from a presentation by Tim Cornwell (given at NRAO SISS 2002)

  33. UV Plane Coverage and PSF images from a presentation by Tim Cornwell (given at NRAO SISS 2002)

  34. Image Deconvolution images from a presentation by Tim Cornwell (given at NRAO SISS 2002)

  35. What emits radio waves? NRAO/AUI/NSF

  36. Recipe for Radio Waves 1. Hot Gases NRAO/AUI/NSF

  37. Electron accelerates as it passes near a proton. EM waves are released NRAO/AUI/NSF

  38. NRAO/AUI/NSF

  39. NRAO/AUI/NSF

  40. NRAO/AUI/NSF

  41. Recipe for Radio Waves 2. Atomic and molecular transitions (spectral lines) NRAO/AUI/NSF

  42. Gas Spectra Neon Sodium 656 nm 434 nm 486 nm Hydrogen NRAO/AUI/NSF

  43. Electron accelerates to a lower energy state NRAO/AUI/NSF

  44. NRAO/AUI/NSF

  45. NRAO/AUI/NSF

  46. Recipe for Radio Waves 3. Electrons and magnetic fields NRAO/AUI/NSF

  47. Electrons accelerate around magnetic field lines NRAO/AUI/NSF

  48. NRAO/AUI/NSF

  49. NRAO/AUI/NSF

  50. NRAO/AUI/NSF

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