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Radio Telescopes and Radiometers

Radio Telescopes and Radiometers. 2015 Single Dish School. Jim Condon. NRAO, Charlottesville. Radio Telescopes and Antennas.

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Radio Telescopes and Radiometers

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  1. Radio Telescopes and Radiometers 2015 Single Dish School Jim Condon NRAO, Charlottesville

  2. Radio Telescopes and Antennas • An antenna is any device that converts electromagnetic radiation traveling through space to electrical currents flowing in a wire (receiving antenna) or vice-versa (transmitting antenna). • Radio telescopes, and only radio telescopes, contain antennas. • Most of a typical radio telescope is not an antenna − the big dish just redirects electromagnetic radiation to the antenna part. Single Dish School 2015 July 6

  3. Electromagnetic radiation is produced by accelerating charged particles Single Dish School 2015 July 6

  4. Dipole antenna Power pattern Single Dish School 2015 July 6

  5. Reciprocity theorem  The receiving and transmitting patterns of an antenna are identical. Single Dish School 2015 July 6

  6. Ground-plane Waveguide horn vertical = ½ of a half-wave dipole Single Dish School 2015 July 6

  7. The waveguide horn used to discover λ = 21 cm HI emission from our Galaxy Single Dish School 2015 July 6

  8. Parabolic reflector:directivity and collecting area Prime focus Single Dish School 2015 July 6

  9. Aperture Single Dish School 2015 July 6

  10. Illumination, field, and power patterns Single Dish School 2015 July 6

  11. Cassegrain subreflector Single Dish School 2015 July 6

  12. 140-foot (43 m)Cassegrain Single Dish School 2015 July 6

  13. Reflector surface errors Single Dish School 2015 July 6

  14. 100 m homology telescope in Effelsberg Single Dish School 2015 July 6

  15. GBT: homology plus active surface Single Dish School 2015 July 6

  16. GBT Offset Gregorian + Prime Focusfor unblocked aperture Single Dish School 2015 July 6

  17. Single Dish School 2015 July 6

  18. GBT feeds andradiometers Single Dish School 2015 July 6

  19. Antenna output noise: voltage and power TA = “antenna temperature” Ae = effective area S = flux density Pν = power per unit frequency k = Boltzmann’s constant ≈ 1.38 × 10−23 Joules per Kelvin Single Dish School 2015 July 6

  20. The simplest radiometer Single Dish School 2015 July 6

  21. Square-law detector: output noise voltage is proportional to input power Single Dish School 2015 July 6

  22. Integrator output noise for: N = 50 samples N = 200 samples Single Dish School 2015 July 6

  23. Differential radiometer Single Dish School 2015 July 6

  24. Superheterodyne receiver Single Dish School 2015 July 6

  25. Spectrometers and software-defined digital back ends  Single Dish School 2015 July 6

  26. To learn more about radio astronomy, Google Essential Radio Astronomy http://www.cv.nrao.edu/course/astr534/ERA.shtml or see the printed book (4 copies are on reserve) The National Radio Astronomy Observatory is a facility of the National Science Foundationoperated under cooperative agreement by Associated Universities, Inc. Single Dish School 2015 July 6

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