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Radio Telescopes and Radiometers 2015 Single Dish School Jim Condon NRAO, Charlottesville Atacama Large Millimeter/submillimeter Array Karl G. Jansky Very Large Array Robert C. Byrd Green Bank Telescope Very Long Baseline Array 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 viceversa (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 Electromagnetic radiation is produced by accelerating charged particles Single Dish School 2015 July 6 Dipole antenna Power pattern Single Dish School 2015 July 6 Reciprocity theorem The receiving and transmitting patterns of an antenna are identical. Single Dish School 2015 July 6 Ground-plane vertical = ½ of a half-wave dipole Waveguide horn Single Dish School 2015 July 6 The waveguide horn used to discover λ = 21 cm HI emission from our Galaxy Single Dish School 2015 July 6 Parabolic reflector: directivity and collecting area Prime focus Single Dish School 2015 July 6 Aperture Single Dish School 2015 July 6 Illumination, field, and power patterns Single Dish School 2015 July 6 Cassegrain subreflector Single Dish School 2015 July 6 140-foot (43 m) Cassegrain Single Dish School 2015 July 6 Reflector surface errors Single Dish School 2015 July 6 100 m homology telescope in Effelsberg Single Dish School 2015 July 6 GBT: homology plus active surface Single Dish School 2015 July 6 GBT Offset Gregorian + Prime Focus for unblocked aperture Single Dish School 2015 July 6 Single Dish School 2015 July 6 GBT feeds and radiometers Single Dish School 2015 July 6 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 The simplest radiometer Single Dish School 2015 July 6 Square-law detector: output noise voltage is proportional to input power Single Dish School 2015 July 6 Integrator output noise for: N = 50 samples N = 200 samples Single Dish School 2015 July 6 Differential radiometer Single Dish School 2015 July 6 Superheterodyne receiver Single Dish School 2015 July 6 Spectrometers and software-defined digital back ends Single Dish School 2015 July 6 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 Foundation operated under cooperative agreement by Associated Universities, Inc. Single Dish School 2015 July 6