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Detection Methods

Detection Methods. Coherent ↔ Incoherent Photon Detection ↔ Bolometric Photon Counting ↔ Integrating. Radio Telescopes. Typical Designs Heterodyne Receivers. Jansky’s First Radio Telescope 1933. Grote Reber: 1937 Radio Telescope. Heterodyne Receivers.

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Detection Methods

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  1. Detection Methods • Coherent ↔ Incoherent • Photon Detection ↔ Bolometric • Photon Counting ↔ Integrating

  2. Radio Telescopes • Typical Designs • Heterodyne Receivers

  3. Jansky’s First Radio Telescope 1933

  4. Grote Reber: 1937 Radio Telescope

  5. Heterodyne Receivers • Mix signal and local oscillator • Mixed signal contains “intermediate frequency” f_signal – f_local but also the sum of the frequencies

  6. Heterodyne Signal Detection

  7. MM and Sub-MM Telescopes • Use both coherent and incoherent detection • Heterodyne receivers for emission-lines • Mostly bolometers for continuum

  8. Millimeter Valley on Mauna Kea

  9. NGC 6334

  10. Detection Methods • Coherent ↔ Incoherent • Photon Detection ↔ Bolometric • Photon Counting ↔ Integrating

  11. Bolometers • Absorb and thermalize photons • Measure temperature change • Balance between heating and cooling results in long time constants • Typically used in chopped operation

  12. Transition Edge Sensors: Extreme sensitivity to small temperature changes allows to build very sensitive bolometer arrays

  13. Photocathods • The photoelectric effect • Quantum nature of light • Photomultipliers, channel plates …

  14. Detection Methods • Coherent ↔ Incoherent • Photon Detection ↔ Bolometric • Photon Counting ↔ Integrating

  15. Photocathod Devices • Cathods • Photomultiplier • Image intensifiers • Microchannel plates

  16. In 1907 Joel Stebbins pioneered the use of photoelectric devices in Astronomy

  17. Photomultiplier tubes: pile up errors Each detected photon produces a pulse of finite duration, t0, which causes a dead time in the detector. The number of pulses (exposure time) is reduced by the amount of overlapping deadtimes. N = n/(1–t0n) N is the true rate, and n the apparent rate Pile-up errors System blocks completely at high light levels

  18. A Revolution in Detectors: Photographic Plates • 1840 J.W. Draper makes a photograph of the moon. Followed by photographs of the Sun by Foucault and Fizeau • Sunspots photographed in 1858 by W. De La Rue • Jansen and Lockyer in the 1870s photographed the solar spectrum and discovered the spectral lines of Helium. • Ainsee Common photographed Orion Nebula and these revealed stars and details you could not see in a telescope • Photographs by Hubble in the early 1900‘s established that some nebula where „island universes“ (i.e. galaxies). His spectral observations of galaxies (exposures of more than one night) led to the discovery of the expansion of the Universe. • For 100 years photographic plates/film dominated the field of astronomical detectors.

  19. Detection Methods • Coherent ↔ Incoherent • Photon Detection ↔ Bolometric • Photon Counting ↔ Integrating

  20. Physics of Semiconductors Basic Quantum Physics Solids Semiconductors PN Junctions

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