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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 • Coherent ↔ Incoherent • Photon Detection ↔ Bolometric • Photon Counting ↔ Integrating
Radio Telescopes • Typical Designs • Heterodyne Receivers
Heterodyne Receivers • Mix signal and local oscillator • Mixed signal contains “intermediate frequency” f_signal – f_local but also the sum of the frequencies
MM and Sub-MM Telescopes • Use both coherent and incoherent detection • Heterodyne receivers for emission-lines • Mostly bolometers for continuum
Detection Methods • Coherent ↔ Incoherent • Photon Detection ↔ Bolometric • Photon Counting ↔ Integrating
Bolometers • Absorb and thermalize photons • Measure temperature change • Balance between heating and cooling results in long time constants • Typically used in chopped operation
Transition Edge Sensors: Extreme sensitivity to small temperature changes allows to build very sensitive bolometer arrays
Photocathods • The photoelectric effect • Quantum nature of light • Photomultipliers, channel plates …
Detection Methods • Coherent ↔ Incoherent • Photon Detection ↔ Bolometric • Photon Counting ↔ Integrating
Photocathod Devices • Cathods • Photomultiplier • Image intensifiers • Microchannel plates
In 1907 Joel Stebbins pioneered the use of photoelectric devices in Astronomy
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
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.
Detection Methods • Coherent ↔ Incoherent • Photon Detection ↔ Bolometric • Photon Counting ↔ Integrating
Physics of Semiconductors Basic Quantum Physics Solids Semiconductors PN Junctions