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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 • Mix signal and local oscillator • Mixed signal contains “intermediate frequency” f_signal – f_local but also the sum of the frequencies Heterodyne Signal Detection MM and Sub-MM Telescopes • Use both coherent and incoherent detection • Heterodyne receivers for emission-lines • Mostly bolometers for continuum Millimeter Valley on Mauna Kea NGC 6334 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 Semiconductors • Conduction in semiconductors • Doping Detection Methods • Coherent ↔ Incoherent • Photon Detection ↔ Bolometric • Photon Counting ↔ Integrating PN Junctions • Formation of pn junction • Rectifying properties • Charge separating properties Electronics • • • • • • PN junctions and photodiodes Field Effect transistors Logic devices Analog switches Operational amplifiers A practical example Field Effect Transistor • Junction FET (JFET) • Metal-Oxide-Semiconductor FET (MOSFET) • CMOS circuits (Complementary Oxide Semiconductor) Fabrication of Integrated Circuits • • • • Doping Depositing metal Growing oxides (as isolators) All controlled by photoresist masking HAWAII-2: Photolithographically Abut 4 CMOS Reticles to Produce Each 20482 ROIC Twelve 20482 ROICs per 8” Wafer Submicron Stepper Options Canon 16mm x 14 mm GCA 20mm x 20 mm ASML 22mm x 27.4 mm Reticle-Stitching: 2048x2048 ROIC 20482 Readout Provides Low Read Noise for Visible and MWIR 3-D Barrier to Prevent Glow from Reaching the Detector HgCdTe Detector p-type n+ Indium Interconnect Low-Noise CMOS Multiplexer Overglass Metal 3 Analog Capacitor Metal 2 Metal 1 Poly 1 CMOS (LOCOS)
