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Transcript
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
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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
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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
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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)