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

• 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.

A Revolution in Detectors: Photographic Plates

Detection Methods

• Coherent ↔ Incoherent• Photon Detection ↔ Bolometric• Photon Counting ↔ Integrating

Physics of Semiconductors

Basic Quantum PhysicsSolids

SemiconductorsPN 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

20482 Readout Provides Low Read Noise for Visible and MWIR

Canon 16mm x 14 mmGCA 20mm x 20 mm

ASML 22mm x 27.4 mmReticle-Stitching: 2048x2048 ROIC

Submicron Stepper Options

3-D Barrier to Prevent Glow from Reaching the Detector

Metal 1

Metal 2

Metal 3

Poly 1

Analog Capacitor

Low

-Noi

se C

MO

S M

ultip

lexe

r

p-type

n+

HgCdTeDetector

IndiumInterconnect

Overglass

CMOS (LOCOS)