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Observational Astrophysics I

Astronomical detectorsKitchin pp. 2-51

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Types of detectors

Integrating detectors Photon counting detectors (PCD)

Accumulate reaction to incoming radiation over time

React to (almost) every incoming photon and produce digital count

Example: photographic plate, CCD

Example: photomultiplier

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Common parameters of detectors Quantum efficiency

(QE) Spectral response Linearity Gain Dynamic range Saturation level Cosmic ray

sensitivity

Modulation Transfer Function (MTF)

Cosmetics Noise

Shot noise Read-out noise Dark current

Memory Flatness

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Charge Coupled Device

Light

Parallel charge transport

Serial charge transport

towards ADC

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Readout sequence

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Cont

inuo

us fl

ow

cryo

stat

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Electron trail

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Critical data flow properties

12 or 16 bits

CCD

Parallel CTE

Serial C

TE

Logarithmicamplifier

Analog signal

Digital signal ADUs

ADC+ bias

Readout

noise

Tem

pera

ture

cont

rol

Voltage on shift register

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Quantum Efficiency

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Improving spectral range QE drops in the blue because the top layer

is too thick and non-transparent. One way to improve it is the remove extra silicon substrate from the back (thinning) and use this side to detect the light (back-illumination).

QE drops in the red because photons have too low energy. Warming up CCD improves response in the red but also increases the noise.

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Dark currentAt T=270 K DC 10 e-/pixel/s

At T=230 K DC 0.1 e-/pixel/s

At T=170 K DC 10 e-/pixel/hour

At T=120 K DC 1 e-/pixel/hour

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Cooling

Liquid N2: 125 150 K

Peltier cooler: -20° -60° C

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Cosmetics

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Fringingλ=650 nm λ=900 nm

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LinearityCCD full well is the number of electrons which can be stored in one pixel (height of energy barrier between pixels).

Typical values are between 30000 and 1000000 which also where the CCD goes non-linear.

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Charge Transfer Efficiency This is examined by measuring the amplitude

of bright points left by a –ray source. Amplitude dependence in the directionof parallel read gives parallelCTE, while the other directionreflects serial CTE. Good CTEis >0.99999.

The same experimentestablishes the relation betweenADU and number of photoelectrons (gain). Same CCD may use more than one gain (e.g. 1.1 and 9).

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CCD noise Shot noise (Poisson distribution σ

≈√N) Dark current is time, depends on

temperature Readout noise, depends on the

temperature, read speed and amplifier(s) used

Cosmic rays destroycontent of a few pixels

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Flatness

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Binning

Step 1

Step 2

Step 3

Step 4

ReadoutExample: 44 binning

Exposure

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Next time… Hybrid detectors IR detectors Photon counting detectors Calibrations