Observational astrophysics i
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Observational Astrophysics I. Astronomical detectors Kitchin pp. 2-51. Types 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

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Observational Astrophysics I

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Observational Astrophysics I

Astronomical detectors

Kitchin pp. 2-51

Types of detectors

Quantum efficiency (QE)

Spectral response



Dynamic range

Saturation level

Cosmic ray sensitivity

Modulation Transfer Function (MTF)



Shot noise

Read-out noise

Dark current



Common parameters of detectors


Serial charge transport

towards ADC

Parallel charge transport

Charge Coupled Device

Readout sequence

Continuous flow cryostat

Electron trail

Parallel CTE

Digital signal ADUs

Voltage on shift register



Serial CTE


+ bias



Analog signal

Critical data flow properties

12 or 16 bits

Quantum Efficiency

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.

Dark current

At 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


Peltier cooler: -20°  -60° C

Liquid N2: 125  150 K



λ=650 nm

λ=900 nm


CCD 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.

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).

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



Example: 44 binning



Step 1

Step 2

Step 3

Step 4

Next time…

  • Hybrid detectors

  • IR detectors

  • Photon counting detectors

  • Calibrations

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