Astronomical Detectors
ASTR 3010
Lecture 7
Chapter 8
SCUBA-2 array
Photoelectric effect
• We want to detect photons!!
• Change photons into electrons and measure the current!
Astronomical Detectors
detector
Detector Characteristics
• detection mode : photon detector, thermal detector, wave detector
• efficiency: QE (quantum efficiency)
• noise: SNR, DQE (detective quantum efficiency)
• spectral response: effective wavelength range
• linearity: threshold and saturation
• stability: deterioration, hysteresis
• response time: minimum exposure time
• dynamic range: hardware and software
• physical size: up to Giga pixels
• sampling: Nyquist sampling
Astronomical Detectors
detector
Detection modes
• Photon detectors: IR and shorter wavelengths
• Thermal detectors: bolometers, IR, radio + X-ray and gamma ray
• Wave detectors: can gauge phase, intensity, polarization (radio)
Efficiency of Detector
• Quantum Efficiency (QE): a common measure of the detector efficiency.
QE =
N detect
N in
• Perfect detector has QE=1.0
(SNR) perfect =
N out
sout
=
N in
sin
= N in
• Detective Quantum Efficiency (DQE): DQE is a much better indication of
the quality of a detector than QE. Why?
(SNR) 2out
N out
DQE º
=
(SNR) 2perfect N in
• For any detector DQE ≤ QE
Detector Performance
• DQE is a function of the input signal.
A certain QE=1 detector produces a background level of 100 electrons per second,
and it was used to observe two sources.
o Obj1 (bright) : 1sec  10,000 electrons  SNRin=100
Since there are two noise sources (Poisson noise and detector noise [proportional
to the sqrt(background level)]),
SNRout=10,000 / sqrt(10,100 + 100) = 99. Therefore, DQE=0.98
; total noise = Poisson noise + detector noise
; Poisson noise = total count from the source and background
o Obj2 (100 times fainter): 100 sec  10,000 electrons  SNRin=100
SNRout=10,000 / sqrt(20,000 + 100*100)=57.8
DQE=0.33
Linearity
• HST WFPC3
Nyquist sampling
• The sampling frequency should be at least twice the highest frequency (of
interest) contained in the signal.
Examples of aliasing
• Moire pattern of bricks
Moire pattern of bricks
Photo-emissive devices
• PMT : Choice of astronomical detector from 1945 until CCD.  fast
response time (few milliseconds). 1 channel
CCD
• Charge coupling = Transfer of all electric charges within a semiconductor
storage element to a similar, nearby element by means of voltage
manipulations.
CCD clocking = charge coupling = charge transfer
CCD readout and clocking
CCD readout : Correlated Double Sampling
• To decrease the readout noise
CCD saturation and blooming
CCD Dark Current
• dark current as a function of temperature
• Device needs to be cooled down
o
o
o
o
LN2 : -196C
Dry ice: -76C
mechanical cooler: -30 ~ -50C
liquid He: 10-60K
• Then, just use liquid He!
 no. charge transfer issue
CCD Charge Transfer Efficiency
• Charge transfer is via electron diffusion  too low Temp means long time to
diffuse.
• Compromised Temp : -100C
o need a heater
o or dry ice + cryo-cooler
• if CTE=0.99 for a pixel, 256x256 CCD,
charges from the most distant pixel
need to be transferred 1 million times!
Total Transfer Efficiency
TTE ≤ (CTE)256=7.6%
If CTE=0.9999,
TTE for a most distance pixel.
TTE=(0.9999)256+256= 0.95
Example of bad CTE
CCD charge traps and bad columns
• charge traps : any region that will not release electrons during the normal
charge-transfer process.
CCD gain, ADC, dynamic range
• If a full well depth of a CCD is 200,000 electrons
• + 16 bit analog-to-digital convertor (ADC).
• 16bit ADC : 0 – 65,535 (1 – 216)
200,000/65,535 = 3.05 electrons/ADU  gain
Even if the gain is set to high, because of the limit in ADC, there is a firm limit
in the upper limit in count (65535)  digital saturation
Noise sources in CCD
• Readout noise (“readnoise”) : present in all images
• Thermal noise (“dark current”) : present in non-zero exposures
• Poisson noise : cannot avoid
• Variance of noise = readnoise2 + thermal noise + poission noise
• How do we measure each of these noise sources?
o Readnoise ?
o Thermal noise?
o Poisson noise?
Sample image of dark current
Microchannel Plate
• MAMA (multi-anode microchannel array detector)
• DQE is very high  Xray to UV
Intensified CCDs
• Mostly military purpose (night vision goggle): 1 photon  104-7 phosphor
photons
• It will always decrease input SNR
Infrared Arrays
• Different from CCDs
• At different wavelengths:
o In-Sb : 1 – 5.5 microns
o HgCdTe: 1.5 – 12 microns
• Hybrid design: IR sensitive layer + silicon layer for readout  nondestructive readout!
• Fundamentally different readout: each pixel has own readout circuit
• Differences from CCDs
o no dead column, no blooming
o non-destructive readout (multiple readouts during an exposure)  various
readout schemes (Fowler sampling, up-the-ramp sampling)
o high background  quick saturation  need for co-add
o linearity is a concern
o dark current
o cold dewar
Different readout schemes…
Uniform Sampling
(“up-the-ramp”)
Fowler sampling
(Fowler & Gatley, 1990, ApJ)
In summary…
Important Concepts
Important Terms
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Photoelectric effect
Types of detectors
CCD
Infrared Arrays
Dark currents and charge tranfer
Nyquist Sampling
QE
DQE
CTE
Dark currents
Charge traps
Chapter/sections covered in this lecture : 8