1. Astronomical Detectors
SCUBA-2 array
ASTR 3010
Lecture 7
Chapter 8

2. Photoelectric effect We want to detect photons!!
Change photons into electrons and measure the current!

3. Astronomical Detectors
photons
signal
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

4. Astronomical Detectors
photons
signal
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)

5. Efficiency of Detector
Quantum Efficiency (QE): a common measure of the detector efficiency.
Perfect detector has QE=1.0
Detective Quantum Efficiency (DQE): DQE is a much better indication of the quality of a detector than QE. Why?
For any detector DQE ≤ QE

6. 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.
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, ) = 99. Therefore, DQE=0.98
; total noise = Poisson noise + detector noise
; Poisson noise = total count from the source and background
Obj2 (100 times fainter): 100 sec  10,000 electrons  SNRin=100
SNRout=10,000 / sqrt(20, *100)=57.8
DQE=0.33
Typical electronic detectors have QE: 20-90%

7. Linearity
HST WFPC3

8. Nyquist sampling
The sampling frequency should be at least twice the highest frequency (of interest) contained in the signal.
True signal of 1 Hz.  sampling at 2Hz, sufficient to capture each peak and trough  at 3Hz, more than enough sample  at 1.5Hz, causing an alias, wrong info…

9. Examples of aliasing
Moire pattern of bricks
Moire pattern of bricks

10. Photo-emissive devices
PMT : Choice of astronomical detector from 1945 until CCD.  fast response time (few milliseconds). 1 channel

11. CCD
Charge coupling = Transfer of all electric charges within a semiconductor storage element to a similar, nearby element by means of voltage manipulations.

12. CCD clocking = charge coupling = charge transfer

13. CCD readout and clocking

14. CCD readout : Correlated Double Sampling
To decrease the readout noise

15. CCD saturation and blooming

16. CCD Dark Current dark current as a function of temperature
Device needs to be cooled down
LN2 : -196C
Dry ice: -76C
mechanical cooler: -30 ~ -50C
liquid He: 10-60K
Then, just use liquid He!
 no. charge transfer issue

17. CCD Charge Transfer Efficiency
Charge transfer is via electron diffusion  too low Temp means long time to diffuse.
Compromised Temp : -100C
need a heater
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) = 0.95
Example of bad CTE

18. CCD charge traps and bad columns
charge traps : any region that will not release electrons during the normal charge-transfer process.

19. 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

20. 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?
Readnoise ?
Thermal noise?
Poisson noise?
Sample image of dark current

21. Microchannel Plate MAMA (multi-anode microchannel array detector)
DQE is very high  Xray to UV
ACS MAMA detector and

22. Intensified CCDs
Mostly military purpose (night vision goggle): 1 photon  phosphor photons
It will always decrease input SNR

23. Infrared Arrays Different from CCDs At different wavelengths:
In-Sb : 1 – 5.5 microns
HgCdTe: 1.5 – 12 microns
Hybrid design: IR sensitive layer + silicon layer for readout  non-destructive readout!
Fundamentally different readout: each pixel has own readout circuit
Differences from CCDs
no dead column, no blooming
non-destructive readout (multiple readouts during an exposure)  various readout schemes (Fowler sampling, up-the-ramp sampling)
high background  quick saturation  need for co-add
linearity is a concern
dark current
cold dewar
HgCdTe : Mercury, Cadmium, Telluride  MerCat detector
In-Sb : Indium Antimonide

24. Different readout schemes…
Uniform Sampling
(“up-the-ramp”)
Fowler sampling
(Fowler & Gatley, 1990, ApJ)

25. Chapter/sections covered in this lecture : 8
In summary…
Important Concepts
Important Terms
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