Astronomical Detectors

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

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

3. 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

4. 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)

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(10,100 + 100) = 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(20,000 + 100*100)=57.8 DQE=0.33

7. Linearity • HST WFPC3

8. Nyquist sampling • The sampling frequency should be at least twice the highest frequency (of interest) contained in the signal.

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)256+256= 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

22. Intensified CCDs • Mostly military purpose (night vision goggle): 1 photon  104-7 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

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

25. In summary… Important Concepts Important Terms QE DQE CTE Dark currents Charge traps • Photoelectric effect • Types of detectors • CCD • Infrared Arrays • Dark currents and charge tranfer • NyquistSampling • Chapter/sections covered in this lecture : 8