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

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

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

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)

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

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 Typical electronic detectors have QE: 20-90%

Linearity HST WFPC3

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…

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 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 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

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

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

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: 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

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

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