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

photons signal

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

detector

Astronomical Detectors

photons signal

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)

detector

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. 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=100SNRout=10,000 / sqrt(20,000 + 100*100)=57.8DQE=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 LN2 : -196Co Dry ice: -76Co mechanical cooler: -30 ~ -50Co 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 : -100Co need a heater o or dry ice + cryo-cooler

• if CTE=0.99 for a pixel, 256x256 CCD, charges from the most distant pixelneed to be transferred 1 million times!

Total Transfer EfficiencyTTE ≤ (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 micronso 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 CCDso no dead column, no bloomingo 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-addo linearity is a concerno dark currento cold dewar

Different readout schemes…

Fowler sampling(Fowler & Gatley, 1990, ApJ)

Uniform Sampling(“up-the-ramp”)

In summary…

Important Concepts• Photoelectric effect• Types of detectors• CCD• Infrared Arrays• Dark currents and charge tranfer• Nyquist Sampling

Important Terms• QE• DQE• CTE• Dark currents• Charge traps

Chapter/sections covered in this lecture : 8