AstrophysicalTechniques V

B. Nikolic

Detectors

Sensitivity

Back matter

Astrophysical Techniques V –Detectors

B. Nikolichttp://www.mrao.cam.ac.uk/˜bn204/mailto:b.nikolic@mrao.cam.ac.uk

Astrophysics Group, Cavendish Laboratory, University of Cambridge

February 2012

AstrophysicalTechniques V

B. Nikolic

Detectors

Sensitivity

Back matter

Outline

Detectors

Sensitivity

Back matter

AstrophysicalTechniques V

B. Nikolic

Detectors

Sensitivity

Back matter

Types of detectors

Photon-counting detectors – measure number of photonsarriving per unit area in focal plane

Bolometer detectors – measure heat deposition per unitarea in focal plane

Coherent detectors – measure the electric field voltage ina mode of the EM field

AstrophysicalTechniques V

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Detectors

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Metrics describing detectors

I Quantum efficiency (QE)I Time constant (τ )I Dark current/noise (σd)I Noise-equivalent power (Pne)I Detectivity (D = 1/Pne)I Dynamic rangeI Spectral responseI Low and high cut-off wavelengths

AstrophysicalTechniques V

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Charge-coupled devices (CCDs)

I Dominate UV/Optical astronomyI Conceptually:

I A 2D array of closely packed side-by-side capacitorsI Incoming photons converted to electrons by

photoelectric effect in silicon and accumulated in thecapacitors

I Manipulating voltages applied to capacitor allowsshifting of charge to neighbouring capacitor→sequential read-out of entire array

I Implemented using standard semiconductortechnology (1st CCD took one week from blackboardto testing!)

I Capacitors→ Minority carriers in a potential wellformed in a depleted region

AstrophysicalTechniques V

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CCDs in a focal plane

ESO VST/OmegaCAM16k×16k pixels – 1 degree ×1 degree on sky

From: http:

//www.eso.org/sci/facilities/paranal/instruments/omegacam/inst/index.html

AstrophysicalTechniques V

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CCD Quantum Efficiency

I Quantum efficiency of Silicon CCDs approaches100%

From http://www.ccd.com/alta_qe.html

AstrophysicalTechniques V

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CCD: MOS principle

metal gate

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In neutral configuration bulk presence of holes due to thedoping

AstrophysicalTechniques V

B. Nikolic

Detectors

Sensitivity

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CCD: MOS principle

metal gate

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depletion region

Applying sufficient positive potential to the metal electrodecreates an electrostatic depletion layer with few majority

carriers

AstrophysicalTechniques V

B. Nikolic

Detectors

Sensitivity

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CCD: MOS principle

metal gate

p-type

insulatorp

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depletion region

e e e e e e e e e e e e ee

inversion

Any electrons forming in the depletion region concentrateclose to the insulator in an inversion layer

AstrophysicalTechniques V

B. Nikolic

Detectors

Sensitivity

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CCD: shifting the signal

Image by Don Groom, from http://www-ccd.lbl.gov/movie.html

AstrophysicalTechniques V

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CCD Advantages/disadvantages

I Very high quantum efficiencyI Good linearity, dynamic rangeI Possible to build very large arraysI Good dark current performanceI Can shuffle the charges representing sky in synch

with the image

I Take time to read outI Read noise prevents single photon counting (but

electron multiplying CCDs overcome this problem)

AstrophysicalTechniques V

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Filters

Typical broad-band filtersThis plot: GEMINI/GMOS

(very close to the “SDSS” filters)

[Hook et al.(2004)Hook, Jørgensen, Allington-Smith, Davies, Metcalfe, Murowinski, and Crampton]

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Near, Mid & Far- Infarred

I ’Band-gap’: difference in energy between valence andconduction bands (of states)

I Photon have to a minimum of this energy to exciteelectron-hole pairs

I Silicon band-gap too large for IR: use other materialsand “bump-bond” to silicon

Material Cutoff (µm)

Silicon 1.11Germanium 1.8

Mercury-cadmium-telluride (HgCdTe) 1–12Indium antimonide (InSb) 6.5

Stressed gallium doped germanium 240

AstrophysicalTechniques V

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VISTA Focal plane array

Large-format NIR detectors now standard!(Focal plan of the VISTA Survey telescope)

http://www.vista.ac.uk/fpa_images.html

AstrophysicalTechniques V

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Bolometers

I Measure heat deposited by incoming radiationI Consist of:

I absorberI heat detection mechanism (often Transition Edge

Superconductor [TES])I Read-out/multiplex (often SQUIDs)

I Advantages: very high bandwidths, low noiseI Examples: Planck HFI, SCUBA-2, MUSTANG

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SCUBA-2

Large-format bolometer array for the JCMT operating at450µm and 850µm

From http://www.roe.ac.uk/ukatc/projects/

scubatwo/images/index.html

AstrophysicalTechniques V

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Detectors

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Outline

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AstrophysicalTechniques V

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Sensitivity (optical)

S = Fν∆νAt (1)

whereS Energy received from an astronomical source

Fν Incident flux (e.g., in Jy)∆ν Filter width/bandwidth

A Effective collecting area (. π(D/2)2)t Integration time

“Detection” when this signal at least about 5 times greaterthen our estimated uncertainty in its value.

AstrophysicalTechniques V

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Uncertainty (optical)

I Self/Photon noise σS =√

S =√

Fν∆νAtI Background noise σB =

√Bν∆νAtΩ

Ω ≡Solid angle on skyI Dark current uncertainty σD =

√Ct

I Read noise σR = constant

Total uncertainty:

σT =√σ2

S + σ2B + σ2

D + σ2R (2)

How does this change for differenced/choppedobservations?

AstrophysicalTechniques V

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Some remarks on sensitivity

I Uncertainties are proportional to square root ofmagnitude of signal (Poisson statistics)

I For excellent CCDs and reasonably long integrationsusually limited by background noise. Only ways ofreducing it is:

I Better site (not for zodiacal background!)I Filters to remove bright lines from the atmosphereI Reduce losses in the mirrors/lenses (near-IR only)I Smaller angle on sky Ω

I In the IR, calibration/atmospheric effects can becomelimiting factors

AstrophysicalTechniques V

B. Nikolic

Detectors

Sensitivity

Back matter

Outline

Detectors

Sensitivity

Back matter

AstrophysicalTechniques V

B. Nikolic

Detectors

Sensitivity

Back matter

References

I. M. Hook, I. Jørgensen, J. R. Allington-Smith, R. L.Davies, N. Metcalfe, R. G. Murowinski, andD. Crampton.The Gemini-North Multi-Object Spectrograph:Performance in Imaging, Long-Slit, and Multi-ObjectSpectroscopic Modes.PASP, 116:425–440, May 2004.doi: 10.1086/383624.