Detectors for Optical and Infrared Astronomy

Kyler KuehnInstrument Scientist, AAO

Observational Techniques Workshop01 April 2014

The Dark Energy Camera

62 2k x 4k full-frame, fully-depleted, backside-illuminated, 4-side-buttable, dual-readout, p-channel LBNL CCDs with 250 micron x 15

micron x 15 micron pixels, possessing 130ke- fullwell, 2.5 ADU/e- gain, “five nine” CTE, totaling 520 Megapixels with >50% QE from 375 nm

to 1000 nm, operating at 173K, with an overall FOV of 3 sq deg.

Motivation• For some of you, astronomical images and spectra are

things that from time to time show up on your screen and/or in your hard drive as FITS files.

• If the materials scientists and optical engineers (and software engineers, and…) have done their job properly, you generally won’t have to worry about most of the details in this presentation.

• But it can be useful to know what’s “under the hood”, so you can (at least) understand the motivations behind each data analysis step, and (possibly) identify and contribute to the resolution of issues arising from your own observations.

Detectors: a History (in one slide)

Sources: Brahe’s Mechanica, Galileo’s telescope from Florence’s Museo Galilei, Image from Sidereus Nuncius

Unaided human eye:Area ~40 mm2, “Integration time” ~0.04 scf. Rudolphine Tables (data acquired by Tycho Brahe up to 1601, published by Kepler in 1627)

First telescopes (circa 1608): <700mm2

(though with significant aberration)

Galileo’s drawing of Jupiter’s “Medicean Stars”: Cosimo, Francesco, Carlo and Lorenzo

(OK, make that two slides)Photographic Plates:• Gelatin film (usually) containing light-

sensitive silver salts, activated by incident light then chemically “fixed” to prevent further change

• Glass plates do not deform like photographic film does

• Area, resolution, & integration time limited primarily by the opto-mechanical precision of telescope and instruments

• Drawbacks: low QE (2% vs. 90%), non-linear response, processing of data is challenging

Source: UKST Southern Sky Survey (IoA, Cambridge)

Charge-Coupled Devices• Photons converted to electronic signal via the

photoelectric effect• CCD comprises an array of pixels, acting as “light buckets”• Shift register reads out pixel values by “clocking” the gate

voltage, “bucket bridgade”-style• For sensitivity to optical wavelengths, detectors are

silicon, doped with B or P• Contain a lattice of overlapping Si e- orbitals• Individual e- can be dissociated from the lattice by an

incident photon and conducted to the pixel electrode

Smith & Boyle shared (half of) the Nobel Prize in 2009 for the invention of the CCD.

CCD Fabrication

• Start* with bare Si wafer• Slicing, polishing• Thinning• Doping• Plasma etching• Chemical vapor deposition• Photolithography• Dicing• Packaging• TestingSource: LBNL

* There’s a whole talk hidden in this one word too.

A Single CCD Picture Element (Pixel)

Source: Dark Energy Survey/LBNL

Fully Depleted

Deeply Depleted

CCD Quantum Efficiency

GMOS-S (Hamamatsu Red/Blue CCDs)

Source: LBNL

Non-Imaging Regions of a CCD

Source: LBNL

Source: FNAL Silicon Detector Facility (Gordie Gillespie)

Shift Register

Imaging Array

Channel Stop

Voltage Bus

VerticalClockVoltages

Source: Simon Ellis (AAO)

Charge Transfer (In)Efficiency

Standard CCDs usually have 99.999% (“five nines”) charge transfer efficiency in the shift register. That sounds pretty good, but…

(0.99999)(4096+1024) = 95%

(0.999999)(4096+1024) = 99.5%Six nines is significantly better!

Noise Characteristics: Bias Voltage

GMOS-N CCDs

A baseline voltage is applied to all pixels so that all random noise is by definition positive.But this requires subtracting this “base level” read noise from all science-related exposures.Bias frames can also be used to identify the effects of some physical defects in the CCD.

650 μm thick CCD 200 μm thick CCD

Not fully depleted at 100V Fully depleted at 100V

Depletion Fraction/Voltage Effects

Source: LBNL

CCD Cold-Probe Testing at FNAL

Source: FNAL Silicon Detector Facility

Errors Arising from CCD Fabrication

• Tape bumps (from physical assembly process)• Glowing edges (e- γ in amplifier near pixels)• Hot spots (substrate voltage leakage)• Bad pixels (the “bucket” has a hole…)• Bad columns (“traps” – the voltage or shift

register connection is broken)• And many more….

From Pixels to CCDs: Choices• Will you be taking images or spectra?• Do you want a full frame CCD (needs mechanical shutter) or a frame transfer CCD (faster readout,

but 1/2 active Si area)? Do you want front- or back-side illumination?• How thick is your photoactive region (i.e., how red-sensitive)? Do you want deep/full depletion?• Does your telescope track (non-)sidereal objects, or do you observe in drift scan mode? For the

latter, your readout must be aligned precisely with the scan direction.• Do you want to cool your instrument? To what temperature -- do you need a cryogenic system or

will thermoelectric cooling provide sufficiently low dark current?• What wavelength region will you observe? What detector material and A/R coating will you use?• What size pixels do you require in microns, and in arcseconds on the sky (the “plate scale”) given

your telescope optics? Are you over- or under-sampling (2 * pixel scale > minimum seeing)?• What field of view do you want to cover in an exposure? How many CCDs will be in your

detector? How close do they need to be? This determines whether to use picture frame or 2-side buttable or 4-side buttable devices.

• What level of cosmetic defects will hamper your science? What voltages can the silicon withstand?

• Your readout likely saturates at 65535 (216 – 1 for a 16 bit system); what fullwell capacity (between say, 100ke- to 300 ke-) is optimal? What is the requisite gain?

• At what stage does saturation/non-linearity begin to affect your measurements?

Source:Dark Energy Survey/LBNL

Fabricating Devices is Tricky!

Device YieldFabrication: 80% (“Mechanical CCDs”)

Cosmetic Defects/ Cold-Probe Tests: 60%(“Engineering CCDs”)

“Science Grade”: <25%Source: DES/FNAL

The finished product (sort of)

Source: DES/FNAL

CCD Focal Plane Installation

Source: DES/FNAL

Instrument Installation

Source: DES/CTIO

Data Acquisition (DAQ)IRAF/IDL (and even cfitsio) are still far away. Between the shift register and the image is the DAQ

For example: Monsoon crate containing Clock & Bias Boards, Master Control Boards

Also, the electronic signal may be turned back into photons via S-Link fibre optic cable to the data PC.

Then telescope/observation control and telemetry data are added

Then you have a (FITS) file with pixel array data and header information

Then you can test your on-sky performanceASIC Readout for SNAP

Source: LBNL

Source: DES/PreCam

Fullwell/Saturation(~120ke- = 65535 ADU?)Fullwell degradation from over-exposure?

Gain (ADU/e-)

Linearity (fraction of FW?)

Shutter Vignetting

Shutter Vignetting, Saturation

Source: DES/PreCam

Dark Current

• Integrate for the same amount of time as a science exposure, but with the shutter closed.

• This determines the (thermal) noise rate that is then subtracted from each pixel.

• Cryogenic cooling to ~173K is common for many astronomical instruments, some can get away with 183 or warmer without swamping the detectors with thermal noise.

Flat FieldingPixel-to-pixel variation in (quantum) efficiency means uniform illumination will not result in uniform counts from every pixel

Thus we divide each science image by a flat field image so that a pixel with, say, 2x the efficiency is weighted by ½ relative to the others.

Dipoles?AAT Wide Field Imager + Prime Focus Unit in z band

FringingScattered light from within the CCD, especially noticeable in redder wavelengths

GMOS-S CCDs

Cosmic Rays

Source: DES/PreCam

What’s the source of this noise?

Source: DES/PreCam

Spoiler: readout cable/pin connector micro-fracturesresulting in intermittent additive (common mode?) noise

What’s the source of this noise? II

Sources of Counts in a Pixel

• Read Noise (Hopefully subtracted properly)• Dark Current (Hopefully subtracted properly)• Sky Noise (Ambient and Anthropogenic)• Scattered light/ghosting (LEDs in instrument)?• Photons from a source (Signal + Shot Noise)• Signal to Noise calculators take all of these (as well

as system throughput) and determine exposure times

• This feeds into observing proposals…

375 400 425 450 475 500 525 550 575 600 625 650 675 700 725 750 775 800 825 850 8750.00%

5.00%

10.00%

15.00%

20.00%

25.00%

30.00%

35.00%Total System Throughput Includes:

Spectral Cross-ContaminationCCD QESpectrograph Optics/CoatingsFibre Output/ConnectorFibre LengthFibre Injection LossFibre AlignmentInterface Thermal AberrationFocal Plane Thermal AberrationFocal Plane OpticsVignettingTelescope OpticsAtmosphere

TAIPAN: A Case Study

TAIPAN: A Case StudyFrom Survey Requirements through Exposure Time Calculator to CCD Specifications• Magnitude 18 galaxy with S/N ~10 in 2.5 hours• Magnitude 12 star with S/N ~100 in 2.5 minutes

– Long exposures for faint objects– Short exposures for bright objects– Medium exposures for medium objects

• Under what conditions do Read Noise, Dark Current, Sky Noise, or Shot Noise dominate?

• TAIPAN Requirements:– Read Noise < 3 e-/pix– Dark Current < 0.01 e-/pix/s

Wavelength 4380Angstroms 5384 4380Mag zero point 4063Jy (Vega mags)Galaxy mag at wavelength 15magsLine Flux in SRE 401e-17 ergs/cm2/s Sky brightness 18.5mags/arcsec^2Line Width 120km/s 1.75A

Source Area 171.21arcsec^2 Fnu from sky 161750.9434nJy/m^2/arcsec^2Fraction of light per fibre 0.04129

Mag on fibre 18.46 mags Flambda from sky 2.52941E-19W/m^2/A/arcsec^2

Fnu from object cuum 167748.5232nJy/m^2 Photons from sky 0.557005115ph/m^2/A/sec/arcsec^2

Total Flambda from object cuum 6.3536E-18W/m^2/A 6.3536E-15erg/s/cm2/A Fibre Area 7.068583471arcsecs^2

Flambda from object cuum/fibre 2.6232E-19W/m^2/A 262.310-18 erg/s/cm2/A True sky/Fibre 3.93723715ph/A/sec/fibre/m^2

Flambda from object line/fibre 1.65147E-13ergs/m^2/s/fibre 1.65147E-17erg/s/cm2 Pixel spectral size 0.952173913Angstroms

True sky per pix 3.748934503ph/sec/SREPhotons from object cuum/fibre 0.578ph/A/sec/fibre/m^2 Pixels per SRE 2pixelsPhotons from object line/fibre 0.036ph/sec/fibre/m^2 Instrument PSF 2pixelsLine width 1.752Angstroms Dark count rate 0.003electrons/sec/pixPhotons from object line/fibre/angstrom 0.021ph/A/sec/fibre/m^2 Scattered OH rate 0electrons/sec/pix

Readnoise 3electrons/pixGain 1.788electrons/ADU

Telescope area 1.13m^2 11300cm^2System efficiency atm->detector 13.5%

Exposure time for one integration 1600seconds on targetDetected sky electrons 1830.079867per SRE

Det .back. electrons 1839.679867per SRE

Detected cuum electrons 269per SRE 150.1695991ADU/SRE Back. noise 43.30911991per SRE

Detected line electrons 5per SRE 2.702175787ADU/SRE Sky subtraction fac. 1sqrt(2) or 1

Total Detected Line electrons 8 Sky/Object cuum 23.46183742per SRE

Cuum Signal/noise per integration 5.8per SRE Spec Resolution R= 2,300Line Signal/noise per integration 0.1per SRE Dark Current Sky Noise Read Noise Object NoiseTotal Line S/N per Integration 0.2 3.098386677 42.77943276 6 16.38606857Number of integrations 3

Spec Res final = 575Cuum Final Signal/noise 10.0per SRE

Other Detector Technologies• HgCdTe (“MerCadTel”): IR-sensitive (1.5-12 um) detectors.

– Hawaii 2-RG detectors (e.g. in the Gemini Planet Imager) have a (non-destructive!) readout layer made of Si.

– Thermal noise (infrared photons) is more of an issue, requiring high-performance cryogenic cooling.

– Much of what has been described here for Si is applicable to HgCdTe.

• InSb (e.g., ESO’s ALADDIN sensor arrays on VLT), or InGaAs• Photomultiplier tubes (“avalanche” detectors)• Electron Multiplying Charge-Coupled Device (Si CCD with increased Gain like PMT, cf. NuVu Cameras)• Other wavelengths? (gamma, X, UV, microwave, radio)• Other astrophysical messengers? (neutrinos, gravitational)

• Microwave Kinetic Inductance Detectors (MKIDs)…

The Future(?): MKIDS

Source: Mazin Lab (http://web.physics.ucsb.edu/~bmazin/Mazin_Lab/MKIDs.html)

LC Circuit gives energy and timing information of individual photonsbut requires (even more) sophisticated readout and precise cooling

than a standard Si-based device

The Dark Energy Camera

62 2k x 4k full-frame, fully-depleted, backside-illuminated, 4-side-buttable, dual-readout, p-channel LBNL CCDs with 250 micron x 15 micron x 15 micron pixels, possessing 130ke- fullwell, 2.5 ADU/e- gain, “five nine” CTE, totaling 520 Megapixels with >50% QE from 375 to 1000 nm, operating at 173K, with an overall FOV of 3 sq deg.

Only after this entire process can your detectors obtain results like this:

Coma Cluster; Source: DES

Or this:

NGC 1365; Source: DES

Thank You!

Source: Marcelle Soares-Santos, FNAL

Useful References• AY257 Modern Observational Techniqueshttp://www.ucolick.org/~bolte/AY257/s_n.pdf• Dark Energy Survey Instrumentationhttp://www.darkenergysurvey.org/DECam/ (see also http://www.ctio.noao.edu/noao/content/DECam-Known-Problems)

• Mazin Labhttp://web.physics.ucsb.edu/~bmazin/Mazin_Lab/MKIDs.html

• Gemini Instrumentationhttp://www.gemini.edu/node/10002

• Me (Follow up questions? Want more details?)kkuehn@aao.gov.au• Wikipedia/Amazon (no kidding)

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