Photon Counting EMCCDs: New Opportunities for High Time Resolution

Astrophysics

Craig Mackay, Keith Weller, Frank Suess Institute of Astronomy, University of Cambridge.

1 July 2012: SPIE 8453-1

• Our knowledge of the Universe has been transformed in the last 40 years.

• The biggest contribution has been the development of solid-state imaging detectors.

• We can now observe objects 10,000 times (10 magnitudes) fainter than we could.

• CCDs have been extraordinarily successful with near 100% efficiency and superb quality.

Introduction and Outline

1 July 2012: SPIE 8453-1

Introduction to EMCCDs: General Characteristics

• Electron multiplying CCDs (EMCCDs) now offer a wider range of opportunities.

• High time resolution astronomy and ways of managing atmospheric turbulence are key applications.

• Electron multiplying CCDs have all the characteristics of frame transfer conventional CCDs (DQE, CTE, dark current).

• The addition of an extended output register allows internal gain before the (relatively noisy) output amplifier.

22 March, 2012: Open University

Introduction to EMCCDs: General Characteristics• EMCCDs are

standard CCDs plus an electron multiplication stage.

• One serial electrode runs at high voltage (~45 volts).

• An electron has a low probability (~1-2%) of a 1 electron avalanche.

• Gives 604 1.01 or 1.02 ~few x100 gain.

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Photon Counting with EMCCDs• All these tests are

with our own 30 MHz camera (from www.pixcellent.com).

• A big overscan separates parallel register effects from serial register effects.

• A cut across an EMCCD image at very low signal level and high gain shows a wide range of events sizes.

1 July 2012: SPIE 8453-1

Photon Counting with EMCCDs• The additional variance introduced by the multiplication stage

increases the amount of noise.• Normally expect the SNR=√ (N). • With the EMCCD the SNR=√ (2N).• Equivalent to halving the DQE of the device.

• If we threshold the image and replace each event by a single value then added noise is eliminated and DQE largely restored.

• At high gains, 0.3 volts change can double the gain.• In photon counting mode, long-term absolute gain stability is

much less important. An event is an event is an event.• This makes camera design significantly easier.

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Clock Induced Charge• Even at temperatures where the dark current is negligible,

exercising the clocks can generate spurious events.• This is Clock Induced Charge (CIC).• Very sensitive to clock swing amplitude, so increasing from

12.5 V to 16.5 V increases CIC 10-fold.• In photon counting mode, parallel register peak well is

negligible so even lower (10-11 V) parallel clocks work fine.• Clock drivers must be carefully designed to minimise overshoot

while maintaining clock speed.• Faster clocks minimise CIC.• With care, parallel CIC can be <0.001 events/pixel/frame.

1 July 2012: SPIE 8453-1

Serial Register Clock Induced Charge• The high clock amplitude in the output register makes serial

CIC more important.• Serial clock amplitudes cannot be reduced at high gains.• Serial CIC is, on average, generated halfway along

multiplication register, so gain is the square root of the true gain.

• Its effects are therefore much less serious.• The most probable gain for an electron is unity.• So the selection of a photon threshold always discards some

genuine events.• With care, ~90% of the true device DQE may be achieved.

1 July 2012: SPIE 8453-1

Photon Counting with EMCCDs

• Top curve is from photon events in image area, lower curve from overscan area so only serial CIC events.

• The slope is different.• Threshold selection can

minimise loss of real events, traded off against counting spurious serial CIC events.

1 July 2012: SPIE 8453-1

EMCCD Camera Electronic Design• High-speed (> few MHz pixel rate) camera design very

different from slowscan design.• Attempting to speed up slowscan design really does not work

very well.• Wide range of very sophisticated integrated circuits have been

developed for commercial digital cameras.• Their performance is quite remarkable: in particular high-speed

clock drivers, sequencers and analog front-end components.• Also a great deal of help available on manufacturers websites

(suggested circuits, layout guidance, PCB pad design, simulation models, etc.).

1 July 2012: SPIE 8453-1

EMCCD Camera Mechanical Design• At the higher speeds (10-30

MHz), the physical layout of wiring becomes important.

• Very difficult to design drivers that are well terminated to minimise reflections and overshoots (which generate CIC).

• We now avoid vacuum dewars with internal flexible wiring.

• Prefer a rigid PCB with tracks on internal layers connecting CCD directly to driver cards.

22 March, 2012: Open University

EMCCD Camera Mechanical Design

• Also works for more complicated, multi-CCD designs • This shows an example of one we are using for the new AOLI

(Adaptive Optics Lucky Imager) instrument. This will be used on the WHT 4.2 m and GTC 10.4 m telescopes on La Palma.

• Uses 4-off CCD201, liquid nitrogen cooled in Kadel dewar.

High Time Resolution Astrophysics• Some of the most extreme astrophysical objects show

variations in brightness and spectral characteristics on very short (seconds to milliseconds) timescales.

• Examples are accretion discs, white dwarfs, neutron stars, x-ray emitters, etc.

• EMCCDs are now being used to search for correlations between visible and x-ray emission from compact objects.

• One of the biggest problems that astronomers face is managing the atmospheric turbulence that degrades images in the visible so dramatically.

• An example of what can be done is given by Lucky Imaging.

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EMCCD Application:Lucky Imaging• Atmospheric turbulence smears the images created with

ground-based telescope.• Using an EMCCD camera running at high frame rate allows

the motion to be frozen.• With a moderately bright reference star in the field, the

sharpest images may be selected.• These are shifted and added to give output images.• With selection percentages in the 3-30% range, near

diffraction limited images may be obtained.• This allows Hubble resolution images to be taken in the

visible from ground-based telescopes of ~ Hubble size.• At present, the only technique to deliver HST resolution.

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• The image on the left is from the Hubble Space Telescope Advanced Camera for Surveys (ACS) while the image on the right is the lucky image taken on the NOT in July 2009 through significant amounts of dust.

• The central slightly fuzzy object is the core of the nearby Zwicky galaxy, ZW 2237+030 that gives four gravitationally lensed images of a distant quasar at redshift of 1.7

The Einstein Cross

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• Globular cluster M13 on the Palomar 5m.

• Seeing ~650 mas.• PALMAO system and

our EMCCD Camera. • Achieved 17% Strehl

ratio in I-band, giving ~35 mas resolution.

• This is the highest resolution image ever taken in the visible.

Large Telescope Lucky Imaging.

1 July 2012: SPIE 8453-1

22 March, 2012: Open University

• Compare Lucky/AO and Hubble Advanced Camera (ACS) is quite dramatic.

• The Lucky/AO images have a resolution ~35 milliarcseconds or ~3 times that of Hubble.

Large Telescope

Lucky Imaging.

Conclusions• The development of CCDs has impacted almost every scientific

discipline.• Now very widely used and of extraordinary quality.• EMCCDs offer very high performance at the lowest signal

levels ever with two-dimensional imaging systems.• In photon counting mode we achieve maximum DQE.• Even in analog mode the excellent read noise achievable can

allow operation at extremely low signal levels indeed.• These cameras can offer astronomers and scientists in other

areas the opportunity to carry out entirely new kinds of research at the very faintest signal levels.

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Instrumentation Group Institute of Astronomy

University of Cambridge, UK

cdm@ast.cam.ac.uk

1 July 2012: SPIE 8453-1