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Photon Counting EMCCDs: New Opportunities for High Time Resolution Astrophysics

Photon Counting EMCCDs: New Opportunities for High Time Resolution Astrophysics. Craig Mackay, Keith Weller, Frank Suess Institute of Astronomy, University of Cambridge. Introduction and Outline. Our knowledge of the Universe has been transformed in the last 40 years.

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Photon Counting EMCCDs: New Opportunities for High Time Resolution Astrophysics

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  1. 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

  2. Introduction and Outline • 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. 1 July 2012: SPIE 8453-1

  3. 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

  4. 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. 22 March, 2012: Open University

  5. 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

  6. 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. 1 July 2012: SPIE 8453-1

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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 Kadeldewar.

  13. 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. 1 July 2012: SPIE 8453-1

  14. 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. 1 July 2012: SPIE 8453-1

  15. The Einstein Cross • 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 1 July 2012: SPIE 8453-1

  16. Large Telescope Lucky Imaging. • 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. 1 July 2012: SPIE 8453-1

  17. Large Telescope Lucky Imaging. • 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. 22 March, 2012: Open University

  18. 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. 1 July 2012: SPIE 8453-1

  19. Instrumentation Group Institute of AstronomyUniversity of Cambridge, UKcdm@ast.cam.ac.uk 1 July 2012: SPIE 8453-1

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