Transcript No Slide Title

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
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
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 (~12%) of a 1 electron
avalanche.
• Gives 604 1.01 or 1.02
~few x100 gain.
22 March, 2012: Open University
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.
1 July 2012: SPIE 8453-1
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.
1 July 2012: SPIE 8453-1
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
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
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
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
Instrumentation Group
Institute of Astronomy
University of Cambridge, UK
[email protected]
1 July 2012: SPIE 8453-1