Bettina Mikulec, Allan Clark
University of Geneva
John Vallerga, Jason McPhate, Anton Tremsin, Oswald
Siegmund
Space Science Laboratory, University of California
CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
Photon Counting
with a 512 x 512 Pixel Array
at 1 kHz Frame Rate
A New Wavefront Sensor
for Adaptive Optics
Introduction into Adaptive Optics*
– Temperature fluctuations in the air
cause changes in the index of
refraction
– light rays are no longer parallel when
they reach telescope and can
therefore not anymore be focused to a
point
Point
focus
Parallel light rays
 blur
Light rays affected
by turbulence
Even the largest ground-based astronomical
telescopes have no better resolution than an 8" telescope!
Bettina Mikulec
*adapted from AO lectures of Claire Max, Astro289C, UC Santa Cruz
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
• Turbulence in the earth’s atmosphere
makes stars twinkle
• More importantly, turbulence spreads
out the star light making it a blob
rather than a point
Adaptive Optics
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
proposal for a
new WFS Optical Medipix tube
Adaptive Optics
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
Example for the enormous improvements using AO
(Lick Observatory).
The new generation: adaptive optics on 8-10 m
telescopes
Subaru
2 Kecks
ESO VLT
Gemini South
Gemini North
And at other places: MMT, VLT, LBT, Gemini South
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
Summit of Mauna Kea volcano in Hawaii:
Next Generation of Large Telescopes
(proposed)
TMT
All propose AO systems with > 5000 actuators
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
30 m diameter:
– California Extremely Large Telescope (CELT) – Thirty Meter Telescope (TMT)
50 m diameter:
– EURO50 on La Palma
100 m diameter:
Palomar
– European Southern Observatory’s “OverWhelmingly Large Telescope”
Hale 5m
(OWL)
telescope
Adaptive Optics
Shack-Hartmann wavefront sensor
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
• Determine the distortions with the help of a natural or laser guide
star and a lenslet array (one of the methods). Deviations of the spot
positions from a perfect grid is a measure for the shape of the
incoming wave-front.
Wavefront Sensor Requirements
•
•
Large pixel array, high frame rate and no readout noise
not simultaneously achievable with CCDs!
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
High QE for dimmer guide stars (~80% optical QE)
Gate the detector in 2-4 s range for operation with laser guide
stars
o Many pixels in the order of 512 x 512; future large telescopes will
have about 5000 actuators (controlled via 70 x 70 centroid
measurements)
• 1000 photons per spot to get a 3% centroid rms error with respect to
the stellar image size.
o 1 kHz frame rate (light integration, readout, calculations, send out
5000 signals and ready for new frame); faster than the timescale of
the atmospheric turbulences
o Very low readout noise (< 3e-)
Proposal for a New Wavefront Sensor
Photocathode
Photon
e-
Q = 104e-
Pij = Pij + 1
Window

MCP
Medipix2


2 µm pores on 3 µm
centers (Burle Industries)
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
 High-QE GaAs photo-cathode
 Matched pair of microchannel plates (MCP) with 10 m pore
diameter in chevron configuration
 Medipix2 counting CMOS pixel chip
 Integrate photon events on pixel,
noiseless chip readout
The Medipix2 Photon Counting Chip
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
m CMOS technology (33M transistors/chip)
square pixel size of 55 µm
256 x 256 pixels
sensitive to positive or negative input charge (free choice of different detector materials)
pixel-by-pixel detector leakage current compensation
window in energy
discriminators designed to be linear over a large range
14-bit counter per pixel
count rate: ~1 MHz/pixel (0.33 GHz/mm2)
3-side buttable
serial or parallel I/O (min. readout time of full matrix 266 µs)
0.25
Measurement Setup
• A Medipix2 photon counting chip
• A matched pair of MCPs:
Photonis MCPs with 33 mm diameter
10 m hole diameters, L/D = 40/1
low resistivity (~22 MOhms per plate)
gain was varied between 20k and 200k (1430 - 1680 V)
• Vacuum tank pumped down to ~10-6 torr
• Hermetic feed-throughs (50-pin connector for Medipix signals)
• A standard UV Hg pen-ray lamp with collimator (~10 counts/s -500M
counts/s)
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
–
–
–
–
Feasibility Tests
06 April 2004
gain 106, rear field 427 V
gain 50k, rear field 980 V
It works!
• Event size function of MCP gain, rear field, MCP-Medipix distance
and Medipix threshold
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
single photon
events
Flood Fields
• Take image with collimated UV source at 50ke gain and 1600 V rear
field (~5000 counts/pixel). Average single spot area: 2.4 pixels
Ratio = flood1 / flood2.
Histogram of ratio
consistent with counting
statistics (rms 0.02)
– Fixed pattern noise from dead spots on the MCPs and MCP multifibres divides
out.
Bettina Mikulec
CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
take 2 independent
uniform illuminations
(flood fields at
~500Mcps)
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Resolution
• The Air Force test pattern was used to demonstrate the imaging
properties of the detector, in particular the resolution.
100 s exposure; the spots
correspond to individual
photon events.
Bettina Mikulec
1 s exposure.
Group 3-2 visible (~9 lp/mm
corresponding to the Nyquist
limit of 55 m pixels)
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
increase
shutter
time
Event Centroiding
• Centroiding individual photon events to achieve sub-pixel resolution:
centroiding
Group 4-2 starts to be resolved
(17.95 lp/mm; 55.7 m
corresponding to ~28 m pixels).
Could be useful for
low rate imaging applications!
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
– Take many very low count rate images with larger spot area to avoid
overlapping spots. (~100-150 counts/frame; 1000 frames)
– Identify unique spots and reject overlapping events (counts  2), count
spots, record their size and calculate the centroids.
UV Photon Counting Movie
Air Force resolution mask, 100 ms exposures
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Spot Size vs. Gain
Pinhole grid mask (pitch 0.5 mm x 0.5 mm) to simulate ShackHartmann spots:
Gain: 200 000
Bettina Mikulec
Gain: 20 000
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
Rear field: 1600V, gap 500 m, threshold 3 ke-
Sub-Pixel Spatial Linearity
Pinhole
Detector
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
Lamp
Average Movement of ~700 Spots
0
Centroid Position (µm)
Delta Y
-20
-30
1 pixel
-40
-50
-60
-70
-80
-90
-100
0
5
10
15
20
25
Lamp Position (mm)
• Achieved a 2 m rms centroid position error with ~550 events/spot.
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
Delta X
-10
Electron Detection
• First test results with beta sources
63Ni,
67 keV max.
~300 counts/pixel
204Tl,
764 keV max.
~100 counts/pixel
 QE ~46% for Ni and ~63% for the Tl image;
increasing efficiency with e- energies above ~50 keV consistent with
literature.
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
Gain ~60k,
rear field 1600 V
Medipix threshold
~38 ke-
Conclusions
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
• New detector concept proven to work!
• Performed systematic tests varying different detector
parameters
• No fixed pattern noise yet detectable except MCP
imperfections
• Resolution at Nyquist limit and below (for event-by-event
centroiding) demonstrated
• Images presented with both UV and electron sources 
detector has a great capacity to be used for various
wavelengths and particles
Ongoing work
– reduce output bandwidth by using an FPGA; goal: 1 kHz
continuous frame rate with 2x2 chip arrangement
• Test prototype tubes at the AO laboratory at CFAO, U.C.
Santa Cruz
• Final test at a telescope
SSL received 3-year NOAO grant, 1.5 more years to go…, some
funding for Univ. Geneva from Fonds National and F. Schmidheiny
• Consider other applications for such a detector
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
• Test new ceramic chip carrier (= tube backend); thermal
cycling tests with Medipix2 chip mounted
• Tube fabrication at SSL Berkeley and at commercial firm
with GaAs photo-cathode
• Test prototype parallel readout board designed in
collaboration with ESRF
Parallel Readout Board
CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
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Bettina Mikulec
Tube Design
CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
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Bettina Mikulec
Tube Design
CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
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Bettina Mikulec
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
Backup Slides!
The Setup at SSL - Photos
CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
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Bettina Mikulec
Spot Size
Rear Field = 1600V
Spot Area vs Rear Field
20
35
Gain 50k
Gain 100k
30
Spot Area (pixel)
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Mean Spot Area (pixel)
Gain 25k
Gain 200k
25
Gain 400k
20
15
10
5
16
G=20k, Area
14
G=50k, Area
12
G=100k, Area
10
G=200k, Area
8
6
4
2
0
0
200
400
600
800
1000
1200
Rear Field (V)
Spot area versus rear field.
Bettina Mikulec
1400
1600
0
0
5
10
15
20
25
30
35
40
Lower Threshold (ke -)
Spot area versus Medipix2
low threshold.
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
40
100
Cs Br
KI
80
60
40
20
0
0.1
Bettina Mikulec
Energy (keV)
1
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
Quantum Detection Efficiency (%)
Soft X-Ray Photocathodes
EUV and FUV
CsI 1985 vs 1999
0.7
0.6
0.5
QDE
0.4
0.3
0.2
0.1
0
0
500
1000
1500
2000
Wavelength (Å)
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
CsI 1985 30°
CsI 1985 20°
CsI #3 2/99 20°
CsI #3 2/99 30°
CsI #2 1/99 20°
CsI #2 1/99 30°
GaN UV Photocathodes, 1000- 4000Å
CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
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Bettina Mikulec
Isoplanatic Angle (0) & Sky Coverage
Telescope
Primary
mirror
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
h
Laser Guide Stars
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
Can achieve
>70% sky
coverage with
laser guide star
adaptive optics!
Laser Guide Star Parallax
589.2 nm
L
d
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
• “Star” more of a streak
• Shape changes over pupil
• Can use pulsed laser to limit spatial
extent
• Requires gated detector
Advantages of Multi-Pixel Sampling of Shack Hartmann Spots
5x5 algorithm error for Gaussian input
Quad cell (2x2) algorithm error for Gaussian input
0.5
=
=
=
=
=
Sigma
Sigma
Sigma
Sigma
Sigma
0.2
0.4
0.6
0.8
1.0
Calculated position
Calculated position
Sigma
Sigma
Sigma
Sigma
Sigma
0
0.2
0.4
0.6
0.8
1.0
0
-0.5
-1
-0.5
=
=
=
=
=
0
Gaussian Centroid true position
2x2
0.5
-1
-0.5
0
Gaussian Centroid true position (center pixel)
5x5
Linear response off-null
Insensitive to input width
More sensitive to readout noise
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
1
Deformable Mirrors
• Range from 13 to > 900 actuators (degrees of freedom)
CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
~ 300mm
~ 50 mm
Bettina Mikulec
Xinetics
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Position Error (550 Events/Spot)
Number of centroids
45
40
35
30
rms = 2.0 µm
25
20
15
10
5
0
-20
-15
-10
-5
0
5
10
15
20
Centroid difference (microns)
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
50
Medipix readout of semiconductor arrays
CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
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Bettina Mikulec
X-ray of Fish
(… with silicon detector)
Bettina Mikulec
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CHIPP Astro-Particle Physics Workshop, Versoix, May 3rd 2005
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.