Transcript EUVE Measurements of Interplanetary Helium

Optically sensitive Medipix2 detector
for adaptive optics in very large
telescopes
John Vallerga, Jason McPhate, Anton Tremsin
and Oswald Siegmund
Space Sciences Laboratory, University of California, Berkeley
Bettina Mikulec and Allan Clark
University of Geneva
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Adaptive optics tutorial*
Turbulence in earth’s
atmosphere makes stars
twinkle
More importantly, turbulence
spreads out light; makes it a
blob rather than a point
*Adapted from AO lectures of Claire Max, Astro 289C, UC Santa Cruz
Even the largest ground-based astronomical
telescopes have no better resolution than a 20 cm telescope!
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Optical consequences of turbulence
• Temperature fluctuations in small patches of air cause
changes in index of refraction (like many little lenses)
• Light rays are refracted many times (by small amounts)
• When they reach telescope they are no longer parallel
• Hence rays can’t be focused to a point:
Point
 focus
Parallel light rays
 blur
Light rays affected by turbulence
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How a deformable mirror works
(idealization)
BEFORE
Incoming
Wave with
Aberration
AFTER
Deformable
Mirror
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Corrected
Wavefront
Adaptive optics increases peak
intensity of a point source
Lick
Observatory
No AO
With AO
Intensity
With AO
No AO
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Schematic of adaptive optics system
Feedback loop:
next cycle corrects
the (small) errors
of the last cycle
Optical Medipix tube goes here
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Lick adaptive optics system at 3m
Shane Telescope
DM
Wavefront
sensor
Off-axis
parabola
mirror
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IRCAL infrared camera
How to measure turbulent distortions
(one method among many)
“Shack-Hartman” WFS
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The new generation:
adaptive optics on 8-10 m telescopes
Summit of Mauna Kea volcano in Hawaii:
Subaru
2 Kecks
ESO VLT
Gemini South
Gemini North
And at other places: MMT, VLT, LBT, Gemini South
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Neptune in infra-red light (1.65 microns)
With Keck
adaptive optics
2.3 arc sec
Without adaptive optics
May 24, 1999
June 27, 1999
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VLT NAOS AO first light
Cluster NGC 3603: IR AO on 8m ground-based
telescope achieves same resolution as HST at 1/3
the wavelength
Hubble Space Telescope
WFPC2,  = 800 nm
NAOS AO on VLT
 = 2.3 microns
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Faint companions around bright stars
Two images from Palomar of a
brown dwarf companion to GL 105
Credit: David Golimowski
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End Tutorial
Vision Science
End Tutorial
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Next generation of large telescopes (proposed)
30 m diameter:
– California Extremely Large Telescope (CELT) – Thirty Meter Telescope (TMT)
50 m diameter:
– EURO50 on La Palma
100 m diameter:
– European Southern Observatory’s “OverWhelmingly Large
Telescope” (OWL)
All propose AO systems with > 5000 actuators
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WFS detector requirements
• High optical QE for dimmer guide stars
• Lots of pixels - eventually 512 x 512
• Very low readout noise
• kHz frame rates
The last three are not simultaneously
achievable with the current
generation of CCDs
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MCP Detectors at SSL Berkeley
COS FUV for Hubble (200 x 10 mm windowless)
25 mm Optical Tube
GALEX 68 mm
NUV Tube (in orbit)
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Imaging, Photon Counting Detectors
Photocathode converts photon to electron
MCP(s) amplify electron by 104 to 108
Rear field accelerates electrons to anode
Patterned anode measures charge centroid
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Photocathode type determines wavelength response
• Soft x-ray to near IR
GaAs Photocathodes (GenIII)
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Wavefront Sensor Event Rates
(example for big telescope)
• 5000 centroids
• Kilohertz feedback rates (atmospheric
timescale)
• 1000 detected events per spot for sub-pixel
centroiding

5000 x 1000 x 1000 = 5 Gigahertz
counting rate!
• Requires integrating detector
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Our concept
• An optical imaging tube
using:
– GaAs photocathode
– Microchannel plate to
amplify a single
photoelectron by 104
– Bare Medipix2 to count
these events per pixel
Photocathode
Photon
e-
Q = 104e-
Pij = Pij + 1
Window
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MCP
Medip ix2
Vacuum Tube Design
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Vacuum Tube Design
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Vacuum Tube Design
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Vacuum Tube Design
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First test detector
• Demountable detector
• Simple lab vacuum, no photocathode
• Windowless – UV sensitive
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Initial Results
It Works!
First light!
Lower gain, higher
rear field
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MCP event spot area
200V
Rear Field = 1600V
20
Mean Spot Area (pixel)
18
16
G=20k, Area
G=20k, Area
14
G=50k, Area
G=50k, Area
12
G=100k, Area
G=100k, Area
10
G=200k, Area
G=200k, Area
8
6
4
2
0
0
5
10
15
20
25
30
-
Lower
Lower Threshold
Threshold (ke
(ke )
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35
40
MCP charge cloud size
1600V rear field
160
Normalized Charge
140
200K
100K
50K
20K
120
100
80
60
40
20
0
0
20
40
60
80
100
R (microns)
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120
140
Spatial Resolution
100 µs
1s
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Group 3-2 visible
9 lp/mm = 55µm
(Nyquist limit)
Interesting tangent - sub pixel resolution
•Use single spot events and calculate centroids
•Accumulate event x,y list
•2-d histogram on finer pitch
9 lp/mm
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Interesting tangent - sub pixel resolution
• Calculate centroids of each event
• Accumulate event x,y list
• 2-d histogram on finer pitch
16 lp/mm
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Flat Field
MCP deadspots
Hexagonal multifiber
boundaries
1200 cts/bin - 500Mcps
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Flat Field (cont)
Ratio Flat1/Flat2
Histogram of Ratio
consistent with counting
statistics (2% rms)
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Future Work (3 yr. NOAO grant)
• Optimize MCP-Medipix2 interface design
• Design and build tube with Medipix2 and GaAs
• Develop parallel readout with European
collaborators
• Develop FPGA to reduce output bandwidth
– 5 million centroids/s vs. 262 million pixels/s.
• Test at AO laboratory at CFAO, U.C. Santa Cruz
• Test at telescope
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Acknowledgements
This work was funded by an AODP grant managed by
NOAO and funded by NSF
Thanks to the Medipix Collaboration:
•
Univ. of Barcelona
•
University of Napoli
•
University of Cagliari
•
NIKHEF
•
CEA
•
University of Pisa
•
CERN
•
University of Auvergne
•
University of Freiburg
•
Medical Research Council
•
University of Glasgow
•
Czech Technical University
•
Czech Academy of Sciences
•
ESRF
•
Mid-Sweden University
•
University of Erlangen-Nurnberg
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Quantum Detection Efficiency (%)
Soft X-Ray Photocathodes
100
CsBr
KI
80
60
40
20
0
0.1
Energy (keV)
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1
EUV and FUV
CsI 1985 vs 1999
0.7
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°
0.6
0.5
QDE
0.4
0.3
0.2
0.1
0
0
500
1000
1500
Wavelength (Å)
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2000
GaN UV Photocathodes, 1000- 4000Å
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Isoplanatic Angle (0)
& Sky Coverage
h
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Telescope
Primary
mirror
Can achieve
>70% sky
coverage with
laser guide star
adaptive optics!
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Laser Guide Star Parallax
• “Star” more of a streak
• Shape changes over
pupil
• Can use pulsed laser to
limit spatial extent
• Requires gated detector
589.2 nm
L
d
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Deformable mirrors come in many sizes
• Range from 13 to > 900 actuators (degrees of freedom)
~ 300mm
~ 50 mm
Xinetics
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30 m telescope capability
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