for Adaptive Optics CCD based curvature wavefront sensorrdorn/papers/ccdcws.pdf · 2004. 9. 20. · Property APDs CCD Peak quantum efficiency 70% 80%* Dark current [electrons/s] 250

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CCD based curvature wavefront sensorfor Adaptive Optics

Reinhold J. Dorn

European Southern Observatory - Instrumentation Division

7/2/01 2

Reinhold Dorn A CCD based curvature wavefront sensor

Contents

• A simple AO-system and basic requirements for the wavefront sensor• Wavefront sensing - Shack Hartmann and Curvature• Implementation of curvature wavefront sensing• CCD design• CCD performance - Simulation results (versus APDs)• Prototype laboratory system• Current results with the frontside CCD in the lab• Implementation with the MACAO system (what needs to be done?)• Conclusions and outlook

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• Most precious signal is the signaldetected by the wavefront sensor.

=> Primary limit of performance is photon noise of the signal at the wavefront sensor.

• Atmospheric distortions changeson timescales of 10 to 30 msec.

=> very short exposures needed (1 msec)

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Basic requirements for wavefront sensors

• High Quantum efficiency• Low readout noise• Ability to take very short exposures• Fast readout must be possible• Minimum phase lag (minimum delay)

Limit of the wavefront sensor should only be photon noise!

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Curvature sensing (1)

• A flat wavefront focused bya lens, showing the

intrafocal and extrafocal image planes on either side

of the focal plane.

• Propagation of a flat buttilted wavefront and theresulting curvature signal.

• Grey is a curvature signalof zero, white is positiveand black is negative.

• The dashed line showsthe outline of the pupil.

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Curvature sensing (2)• The change in intensity from intrafocal to extrafocal planes is due to the curvature of the wavefront over

the pupil and the tilt of the wavefront at the pupil edge.• The usefulness of this signal for adaptive optics was first recognized by Francois Roddier (1987) and he

first presented a system concept for wavefront sensing (Roddier, 1988).• Using the vector r for the location x,y in a z-plane, I1, I2 for the intrafocal and extrafocal images, and l for

the intra- and extrafocal distances, Roddier used the geometrical optics approximation to derive thecurvature signal as,

• where f is the telescope focal length, is the derivative in the outward pointing radial direction and is alinear impulse function at the edge of the pupil (both at the outer edge of the primary and the inner edgedue to secondary obscuration).

• The radial wavefront tilts provide the boundary conditions necessary to determine the phase from thecurvature signal. Normally, in an operating curvature AO system, the phase is never computed as thePoisson equation is solved automatically by the bimorph mirror response function. Since tilt is onlysensed at the pupil edge and the curvature signal is sensed over the entire pupil, this approach of sensingwavefront distortions is called curvature wavefront sensing.

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Curvature sensing - Computer simulation of curvature wavefront sensing

• AO loop open:

(a) wavefront distortion,

(b) intrafocal image,

(c) extrafocal image

(d) curvature signal athigh resolution,

(e) curvature signalbinned into 60subapertures.

• Simulation parameters: 0.66 arc sec seeing (at 500 nm), sensingwavelength = 700 nm (monochromatic), infrared image wavelength= 2.2 µm, out of focus distance = 25 cm, telescope focal length = 400m, telescope diameter = 8 m with 14% obscuration from 1.12 mdiameter secondary. Photon noise has not been simulated – allsignals are “infinite” light level.

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Implementation - AO curvature wavefront sensor

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Membrane function

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Time [ms]

Foc

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ngth

of m

embr

ane

[m]

Membrane position

Focal length

Intrafocal plane

Extrafocal plane

0.25 0.5 0.75 1

• Membrane amplitude (dashed line) and membrane focal length (solid line) as a function of time for a 2 kHzmembrane frequency with the minimum focal length set to 25 cm. Typical parameters for a membrane are: d = 10mm, fmin = 25 cm, A = 100 µm. It is amazing what an oscillation of a small mirror by the width of a human hair cando!

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Subaperature geometry

• Subaperture geometry for ESO's 60 element curvature AO systems

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CCD design - design considerations• 60 integration areas• Very short exposure times (250 µsec) with “long” integration times (1 to 20 msec, 2

to 40 membrane cycles)• Ability to switch between half-cycle integrations within 10 µses• Lowest possible noise: 2 electrons maximum, < 1 electron desired (including all

sources – dark current, readout noise, etc.)• Ability to store half-cycle frames on-chip while integrating other half-cycle frames.

The key to making a CCD work in this application is on-chip integration. In orderto fulfill the requirements, we have utilized the following design options:

• Use superpixels, i.e. bin on-chip, to loosen alignment tolerances• Layout pixels on a grid to lower risk (keep it simple!), using fibers to feed from the

lenslet array to the CCD, as is done with APD modules• Use multiple readout ports to have slower readout rates and lower readout noise.

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CCD design -unit cell

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7/2/01 15


CCD design -CCD array

• Curvature wavefrontsensor array.

• The design consists of 80unit cells. Ten unit cellsare combined into a unitcolumn.

• Each of these unitcolumns has an amplifierat the "bottom" end ofthe serial register.

• One the right side of thedevice is the tip/tiltsensor.

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ESO / MIT/LL CCID-35

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Expected QE for the thinned CCD (versus APD)

• High Quantum efficiency (peak > 90%)

+ the ability to store charge while reading and integrating

• Thinned versions of the CCD are expected by the end of 2001


Measured quantum efficiency of an EG&G APD module

0

10

20

30

40

50

60

70

80

90

100

300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100

Wavelength [nm]

QE

[%]

Expected quantum efficiency of the CCID-35 for curvature wavefront sensing

0

10

20

30

40

50

60

70

80

90

100

300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100

Wavelength [nm]

QE

[%]

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System performance - Simulation results

Strehl ratio in K-band for the different detectors. Specifications of the detectors.

Property APDs CCDPeak quantum efficiency 70% 80%*Dark current [electrons/s] 250 0Read-out noise [electrons RMS] 0 2Read-out delay [µs] 0 250

* includes light loss in the additional relay optics required for the CCD.

Measuring the integrated Strehlratio in K-band achieved by the

system for guide stars ofmagnitude 10 to 18, these curves

are obtained.

The difference in performancebetween APDs and the CCD is

only significant at magnitude 15and 16 where the APDs performbetter than the CCD by five andtwo percent Strehl respectively.

For all other magnitudes, thedifference amounts to less than

0.6 percent Strehl.

667k 265k 106k 42.1k 16.7k 6670 2650 1060 421

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Photon counts per subaperture per second

Str

ehl r

atio

10 11 12 13 14 15 16 17 180

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Guide star magnitude

Str

ehl r

atio

APDs with 250 dark current counts per second

CCD with 2e read−out noise and 0.25 ms read−out delay

1


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Lenslet array

CCID-35

CryostatWindow

MIrror

Reflecting surface

Reflecting surface

Fiber input

Fiber bundle

Laboratory system design - Relay optics

• An Offner Relay designconsisting of two spherical,reflecting surfaces will be usedto re-image the light of thefibers 1:1 onto the superpixels

•Optics has a maximum blur of60 microns (100% encircledenergy)

•Fiber is comparable to a 3arcsec field of view in the VLTcurvature AO system design


7/2/01 20

Laboratory system design - Cryostat

Cryostat design by J.L. Lizon

•The CCD must be cooled to a temperature of 192 Kelvin or- 81oC to achieve a dark current of 0.25 electrons per superpixel

at 50 Hz frame rate.

•Thus liquid nitrogen and a cryostat is used to cool the CCD.


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Laboratory system design - Detector board

• Detector board important for

low noise performance

• No connectors inside the

cryostat

• No active electronics inside

the cryostat


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Laboratory system design - Fiber Feed and Simulation setup


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Laboratory system design - Picture of the lab system


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Results - CCD readout modes


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Results - CCD readout modes


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Results - CCD readout noise performance


Drain voltage versus readnoise

0

1

2

3

4

5

6

7

8

18 18.5 19 19.5 20 20.5 21 21.5 22 22.5

drain volttage (volts)

read

nois

e (e

lect

rons

)

readnoise ch1 readnoise ch2 readnoise ch3 readnoise ch4

Readout noise < 1.5 electrons at 4000 frames per second

Conversion factor 0.25 electrons/ADU

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Results - Photon Transfer Curve


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Results - moving small charge packages


Single exposure ~ 1 electron/pixel (readout noise 1.3 electrons @ 50 kps)

Sum of 256 exposures and then normalized

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Results - CCD Linearity


Linearity curve up tofull well @ 20000 electrons) Residual non Linearity curve

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From a prototype system to a science system

• Evaluation of the thinned versions of the CCD. (Difference should only be higher QE for the thinned CCDs!)• Requires new PCB inside the Cryostat.• FIERA DSP/control software adaptation.• Synchronization of the readout modes with the membrane

frequency. (Hardware already foreseen in the new FIERA PCI - SLCU).• Interface to MACAO OS.

• Timing board /RTC interface board +software.

7/2/01 31


Conclusions

• This CCD achieves nearly the same performance as APDs and thinnedversions have the potential to work as well as APDs with reduced costand reduced complexity (no neutral density filters needed, simpler).

• CCD has a higher quantum efficiency than APDs and greater dynamicrange than APDs (factor 1000).

• A readout noise of less than 1.5 electrons at 4000 frames per second hasbeen demonstrated.

• This research showed that detectors can be successfully designed forspecial purposes (instruments).

• Special lenslet array that images the pupil image directly onto the gridstructure of the CCD is under investigation (hence no light loss due torelay optics or fibers).

Documents

for Adaptive Optics CCD based curvature wavefront sensorrdorn/papers/ccdcws.pdf · 2004. 9. 20. · Property APDs CCD Peak quantum efficiency 70% 80%* Dark current [electrons/s] 250