spie instrumentation for astronomy ao - june22, 2004 john vallerga, jason mcphate, anton tremsin and...

SPIE Instrumentation for Astronomy AO - June22, 2004

John Vallerga, Jason McPhate, Anton Tremsin and Oswald Siegmund

Space Sciences Laboratory, University of California, Berkeley

Bettina Mikulec and Allan Clark

University of Geneva

A noiseless 512 x 512 detector for AO with kHz

frame rates


Future WFS Requirements*

• High (~80%) optical QE

• Lots of pixels - eventually 512x512

• Very low readout noise (< 3 e-)

• kHz frame rates

The last three are not simultaneously achievable with the current generation of CCDs

*Angel et al “A Road Map for the Development of Astronomical AO”


Imaging, Photon Counting Detectors

• Detects individual quanta of light via photoelectric effect

• Microchannel plate amplifies single electron to large charge cloud

• Signal per photon >> noise

• Readout gives X,Y of every event

• Time of every event also available

Charge distribution on stripsCharge CloudMCP stackTube Window withphotocathodeγ


Microchannel Plates

2 µm pores on 3 µm centers (Burle Industries)


MCP Detectors at Berkeley

85 mm

10 mm

400 pxl

14000 pxl

COS FUV for Hubble (2004,2005,???)

25 mm Optical Tube

68 mm

GALEX NUV Tube (in orbit)


GaAs Photocathodes (GenIII)

• Developed for night vision tubes

• Slight cooling required (104 cps at room temp)

• Only fabricated in USA and Japan


Advantages of multi-pixel sampling of Shack Hartman spots

Linear response off-nullInsensitive to input widthMore sensitive to readout noise

2 x 2 5 x 5


Wavefront Sensor Photon Rate

• Future large telescopes need > 5000 actuators

• Kilohertz feedback rates

• 1000 detected events per spot for sub-pixel centroiding

5000 x 1000 x 1000 5 Gigahertz counting rate!

• 104 time faster than existing photon counting imagers– Requires integrating readout


Our detector concept

An optical imaging tube using:

– GaAs photocathode

– Microchannel plate to amplify a single photoelectron by 104

– ASIC to count these events per pixel


Medipix2 ASIC Readout

Pixellated readout for x and gamma ray semiconductor sensors (Si, GaAs, CdTe etc)

Developed at CERN for Medipix collaboration

55 µm pixel @ 256x256 (buttable to 512 x 512).

Pixel level amp, discriminator, gate & counter.

Counts integrated at pixel No charge transfer!

14mm

16mm

Applications: Mammography, dental radiography, dynamic autoradiography, gamma imaging, neutron imaging, angiography, xray diffraction, dynamic defectoscopy, etc.


Single Medipix2 pixel

Input

Preamp

Disc.

Disc. logic Mux. 13 bit

counter –ShiftRegister

Clock out

Shutter

Lower Thresh.

Disc.

Mux.

Previous Pixel

Mask bit

Analog Digital

Upper Thresh.

Next Pixel

Mask bit

Polarity

Each 55µm Pixel has ~ 500 transistors using 0.25µm CMOS technology


Readout Architecture33

28 b

it P

ixel

Col

umn

0

3328

bit

Pix

el C

olum

n 25

5

3328

bit

Pix

el C

olum

n 1

256 bit fast shift register

32 bit CMOS output LVDS out

• Pixel values are digital (13 bit)

• Bits are shifted into fast shift register

• Choice of serial or 32 bit parallel output

• Maximum designed bandwidth is 100MHz

• Corresponds to 266µs frame readout


First test detector

• Demountable detector

• Simple lab vacuum, no photocathode

• UV sensitive


Initial Results

It Works!

First light! Lower gain, higher rear field


Spatial Resolution

100 µs 1 s Group 3-2 visible 9 lp/mm = 55µm

(Nyquist limit)


Flat Field

1200 cts/bin - 500Mcps

MCP deadspots

Hexagonal multifiber boundaries


Flat Field (cont)

Histogram of Ratio consistent with counting

statistics (2% rms)Ratio Flat1/Flat2


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


Acknowledgements

• Univ. of Barcelona

• University of Cagliari

• CEA

• CERN

• University of Freiburg

• University of Glasgow

• Czech Academy of Sciences

• Mid-Sweden University

• University of Napoli

• NIKHEF

• University of Pisa

• University of Auvergne

• Medical Research Council

• Czech Technical University

• ESRF

• University of Erlangen-Nurnberg

Thanks to the Medipix Collaboration:

This work was funded by an AODP grant managed by NOAO and funded by NSF

spie instrumentation for astronomy ao - june22, 2004 john vallerga, jason mcphate, anton tremsin and...

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