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High Speed Wavefront Sensors for ELT Adaptive Optics

This article discusses the use of high-speed wavefront sensors in adaptive optics for Extremely Large Telescopes (ELTs), which help to compensate for the distortion caused by the Earth's atmosphere. The specifications, architecture, design, and implementation of the wavefront sensors are explored.

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High Speed Wavefront Sensors for ELT Adaptive Optics

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  1. High Speed Wavefront Sensors for ELT Adaptive Optics Mark Downing, Mark Casali, Gert Finger, Enrico Marchetti, Leander Mehrgan, Javier ReyesEuropean Southern Observatory ESO (http://www.eso.org)Denis Bourke, Ryan Cassidy, Paul Jerram, Paul Jorden, Rafael Gil Otero, Jérôme Pratlong, Wolfgang Suske, Nick SwiftTeledyne-e2v technologies (http://www.e2v.com) LVSM SDW2017

  2. Adaptive Optics (AO)- removing the twinkle of the stars 1 Wavefronts from astronomical objects are distorted by the Earth’s atmosphere, reducing the spatial resolution of large telescopes to that of a 10 cm telescope Deformable mirror compensates the distorted wavefront, achieving diffraction-limited resolution OFF 4 ON 3 2 Wavefront Sensor measures deviation of wavefront from a flat (undistorted) wave Control System computes commands for the deformable mirror(s) LVSM SDW2017

  3. Outline • ELT WFS Specifications of the ELT • LVSM Architecture and Design • LVSM Package • Cameras • Summary LVSM SDW2017

  4. ELT WFS specificationsour best knowledge; evolves with maturing of designs 0.5 mm < l < 1 mm High-Order NGS WFS HARMONI-SCAO: 1 / 160x160 px / 1kHz MICADO: 1 / 160x160 px / 1 kHz METIS-SCAO: 1 / 160x160 px / 1 kHz PFS M4/char: 1 / 800x800 px / 500 Hz PFS scalloping: 1 / 800x800 px / 500 Hz PFS LOO: 3 / 800x800 px / 500 Hz 1.3 mm < l < 2.5 mm l = 0.589 mm High-Order LGS WFS MAORY: 6 / 800x800 px / 700 Hz HARMONI-LTAO: 6 / 800x800 px / 700 Hz METIS-LTAO: 6 / 800x800 px / 700 Hz Low-Order NGS WFS MAORY: 3 / 160x160 px / 1 kHz 3 / 250x250 px / 1 kHz HARMONI-LTAO: 1 / 250x250 px / 1 kHz METIS-LTAO: 1 / 250x250 px / 1 kHz PFS fast Guiding: 3 / 200x200 px / 500 Hz PFS slow Guiding: 1 / 200x200 px / 500 Hz LVSM SDW2017

  5. Type/Number of WFS Cameras 6 (+spares) Small NIR SAPHIRA 320x256 pixels 9 (+spares) Small Visible CCD220? MICADO-SCAO METIS-SCAO HARMONI- SCAO HARMONI-LTAO METIS-LTAO PFS MAORY 23 (+spares) Large Visible LVSM LVSM SDW2017

  6. T ~ 10km Sodium layer H ~ 80km Predicted spot elongation pattern LLT Pupil plane Detector plane Distance from launch site LVSM is needed to sample the spot elongation Sodium Laser Guide Stars • Frame rate ~1 kframe/sec • require bright “guide stars” • With natural guide stars only 1% of the sky is accessible • Sodium layer at 80-90 km altitude can be stimulated by Laser to produce artificial guide stars anywhere on the sky LVSM SDW2017

  7. AOF LGS GLAO IC 4406 -> Retina Nebula planetary nebula NGC 6563 * http://www.eso.org/public/news/eso1724/ LVSM SDW2017

  8. Location of Wavefront sensors on the ELT 38 m diameter Pre-Focal Station PFS WFS arms (contain WFS detectors) 65 m high Deformable Mirror Instruments contain WFS detectors Pre-Focal Station Instruments LVSM SDW2017

  9. While the Telescope is large, the volume for cameras are modest Camera Volumes Housing 110*190*215 mm3 LVSM SDW2017

  10. LVSM Format Size • Would like 80x80 sub-apertures of 25x25 pixels to fully sample the spot elongation  2kx2k pixel LGSD, however, • to ensure devices available on-time for first light instruments, 80x80 sub-apertures of 10x10 pixels  800x800 pixel imager called the Large Visible Sensor Module, LVSM. • Studies are underway to develop techniques of how to handle truncation of the spot elongation or to use pixels more efficiently. LVSM SDW2017

  11. LVSM Floor Plan • 800x800 continuous array of 24µm pixels • 20 columns of dark reference pixels each side for tracking dark level or for row noise subtraction Highly integrated readout • All analog processing on-chip: • correlated double sampling (CDS), • programmable gain of x1/2/4/8 settable on the fly, • 9/10 bit single slope ADCs, • for total effective 12 bit data conversion • 4 rows top + bottom processed in parallel to slow the read out per pixel and beat down the noise • 14us/row = 12µs to sample the analog + 2µs ADC • 24 line at 128MHz DDR LVDS serial interface to outside world • simple easy digital interface to FPGA LVSM SDW2017

  12. Photodiode Design • No white spots - uniform intrapixel response– high QE – good PSF – low image lag • Photodiode - double-N implant: • a large square implant to guarantee uniform response and good PSF • a tapered finger implant, that provides a potential gradient towards the TG, to improve image lag • At least 50% area of reflective metal to give high QE in the “red” • Back-illuminated, high resistivity silicon >1000 ohm-cm, multi-2 AR Coating • 12 µm thick: • best trade validated in NGSD between PSF and QE     LVSM SDW2017

  13. Highly Parallelized Readout • At top and bottom, in the shadow of each pixel column, is a single row of four readout channels (each 6µm wide) for total of (4x840) 3360 channels across the device. • Clean/simple layout with good separation and no capacitive cross-coupling between analog and digital signals • Good synchronicity and low latency within a 10x10 pixel sub-aperture (< 2% of exposure time) Read out direction compatible for both Shack Hartmann and pyramid? WFS LVSM SDW2017

  14. Video Chain – single slope ADC VSS_SH VDD_SH Column bus Sample/Hold Ping Pong Latches VRST VSF Parallel to Serial Gray Code 9/10 A reset 1 select 4T pixel Pre-Amp 2 Comparator 3 shr + mux PPD x1 - + B - n+ x2 x4 x8 p+ transfer shs p-Si 4 Ramp Q Q D D LVDS Out reset Clk Clk transfer signal SN reset video Latch code ramp offset /Ramp comparator output Gray code 0 512 LVSM SDW2017

  15. LVSM Required Performance LVSM SDW2017

  16. Package – Integral Peltier to cool to ~ -30DegC Kovar body with: • Flat metal base with holes for firmly bolting (to ensure good thermal contact) to the heat exchanger to cool the hot side of the Peltier • Kovar lid with seam welded window to enable lenslet array to be glued. • Low thermal conductivity inert gas filling • Pinched-off pump tube after filling with inert gas. • Pin grid arrays each side of baseplate • Internal Peltier cooler • AD590 Temperature sensor Window Temperature Sensor Lid LVSM glued to Ceramic insert IPGA Bracket Peltier IPGA Ceramic Pump Tube Kovar Body Mating IPGA Socket Peltier pins LVSM SDW2017

  17. LISA (Large vISble cAmera) • LISA (the LVSM Camera) is just one of three WFS cameras under development at ESO, others are: • FREDA (inFraRrED cAmera) which uses the Leonardo SAPHIRA of 320x256 of 24µm pixels • ALICE (smALl vIsible CamEra) Teledyne-e2v CCD220-DD? of 240x240 of 24µm pixels ESO ELT Adaptive Optics High‐Speed Wavefront Sensors ‐ Camera and Control Electronics.Javier Reyes Wednesday 27th Sept 16:20 Wavefront Sensing and Demonstration of e‐APD Pixel Performance to Improve the Sensitivity of Large Science Focal Planes.Gert Finger Thursday 28th Sept 8:40 LVSM SDW2017

  18. ELT WFS DETECTOR Multi-phase plan to progressively retire risk 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 Design Study Design Study Retire Pixel Risks Technology Validation Retire Architecture/ Process Risks Technology Demonstrators 840x840 pixel Natural Guide Star Detector NGSD 800x800 pixel Large Visible Sensor Module LVSM 2kx2k LGSD Development Authorize Production Testing/ Acceptance Production Phase LVSM Production 28 Science Devices SDW2017 LVSM SDW2017

  19. Summary • Design of LVSM is at an advanced stage and on track to: • go into manufacture early 2018; • with front-side device available Q3/2018; • BSI characterization devices in early 2019; and, • the 28 production devices in mid-2020; • Devices will be validated both at Teledyne-e2v and ESO. • High confidence of success from using all lesson’s learnt from previous prototypes. • LISA (the LVSM Camera) will be developed in parallel at ESO. • Production of LISA will be out-sourced to industry through CfT with Cameras available early 2022 to meet first light needs of ELT instruments. LVSM SDW2017

  20. Thank You This work has been "partially funded by the OPTICON-JRA2 project of the European Commission FP6 and FP7 program, under Grant Agreement number 226604" LVSM SDW2017

  21. LVSM Physical Characteristics LVSM SDW2017

  22. LVSM Read out LVSM SDW2017

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