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A visible-light AO system for the 4.2 m SOAR telescope

A visible-light AO system for the 4.2 m SOAR telescope. A. Tokovinin, B. Gregory, H. E. Schwarz, V. Terebizh, S. Thomas. Outline of the talk…. Case for visible-light AO at SOAR Performance estimates System concept. SOAR telescope. Built and operated by a consortium

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A visible-light AO system for the 4.2 m SOAR telescope

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  1. A visible-light AO system for the 4.2 m SOAR telescope A. Tokovinin, B. Gregory, H. E. Schwarz, V. Terebizh, S. Thomas

  2. Outline of the talk… • Case for visible-light AO at SOAR • Performance estimates • System concept

  3. SOAR telescope • Built and operated by a consortium • Located at Cerro Pachon, Chile • Optimized for high angular resolution • First light: April 2003

  4. Drivers for visible-light AO SOAR should complement 8-m Gemini (IR-optimized) and 4-m Blanco (wide field): high angular resolution in the visible is required! Problems: • Lack of bright guide stars for AO • Small isoplanatic field and cone effect • Competition with Hubble Space Telescope • Competition with Gemini, VLT in the IR

  5. Concept for SOAR AO • High-resolution mode: NGS up to 12 mag., small field, diffraction-limited resolution, 3-D spectroscopy • Low-resolution mode: ground layer compensation (improved seeing) with Rayleigh LGS, 3 arcmin. field, 100% sky coverage

  6. Ground layer compensation Rayleigh LGS is better than sodium LGS for ground-layer turbulence sensing

  7. Sciencecase

  8. Resolution: 0.3” and 0.7”

  9. Performance 1. Seeing at Pachon • Median seeing: 0.67” (r0=15cm at 500nm) • Good seeing: 0.50” (r0=20cm) • Outer scale 25m • Average profile (65% near the ground) • >25000 profiles at CTIO with MASS A good night: June 20, 2002

  10. Performance 2. High resolution Good seeing, 660 nm Good seeing, 660 nm, R=12 NGS

  11. Performance 3. Low-resolution AO with LGS Tip-tilt Stacked PSFs (good seeing, 660 nm)

  12. Performance 4.Summary • FWHM vs. wavelength: median and good seeing • More gain for favorable turbulence profiles!

  13. AO instrument concept • Compensation order 10 (40-cm sub-aperture size) • Dedicated science instruments (not adaptive secondary) • Small Deformable Mirror (DM) • Shack-Hartmann WFS • Compact refractive optics • UV laser

  14. Dedicated science instruments

  15. Deformable mirror • Small electrostatic (OkoTech) • 35 mm pupil • 70 actuators • Enough stroke for 1” seeing • Biased, R=25 m • DM-37 studied

  16. Wave-front sensor • Shack-Hartmann type • 10x10 format (8 pixels per sub-aperture) • CCD-39 from E2V corp. most likely • No offsets resp. to science instruments • 4 TTS for LGS (APD-based)

  17. Optical design • Refractive design (cheap, compact) • Field lens, collimator, DM, camera • Two cameras: low and high resolution • Low Res.: FWHM <0.1” over 3 arcmin. • High Res.: diffraction-limited • Wavelength range 0.4-1 micron • Transmission at 355 nm 0.74

  18. Spot diagrams (LR mode)

  19. Layout

  20. Laser Guide Star • Solid-state Nd:YAG laser, 355 nm • Power from 1 to 8 W • Focused at 10 km, range gate 1 km • Flux 400-3000 photons per sub-aperture per millisecond • Small launch telescope behind the SOAR secondary • No danger to airplanes and satellites • Tip-tilt on 2-4 stars to 18-19 mag, 100% sky coverage

  21. Conclusions • Astronomy-driven AO for SOAR • Cheap AO system • Visible-light AO • Improved seeing with Rayleigh LGS: test-bed for larger telescopes

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