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• Located at Cerro Pachon, Chile• Optimized for high angular

resolution• First light: April 2003

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!

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

Problems:

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

Ground layer compensation

Rayleigh LGS is better than sodium LGS for ground-layer turbulence sensing

Science case

Resolution: 0.3” and 0.7”

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

Performance 2. High resolution

Good seeing, 660 nm, R=12 NGS

Good seeing, 660 nm

Performance 3. Low-resolution

Stacked PSFs (good seeing, 660 nm)

Tip-tilt

AO with LGS

Performance 4. Summary

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

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

Dedicated science instruments

Instrument Format Pixel size,arcsec

Field,arcsec

CCD, High resol.

2048x2048

0.015 30x30

CCD,Low resol.

2048x2048

0.077 158x158

IFU spectrogra

ph

50x26 0.015 and 0.1

0.75x0.39 and 5x2.6

Deformable mirror• Small

electrostatic (OkoTech)• 35 mm pupil • 70 actuators• Enough stroke

for 1” seeing• Biased, R=25 m• DM-37 studied

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)

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

Spot diagrams (LR mode)

Layout

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

Conclusions

• Astronomy-driven AO for SOAR• Cheap AO system• Visible-light AO• Improved seeing with Rayleigh

LGS: test-bed for larger telescopes