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• 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 MASSA 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