Adaptive optics for SOAR
Andrei Tokovinin (CTIO)
Do we need AO?
What can and can not be done with AO?
System concept and performance
Science case for AO at SOAR
The case for high angular resolution is evident!
See images with 0.3" and 0.7" resolution.
Improved seeing in a large (3') FOV
Rival 8 m telescopes in background-limited sensitivity
Best ground-based sensitivity in crowded fields
Diffraction limit, small (10") FOV
Optical: UNIQUE capability, 0.03" resolution
IR: lose to Gemini 2 times in resolution, 4 times in sensitivity, 16 times
in exposure time to reach given S/N.
Seeing at Cerro Pachon
Source: Gemini RPT-AO-G0094
r0 at zenith at 0.5 micron
- Median: 15 cm (FWHM 0.67")
- Good (25%): 20 cm (FWHM 0.50")
- 75%: 10.7 cm (FWHM 0.94")
Finite turbulence outer scale
- Less tip-tilt (compared to standard theory)
- Better uncompensated seeing
- More improvement in the IR
- Less improvement from tip-tilt
Revised "isokinetic patch"
4 m/ 16 km = 51"
Mean turbulence profile at Cerro Pachon (1998, 22 nights).
Seeing improvement with AO
Improve wave-front coherence at r0 scale .
Idea 1: Low-order compensation =
incoherent combination of 1 m
sub-apertures. Example: CFHT+PUEO. Gain in FWHM: ~2 times. Small FOV.
Idea 2: Compensate only low layers.
Gain in FWHM: ~2 times. Large FOV.
PSFs under good seeing at 0.67 micron:
- Several NGSs and tomography (but: not enough photons)
- Rayleigh LGS: good, insensitive to high layers!
Full line: uncompensated (leftmost)
and with tip-tilt.
Dashed line: AO+LGS at 10 km.
Improve wave-front coherence across full aperture.
- Need NGS -> small sky coverage (but: LMC, Galactic plane!)
- Small compensated FOV
PSFs under good seeing at 0.67 micron with 12m NGS. The leftmost
PSF is for uncompensated image under same conditions.
Strehl ratio on-axis and at 4" off-axis versus NGS R-magnitude. Good seeing,
PUEO: 1-m sub-apertures -> 15m
SOAR: 0.5-m sub-apertures -> 13m?
Comparing tip-tilt, seeing improvement
and full AO
- Compensation changes the PSF structure: becomes more peaked
- Maximum gain in Strehl and FWHM
- Lower gain in encircled energy and detectivity
FWHM on-axis, median seeing
Good match between seeing-improved resolution in the visible and
seeing limit in the IR.
FWHM on-axis, good seeing
Performance: Central intensity = Strehl
Performance: Encircled energy
- Small (~1 m size) AO module on the optical ISB
- DM: OKO Tech, 35 mm beam diameter, 70-120 actuators
- WFS: Shack-Hartmann, curvature or pyramid (TBD)
- LGS: Rayleigh, solid-state UV laser, 10-20 km altitude
- TTS: three sensors, to be used with LGS
- Three scientific instruments, switchable
- Imager: 2K CCD, red-optimized, pixels 100 and 15 mas
- IFU spectrometer: 2 plate scales
- Third instrument: TBD, reserved for future
2002 - NOAO internal (design study)
2003-2005: NOAO+SOAR? $500K?
Summary: We want to design a USEFUL system which combines unique
high-resolution capability in the optical with seeing improvement
It must be cheap!