Transcript Astronomical Adaptive Optics using Multiple Laser Guide Stars Christoph Baranec Photo courtesy T. Stalcup.

Astronomical Adaptive Optics
using
Multiple Laser Guide Stars
Christoph Baranec
Photo courtesy T. Stalcup
Outline
Quick overview of Adaptive Optics (AO)
History of multiple guide star AO program in
Arizona
Description of MMT laser AO system
Review of recent results
The future of multi-beacon AO
Introduction – Basic AO
Credit: Claire Max, UCSC
Why Laser Guide Stars?
•Traditional AO system use natural guide stars (NGS)
•Severely limited in sky coverage – science targets may not be near a bright
(mV<13) guide star
•Bright laser guide stars (LGS) can be steered towards science target
•Still requires a mV<18 tilt star within 2 arc min diameter field (x100 fainter,
almost full sky coverage at this magnitude)
•Some error due to focal anisoplanatism – laser not fully sampling cylinder of
light from science target
•This can be solved with multiple beacons – doing tomography of the
turbulence on the sky
Volumetric sampling
Natural Star
Laser Spots
Turbulence
Telescope
Credit: T. Stalcup
Multi-beacon AO in Arizona
• First Light with Deformable Secondary at MMT with NGS AO system (2002)
• Development of Dynamic Refocus optics to increase return from Rayleigh
LGS (2002-2003)
• Open-loop multi-NGS wavefront sensing at 1.6 m Kuiper telescope (2003)
• Completion of multiple laser beam projector at MMT (2004)
• Open-loop ground-layer wavefront estimation with multiple LGS (2005)
• Open-loop tomographic wavefront estimation with multiple LGS (2006)
• First closed-loop adaptive optics with multiple LGS (July 2007)
Multi-LGS AO at the MMT
• Proof of concept for ground-layer adaptive optics (GLAO)
and tomographic adaptive optics (LTAO) using laser
guide stars.
• Develop a competitive LGS AO system at the MMT which
can support the current and future suite of AO
instruments.
• Lessons from experiments support the design of adaptive
optics for the 2 x 8.4 m Large Binocular Telescope and
the 25 m Giant Magellan Telescope.
RLGS Beam Projector at the MMT
• Two commercially available 15 W doubled
Nd:YAG lasers at 532 nm pulsed at 5 kHz.
• Mounted on side of telescope
• The laser beams are combined with a polarizing
beam splitter.
• A computer generated hologram creates the five
beacons, 2 arc min diameter on-sky.
Projection
optics
Fast steering
mirror
• Fast steering mirror controls beam jitter
• Projection optics mounted on the telescope axis
behind the secondary mirror
• Photometry Return:
• April ’06: 1.4x105 photoelectrons/m2/J
• Equivalent to five mV ≤ 9.6 stars
Laser box on side of telescope
Credit: T. Stalcup
Facility WFS Instrument
Facility Wavefront Sensor Instrument mounts to MMT Cassegrain focus (beneath
primary mirror cell). Mounts all current and future AO instruments to underside of WFS
instrument in the same way as with the MMT’s NGS AO system.
RLGS WFS:
•Multi-beacon 60 subaperture
Shack-Hartmann WFS on single
shuttered CCID18 detector
NGS WFS:
•108 subaperture S-H WFS
•2 arc minute searchable field.
Single Star Tilt Sensor:
•Electron Multiplying CCD,
looking into upgrading to APD’s.
•Variable beam splitter between
NGS WFS and Tilt Sensor
•Limiting magnitude mV<17 at
200 fps
Facility WFS Instrument
WFS on the MMT
LGS WFS
 460 fps
from April
2007
Wavefront Reconstruction
Wavefront reconstruction of the laser and natural guide stars
•LGS wavefront reconstruction by inversion of synthetic influence
matrix of Zernike modes on our geometry of Shack-Hartmann pattern.
•NGS wavefront reconstruction by using the same reconstructor matrix
as used in the closed-loop MMT NGS AO system. The NGS WFS is
optically the same, so we can use the same reconstructor.
•Stellar tilt measurements made from image motion off of separate tilt
camera.
Ground-layer Reconstruction
Ground-layer estimate based on combination of laser and tilt signals
Five laser wavefronts are averaged to give estimates of Zernike orders
2 through 8
Global tilt information provided from stellar tilt camera
Tomographic Reconstruction
Tomographic reconstruction assumes linear relation between LGS and
NGS wavefronts:
ai
T bi
For each ith set of simultaneous wavefront measurements, âi is an
estimate of NGS zernike coefficients, bi are the measured LGS zernike
coefficients and tilt measurements of field stars, and T is the
tomographic reconstructor.
We derive T by a direct inversion of the data using singular value
decomposition (SVD). T is given by:
T
AB
1
Where B is constructed from ~3000 data vectors b, and inverted with
SVD to give B-1. A is constructed from the corresponding vectors a.
Open-loop Tomographic
Reconstruction
NGS
Tilt Sensor
LGS
NGS
Tomographic
Estimate
Individual
Beacons
Camera Data:
Reconstructed
Wavefronts:
GLAO vs. LTAO
Example of GLAO vs. Tomography in tracking Zernike mode
Defocus
(Dashed Blue) NGS ground truth, (Sold Black) LGS Estimate
April 2006 Results
June 2005
April 2006
Uncorrected: 511 nm
GLAO Residual: 360 nm
Tomographic Residual: 259 nm (single tilt star)
Tomographic Residual: 243 nm (3 field tilt stars)
Uncorrected: 395 nm
GLAO Residual: 277 nm
Tomographic Residual: 157 nm (single tilt star)
*Fitting error on LGS WFS geometry: ~130 nm
Simulated PSFs
Simulated PSFs
from open-loop
LGS data – June
2005
GLAO
Uncorrected
FWHM
(arcsec)
LTAO
Band
Uncorrected
GLAO
LTAO
Diffraction Limit
J
0.77
0.38
0.11
0.04
H
0.68
0.17
0.09
0.05
K
0.55
0.13
0.09
0.07
Data from April ’06
and April ‘07,
suggest actual
performance will
be better. (faster
frame rate, better
optics, higher
return)
Progress earlier this year
•December 27th – January 1st
•Our first attempt at closed loop
operation
•Lost 4 days due to snow and ice,
2 half nights due to DM
contamination, 3+” seeing
•Alignment of all WFS and DM
•Software problems with
reconstructor computer
•March 29th – April 2nd
•New PC based reconstructor computer, modified version of NGS reconstructor
•Full system running
•Closed tip/tilt loop around a natural star!
•Closed AO loop around lasers – loop unstable, due to bad zero centroid offsets in WFS
and excessive beam jitter
Closed-loop test stand work
•Closed-loop tests off
of telescope with
deformable F/15
secondary and LGS
WFS
F/15 secondary
LGS WFS
•Verify reconstructor
computer and optical
feedback
•Found errors and
fixed them!!!
Closed AO loop on test stand, orders 1-8, 200 Hz for 15+ minutes
Results from July 2007
Out of 4 nights, only about 4 useable hours over two nights:
On the second night:
•Closed tilt loop again - then clouds
Beginning last night:
•Closed loop on focus signal from lasers at 200 Hz!
Seeing was terrible r0 ~ 8.0 cm (at λ = 500 nm)
Some recorded telemetry lost (every other frame, beginning and end of
data sets) but we have proof...
July 2007 results
Astigmatism
Open loop = 341 nm
Focus
Closed loop = 130 nm
Closed-loop PS
Immediate future Work
•September/December 2007:
•Close the full high order GLAO loop
• Image clusters on PISCES (2’ FOV, NIR, 0.11”/pixel) – evaluate
corrected PSF with λ, field, time – calculate sensitivity
•Evaluate sensitivity of tilt camera, correction versus tilt star magnitude
•Do calibrations to prepare for LTAO operation: add static shapes to
secondary and measure response of each beacon.
Future Plans at MMT
• Initiate transition to regular science observing with one
shared risk run per trimester. Using science instruments
PISCES, Clio, ARIES and BLINC-MIRAC.
• Test LTAO operation in early of 2008
• Planned system upgrades
– Increase laser power by adding 3 more lasers: one per beacon.
At the same time, add 4 more WFS cameras. Will extend
wavelength coverage down to 1 micron.
– Implement a variable radius beacon constellation to optimize for
GLAO or diffraction limited correction.
– New GLAO optimized NIR camera
Loki: MMT GLAO Camera
Designed for deep,
wide field imaging
•4K x 4K pixels with
2 x 2 mosaic of
JWST NIRCam
detectors
•4 arc minute, 0.06” /
pixel, perfect for
GLAO field
•Achromatic, all
spherical optics
Science:
Star forming regions
High redshift galaxies
Loki optical design
Large Binocular Telescope
Design of multi-LGS AO system at 2 x 8.4m LBT
•Investigating Multi-LGS AO for the LBT
•Design of Laser Beam projector
•Design of Multiple RLGS WFS using
Dynamic refocus technology
•Want to enable GLAO and tomographic AO
•Possibly a hybrid system where single
Sodium and multiple Rayleigh LGS used
•Encouraged by our results at MMT that this
is a viable solution.
Image courtesy John Hill
Dynamic Design
refocus
and LGS WFS
of LBT Dynamic refocus
4 x pickoff mirrors
/ dichroic
Rotating
turn mirror
Tertiary
Giant Magellan Telescope
Design of AO system for the 25m GMT
•Relies heavily on MMT
experience
•Multi-Sodium LGS
•High-order adaptive
secondary
•Allowing LTAO and
GLAO correction with
clean thermal
background
•Expansion to ExAO and
MOAO in 2nd generation
Conclusions
Demonstrated open-loop ground-layer and
tomographic AO
First multi-laser system working in closed-loop
with support for science very soon
Have sights set on putting multiple lasers on bigger
telescopes
LASER AO TEAM
LGS AO TEAM
PI: Michael-Lloyd Hart
Tom Stalcup
Mark Milton
Miguel Snyder
Keith Powell
Vidhya Vaitheeswaran
http://caao.as.arizona.edu
Matt Rademacher
Ground-layer AO with NGSs
Open-loop GLAO wavefront
sensing and estimation with
NGSs in 2003
•GLAO - Average of
wavefront from 3 stars used
to correct 4th star
•1.6 m aperture telescope
•d = 30 cm
•d/r0 = 1.3 (at λ = 1.25 µm)
DSS archive image
WFS image
Ground-layer AO with NGSs
•Created synthetic
PSFs with additional
high order unsensed
modes
Predicted GLAO PSFs from open-loop data
• Calculated FWHM
and encircled energy
improvement
1
4
3
2
3 surrounding
stars used to
correct central star