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OSA Ground Layer AO

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									                              The University of Arizona, Steward Observatory,
                                 Center for Astronomical Adaptive Optics




                                         Astronomical Adaptive Optics
                                                     using
                                          Multiple Laser Guide Stars

                                                          Christoph Baranec




                                                       Tucson - August 7th, 2007
Image courtesy Gabor Furesz
                  Outline


History of multiple guide star AO program in
                   Arizona

   Description of MMT laser AO system

             Review of results

      The future of multi-beacon AO
              Multi-beacon WFS 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)
              Ground-layer AO with NGSs

Open-loop GLAO wavefront
sensing and estimation with
NGSs in 2003


•GLAO - Average of             DSS archive image   WFS image

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)
               Ground-layer AO with NGSs

                        Predicted GLAO PSFs from open-loop data
•Created synthetic
PSFs with additional
high order unsensed
modes

• Calculated FWHM
and encircled energy
improvement
                                                            1
                                                            4
                                                            3
                                                            2

 3 surrounding
 stars used to
 correct central star
            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 LBT and the 25 m GMT.
                    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
                                                       Fast steering   Projection
                                                       mirror          optics
• The laser beams are combined with a polarizing
  beam splitter.

• A computer generated hologram creates the five
  beacons, 2 arc min diameter on-sky.

• 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         Laser box on side of telescope
   • Equivalent to five mV ≤ 9.6 stars
                                                                          Image courtesy Gabor Furesz
                              Facility WFS Instrument

Facility Wavefront Sensor Instrument mounts to MMT Cassegrain focus. 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 Shack-Hartmann
 WFS on single shuttered CCID18
 detector
 NGS WFS:
 •Clone of current MMT NGS AO
 system.
 •Field steering mirror to give
 larger 2 arcminute diameter field.
 Single Star Tilt Sensor:
 •Electron Multiplying CCD,
 looking into upgrading to APD’s.
 •Same 2 arcminute field as NGS
 WFS
 •Variable beam splitter between
 NGS WFS and Tilt Sensor
 •Limiting magnitude V<17 at 200
 fps
LGS WFS


          LGS WFS
           200 fps
          from April
             2006
              -
          Now able
          to run at
           460 fps
               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:
                                            1
                          T        AB
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




Camera Data:




Reconstructed
Wavefronts:




                NGS     Tomographic    Individual
                          Estimate      Beacons
                  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                                 Uncorrected: 395 nm
GLAO Residual: 360 nm                               GLAO Residual: 277 nm
Tomographic Residual: 259 nm (single tilt star)     Tomographic Residual: 157 nm (single tilt star)
Tomographic Residual: 243 nm (3 field tilt stars)
                                                    *Fitting error on LGS WFS geometry: ~130 nm
                              Simulated PSFs

                                                                      Simulated PSFs
                                                                      from open-loop
                                                                      LGS data – June
                                                                      2005
                                                                      Data from April ’06
                                                                      and April ‘07,
                                                                      suggest actual
                                                                      performance will
    Uncorrected              GLAO                   LTAO              be better. (faster
                                                                      frame rate, better
            Band   Uncorrected   GLAO      LTAO   Diffraction Limit
                                                                      optics, higher
 FWHM         J       0.77          0.38   0.11         0.04
(arcsec)      H       0.68          0.17   0.09         0.05
                                                                      return)
              K       0.55          0.13   0.09         0.07
                       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     F/15 secondary            LGS WFS
of telescope with
deformable F/15
secondary and 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...
             Closed-loop Laser Modes

Loop closed at frame ~ 2200




          Focus               Astigmatism (45)
Open loop RMS = 276 nm
                               RMS = 315 nm
Closed loop RMS = 135 nm
 Closed-loop Power Spectra

Focus              Astigmatism (45)
                  Immediate future Work

•October/November 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               Loki optical design
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
                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   of LBT Dynamic refocus
and LGS WFS


                               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
                    Thank you!!!

  Without you, none of this would have been possible:

  Michael Lloyd-Hart, Roger Angel, Mark Milton, Tom
Stalcup, Miguel Snyder, Jamie Georges, Don McCarthy,
Matt Rademacher, Vidhya Vaitheeswaran, John Codona,
Phil Hinz, Michael Meyer, Jim Wyant, Craig Kulesa, Dan
 Cox, Manny Montoya, Keith Powell, Steve Moore, Will
Bronson, Chris Johnson, Mike Alegria, Alejandra Milone,
       John McAffe, Kim Chapman, Doris Tucker

                  and many more...
Following are optional
                  Beam Jitter Stabilization

•Excess beam jitter can corrupt signal in
laser wavefront sensor (LGS WFS)

•Jitter caused by rotational mode of
projector – Secondary hub not originally
designed for stiffness in rotation

•Tried feedback from LGS WFS –
(Gemini N. does this) too slow

•New control, measured by
accelerometers, corrected by fast           Image of star through the
steering mirror at 1kHz:                     laser projection optics,
                                              with and without jitter
                                                     control
Introduction – Basic AO




                          Credit:
                          Claire Max, UCSC
                    Why Laser Guide Stars?

•Traditional AO system use natural guide stars
 •Severely limited in sky coverage – science targets may not be near a bright
 (V<13) guide star



•Laser guide stars can be steered towards science target
 •Still requires a V<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

								
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