Instrumentation Concepts Groundbased Optical Telescopes

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Instrumentation ConceptsGround-based Optical
Telescopes

• Keith Taylor
• (IAG/USP)
• Aug-Nov, 2008

Aug-Sep, 2008
IAG-USP (Keith Taylor)
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Adaptive Optics

• Methods
• (appreciative thanks to Durham University)

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Review just to give you the flavour(I will be
showing you some slides that I barely understand!)

• Definitions and introduction
• Atmospheric turbulence
• Components of an AO system
• Wavefront Sensing
• Wavefront Correction
• Turbulence Conjugation
• Laser Beacons
• AO Modelling

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Adaptive Optics (AO)Real-time correction of
wavefront distortion

• The diffraction limit of an 10m telescope in in
  the visible is approximately 0.01 FWHM
• At the very best astronomical sites in the world,
  youll very rarely see images much better than
  0.4 FWHM.
• Why?!?
• Atmospheric turbulence distorts stellar
  wavefronts
• Turbulence results in blurred images
• Two solutions
• Put your telescope in space
• Limited to a small mirror
• Correct for the atmospheric distortion
• ADAPTIVE OPTICS!

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Strehl ratio
Corrected0.20 FWHM
Uncorrected0.49 FWHM
MARTINIWHT, K-band

• There are two components of the PSF for ?2 lt? 2
  radians2
• So width of the image is not a useful
  parameter, use height of PSF
• Strehl ratio
• For small ?2 R exp (-?2)

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Isoplanatic angle, temporal variation

• Angle over which wavefront distortions are
  essentially the same

• It is possible to perform a similar turbulence
  weighted integral of transverse wind speed in
  order to derive an effective wind speed and
  approximate timescale of seeing
• t0 is the characteristic timescale of turbulence
• Note the importance of Cn2(h) in both cases

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Atmospheric Seeing - Summary

• Dependence on Wavelength

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Components of an AO System
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High order AO architecture

• Wavefront controller
• Typically a deformable mirror (DM)
• May not be optically conjugate to an image of the
  primary
• Wavefront sensor (WFS)
• Shack Hartmann (WFS) or Curvature Sensor (CS)
• Beamsplitter
• Dichroic, multi-dichroic, intensity, spatial or
  combination
• Controller
• Typically multi-processor or multi-DSP
• Interfaces
• Can be complex and include removal of non-common
  path errors to science instrumentation (hence an
  interface to science data path)
• Laser beacons
• Multi-conjugate AO many beacons, DMs

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Astronomical Adaptive Optics
Correcting the fluctuating aberrations caused by
atmospheric turbulence above ground-based optical
and near-infrared telescopes.
Corrected Image
Uncorrected image
Corrected wavefront
Uncorrected wavefront
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Wavefront Sensing
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Wavefront Sensing

• Types of Adaptive Optics Wavefront Sensor (WFS)
• Shack-Hartmann WFS
• Curvature Sensor
• Interferometers
• Others
• Performance comparison of Shack-Hartmann (SH) and
  Curvature Sensor (CS)

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Shack-HartmannWavefront Sensor (WFS)
Microlens Array
Detector
Each xy offset measures the local
wavefront slope across the corresponding lenslet.
Wavefront
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CurvatureWavefront Sensor
Focal Plane
Input Wavefront
Sensing Planes
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Wavefront Sensors and Detectors

• The curvature sensor minimises the number of
  pixels required to remove a given wavefront
  variance
• the use of noiseless fibre-coupled avalanche
  photo-diodes is therefore feasible
• Shack-Hartmann requires more pixels so a CCD is
  normally employed
• low read-noise multi-port specialised devices

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Comparison of SH and CS (0.5 seeing)(Pete
Doel, University of Durham)
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Comparison of SH and CS (1 seeing)(Pete Doel,
University of Durham)
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Wavefront Control
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Wavefront Control

• Deformable Mirror (DM) types
• Continuous
• Bimorph
• Segmented
• Hysteresis
• System order
• Liquid Crystals
• Conjugation

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Types of Adaptive Mirror(J.C.Dainty, Imperial
College)
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Deformable Mirror
One type of Deformable Mirror (DM)
Flexible continuous phase sheet
Minimum physical actuator separation 7mm
Fitting error s2f k (rs/r0)5/3 rad2 rs
projected actuator separation on sky k
fitting coefficient for DM type.
(continuous face sheet 0.35-0.4)
reflective surface
Actuators typically PZT or PMN throw 2-20
microns
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Continuous Face-sheetDeformable Mirror
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Bimorph Mirror(J.C.Dainty, Imperial College)
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BimorphDeformable Mirror
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The ELECTRA Segmented Adaptive Mirror (76
tip-tilt-piston segments) built by ThermoTrex,
San Diego
228 degree of freedom adaptive mirror
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Wavefront Fitting Error Comparison
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Actuator Hysteresis
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Hysteresis

• Effect of high hysteresis
• Continuous mirror 2-3 times more WFS samples
  required
• Segmented mirror makes piston control hard
• Solutions
• low hysteresis actuators
• linearise with motion sensor (e.g., strain gauge)
• linearise with figure sensor
• Example ELECTRA has strain gauges (with
  temperature compensation) which reduce hysteresis
  from 15 to lt0.1

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Liquid Crystals (LCs)General advantages as
adaptive wavefront correctors

• Low cost
• Easy to drive (low voltages)
• Can use open-loop
• Can build very large arrays
• Overcomes minimum actuator separation constraint
• this parameter drives the collimated beam size
  with a stacked actuator DM
• Transmissive
• variable or multi-conjugate systems easy to build
• easy to retrofit

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Liquid CrystalsTwo basic kinds for AO (at
present)

• Nematic
• continuously variable phase screens
• hard to get high OPD stroke and high bandwidth
  with same device
• Ferroelectric
• binary OPD switch
• fully achromatic operation not possible

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LC applications

• Active Optics (oscillations lt10Hz) correction of
  telescope static/flexure errors
• Low cost AO embedded into dedicated portable
  instruments (e.g. coronagraph)

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Generation of astigmatism using a 69-element
nematic LC0.1 (left)-gt1.0 (right) HeNe waves in
steps of 0.1Row 1(top) theoretical,row 2
simulated pixilationrow 3 experiment
(Zygo),row 4 (row3-row1)
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Sky CoverageThe big problem with AO
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You cant observe off-axis!

• Angle over which wavefront distortions are
  essentially the same

• This is a very small angle 5 in the visible
• It means that if you look at an object thats a
  large angular distance away from your guide star,
  you get poor correction!

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Guide Star Availability.All sky.Model D.
Simons, Gemini
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Guide Star Availability.(Galactic Latitude gt 30
degrees)Model D. Simons, Gemini
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NGS sky coveragemodel for ING by Remko Stuik,
Leiden Observatory
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Laser Guide StarsCreating an artificial
wavefront reference
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Types of Laser Guide Star

• Rayleigh (Green or UV)
• uses Rayleigh backscatter
• beacon height up to 20km
• requires time-gating to set beacon height
• Sodium D (Orange)
• uses excitation of mesospheric sodium atoms
• beacon height 80-90km
• no time-gating required
• tuned to sodium D line

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Comparison of Rayleigh backscatterand
sodium-resonance backscatter. (Courtesy of MIT
Lincoln Lab.)
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Rayleigh and Sodium Guide Stars at La Palma(IC
Applied Optics Group Tom Gregory, ING)
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Durhams Rayleigh Laser Guide Star
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Other LGS Systems
Subaru
(Keck)
Keck
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Other LGS Systems
WHT
US Military
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LGS sky coveragemodel for ING by Remko Stuik,
Leiden Observatory
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LGS sky coveragemodel for ING by Remko Stuik,
Leiden Observatory
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Laser Beacon Limitations

• Tilt reciprocity
• no tip-tilt signal from laser beacons
• must use a natural guide star
• focus is complicated for sodium beacons
• low frequency atmospheric focus may be masked by
  changes in effective beacon height
• Focus Anisoplanatism (cone effect)
• Sodium layer saturation
• Safety/site issues

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Tilt Reciprocity(J.C.Dainty, Imperial College)
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Angular and Focal Anisoplanatism
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Focal Anisoplanatism
d03m
d08m
d05m
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Schemes for the use of multiple laser
beacons(J.C.Dainty, Imperial College)
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Turbulence Conjugation(if normal AO is just a
bit too easy)
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Multiple Conjugate AO

• Putting a second DM in a plane conjugated to a
  higher layer of turbulence allows off-axis
  correction
• Requires multiple guide stars

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Multi-Conjugate AO (MCAO)Multiple LGS, Multiple
DM Wide corrected FOV
Rayleigh beacon Telescope
MCAOLaboratory
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Strehl Uniformity vs. FOV

• 0 degree zenith angle, 50 Cerro Pachon
  Turbulence Profile
• 5 LGS, 16 by 16 subapertures, 3 DMs
• No WFS noise or servo lag
• Courtesy of GEMINI

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MCAO Control Loop Architecture
Courtesy of GEMINI
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AO Modelling(or AO on a budget)
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AO Modelling

• Computer Modelling
• Required for performance prediction,
  instrumentation choices, instrumentation and AO
  systems engineering, detailed design.
• 8m Monte Carlo models using 10-12 processor
  Beowulf clusters are available, examples
• ESO/RTN (LeLouarn et al)
• Durham (Wilson et al)
• Ellerbroek/Rigaut
• Memory requirements scale as D4
• CPU requirements scale as D6
• D gt L0 poses new challenges for optimisation of
  WFS sample rate and control law (both in
  performance model and implementation)

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Durham 12-processor cluster simulations (Richard
Wilson)

• DCAO I-band simulation (the full Monte Carlo)
• D Tel diam (m) WFS order Tmx (s)
  Tloop (s)
• 4 8x8 18
  0.14
• 8 16x16 140
  0.52
• 12 24x24 847
  1.4 (MOVIE)
• 16 32x32 4067
  4.1
• Tmx is the time taken to produce the poke/control
  matrix, which as expected goes as something like
  D4.
• Assuming that the wavefront reconstruction
  calculation does not take over as the slowest
  component (ie. we use sparse matrix techniques),
  then we can project the timings to higher orders
  assuming that Tloop goes as D2 and Tmx as D4
  Projecting from the 24x24 case gives
• 32 64x64 42831 (12
  hours) 9.9
• 64 128x128 685296 (190
  hours) 39.6
• 256x256 107 (3045 hours)
  158.4 ELT
• 5000 loops required for 10 seconds of seeing
• Need a factor of 100 speedup.
• Assuming better parallelisation, this could be
  accomplished with order of magnitude larger
  cluster of up-to-date CPUs, and hardware
  acceleration.

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Durham 12-processor cluster simulations (Richard
Wilson)

• 24 x 24 WFS
• r020cm at V
• Science at 1µ.
• Cn2 is just 2 layers (0km, 5km)
• 500Hz simulated sample rate
• Top left 24x24 WFS
• Top right phase map at science pupil
• Bottom left right science PSF at 2 field
  points 30 arcsec apart.

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Does it actually work?
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AO Scientific Potential
Actual AO image
tip/tilt simulation of Galactic Center image (K)
at CFHT
Doug Simons Gemini
0.1 slit
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NGC7469 - Starburst galaxy
PUEO image fov 10x10, resolution0.13
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Io imaged with Keck AO
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The GLAS LGS AO System
INGRID J-band image of M15, 20s exposure, 20
diameter FOV, Open loop FWHM 0.45, Closed loop
0.2 (Moffat fit to PSF)
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What does this mean to an astronomer?
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Observing with an AO System

• Position of target in the sky?
• Nearer zenith is better (less atmosphere to
  correct)
• Is there a suitable guide star near your target?
• What wavelength do you want to observe in?
• Longer is better for AO as turbulence is weaker
• What field of view do you require?
• Current facility-class AO systems are not
  multiconjugate
• What performance can you expect?
• Highly dependent on weather
• How long does it take to set-up the AO system?
• Will a variable PSF across the field affect your
  results?
• What is the throughput to the Science CCD with
  the AO system?
• Extra surfaces in the optical path lower
  efficiency

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Seminar Topics (1-6)

• What are the essential differences between
  refracting and reflecting telescopes?
• Why is one is favoured over the other for modern
  telescopes?
• What are the challenges and advantages presented
  to modern instrumentation by the advent of
  electronic area detectors?
• What are the primary methods for achieving
  diffraction limited imaging on ground-based
  telescopes?
• What are the limitation of these techniques?
• What is the significance of camera f-ratio and
  pupil size to the design of spectrographs?
  (Bruno)
• Discuss the advantage and disadvantages of
  Littrow, Ebert and non-Ebert spectrograph
  configurations.
• What are the noise sources in EMCCDs and how does
  that influence their use for photon counting and
  amplification mode? (Julian)

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Seminar Topics (7-13)

• Discuss the advantages and disadvantages of
  Prisms, Grisms and Vrisms for astronomical
  spectroscopy.
• Discuss the distinctions between the use of
  Fabry-Perots in the pupil and image plane for FP
  imaging spectroscopy.
• Compare and contrast 3D spectroscopy using
  Fabry-Perots and Integral Field Units.
• What causes Focal Ratio Degradation in Fibres and
  what are the consequences to multi-fibre
  spectroscopy?
• What techniques can be used to suppress OH
  emission in infra-red imaging and spectroscopy?
• What new technologies are required for Adaptive
  Optics?
• How do crossed-VPH gratings work to achieve
  tunable filter imaging? (Rene)