AO concepts for extremely large telescopes

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AO concepts for extremely large telescopes

• Rich Dekany
• Jet Propulsion Laboratory
• Richard.Dekany_at_jpl.nasa.gov

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Todays show...

• Extremely Large Telescopes
• Historical perspective
• CELT concept
• Adaptive optics observation modes
• Seeing limited
• Single conjugate
• Multiconjugate
• Extreme
• CELT configuration options

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Lessons of History

• Plot of largest optical/IR telescope size vs.
  time reveals exponential growth
• Remarkable given various social, economic, and
  technical factors
• Extrapolating from Keck 10 m
• 10 m 1993
• 25 m 2034
• 50 m 2065
• 100 m 2097
• History does not explain how future gains will be
  made

Log10 collecting area (meters2)
Courtesy J. Nelson
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Large telescope projects 1950-2020
1949 1990 1995 2000 2005 2010 201
5 2020
Hale Keck1 Keck2 MMT HET Gemini (x2) VLT
(x4) Magellan .others LBT (x2) GTC CELT
HST SIRTF NGST
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California Extremely Large Telescope (CELT)

• Features
• 30 meter class optical / IR observatory
• Filled-aperture, fully pointable
• Advanced adaptive optics
• State-of-the-art instrument complement
• Cost TBD
• Completion date TBD

• Baseline design
• Two-mirror Ritchey-Chretien
• F/1.5 primary mirror
• 1080 hexagonal segments
• 0.9 m (shortest edge) segment diameter
• Segments organized onto rafts for handling
• Non-interlocking edge sensors
• Final F/ 15 (may shorten)
• Nasmyth foci only

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CELT Science
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NGST

• NASA mission currently in the planning/technology
  development stage
• 8-meter class aperture
• Diffraction-limited at 2 microns wavelength
• Optimized for 1-5 microns, but zodi limited to 10
  microns in the currently passively cooled model
• Gold-coated optics detectors will mean short
  wavelength cutoff of 0.6 microns
• FOV of wide-field instruments 3-4 arcminutes
• 5-year lifetime in L2 orbit (no servicing
  possible)
• Launch in 2008 time frame
• Total cost, including technology is 2B
• Instruments
• TBD, but includes 1-5 micron imager and
  spectrograph
• Resolutions to R3000, but optimized for
  low-dispersion

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CELT modes of observation

• Seeing-limited (SL)
• Still requires wavefront information from
  guider star
• Likely to be 10-20 Zernike modes or equivalent
• Single conjugate AO (SCAO)
• Sufficient for observations longward of l 3.5
  mm
• Emphasizes low emissivity, possibly polarimetry
  (dust)
• Multiconjugate AO (MCAO)
• Wide FOV correction
• 2 arcmin diameter optimized for l 1-3.5 mm
• Extreme AO (EAO)
• NGS-based system targeting highest-contrast
  science
• Nearby extrasolar companion searches and study

Observing with CELT by band
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Some CELT killer apps...
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• High SNR, high imaging, and spectral resolution
• Accretion environments
• Study disks to centrifugal radius (Rc) 10 AU at
  distances up to 1 kpc
• R 105 to obtain velocities to 3 km/s to
  estimate mass accretion rate
• Measure infall rate as function of position and
  angular momentum
• Cepheids out to redshift z 0.1
• Measure H0 and omega matter
• Age determination of giants (measure t0)
• Measure age of stars via thorium decay in giants
  below RGB tip
• Requires R 3 x 105 and S/N 1000.
• Geometry of the Universe via SNe at z 3
  (measure q0)
• Break degeneracy of omega matter and omega lamba
• Spectroscopy of z 0.5 - 5 galaxies
• Measure kinematics and chemistry
• Planetary science
• KBO searches
• 20 km resolution imaging of Jupiters
  atmosphere
• Comparable to Galileo images
• Many others

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Courtesy M. Mountain
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IR exosolar planet studies with CELT

• CELT 5 mm point-source sensitivity 0.6 x 10-6
  Jy
• 5s, 1 hour, no IR optimization
• Includes reality factor of 25 seems with Keck
  LWS
• Predicted 5 mm flux from 1 Gyr old Jupiter 3 x
  10-6 Jy
• Planet self-luminous (around low-mass star), at
  10 pc
• 200 stars with 10 pc, mostly K and M dwarfs
• 5 mm flux from M2 star 0.4 Jy
• PALAO demonstrating 1000x contrast at 40 Strehl
  at 1 (2.2 mm)
• Extrapolation to CELT at 5 mm Star flux 3 x
  10-6 Jy
• Fluxes are very model-dependent
• Up to 100 times lower for 5 Gyr old planet
• Up to 106 times higher than blackbody, depending
  on wavelength
• Jovian frequency within 1 AU around G stars 5
  (Marcy Butler, PASP, 112, 137) 10-20 systems
  of interest

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CELT Telescope
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Scaling laws

• Cost
• Recent history Cost D2.3
• Extrapolating Keck costs 30 m cost 1.2B
• Need innovation to get break this law
• Define the scale size of interest as S
• Deflections due to self-weight, d S2
• Deflection due to fixed load/m2, such as thin
  segments, S1
• Angular change due to self-weight d /L S1
• Angular change due to fixed load/m2 S0
• Mass S3 (extremely fast, so often ignored, use
  m S3/2)
• Keck moving mass 250 tons
• GBT (100m radio dish) mass 8000 tons (law
  predicts 250,000 tons)
• Stiffness of structure, k F/ d mG/ d
  S3/2/S2 S-1/2
• Resonant frequency d-1/2 S-1
• Wind loading
• Excitation frequency characteristic of eddy size
  vwind/S S-1
• Same scaling as the structure resonance frequency
• Deflections due to wind F/k S2/S-1/2 S5/2

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Large telescopecost scaling laws
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Recent large telescope costs

• Item Keck1(Y88) Keck2(Y95) HET
• Dome development 6.7 8.6 1.2
• Drive and control system 2.8 2.1 1.3
• Mirror support system 15.2 4.1 (optics)
• Observatory development 9.8 10.3 2.4
• Project managementand engineering 7.7 8.1 1.1
  
• SSC/support 1.7 0.4 0.2?
• Support Facilities 5.6 6.5 0.0?
• Telescope structure 9.2 10.8 1.2
• Operations Testing 0.0 5.5 2.0
• Total 93.7 77.7 16.5

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CELT Primary Mirror
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CELT primary mirror

• Equivalent collecting area to all major existing
  observatories combined

• Fun Fact!
• Approximately the same as ocular collecting area
  of the population of the City of Los Angeles
• Baseline
• Segment thickness 45 mm
• Keck segment thickness is 75 mm
• Non-locking capacitive edge sensors
• Keck sensors interlock
• Segments grouped into rafts

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• CELT

Keck
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Optimal Segment Size (Nelson, Chanan)
Factors favoring many small segments
Lower mass, easier to support Lower
material costs Easier to fabricate Facto
rs favoring fewer large segments Smaller
error propagation (sensors ? actuators/wavefront)
ssurface 0.74 (Nseg)1/2 ssensor 4.4
for Nseg 36 15. for Nseg 1098 Easier
to phase no. edges ? Nseg
C22 astigmatic coefficient a hexagon side
length R off axis distance k primary radius
of curvature
84 for Nseg 36 3168 for Nseg 1098
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Keck Stressed Mirror Polishing Set-up
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title
Proposed CELT Stressed Mirror Polishing Set-up
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Keck Sensor Geometry
R 35 m
Mirror Segment
7.5 cm
Sensor Mount
Sensor Body
Conducting Surfaces
Sensor Paddle
2 mm
L
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title
Proposed CELT Sensor Geometry
Non-Interlocking Sensors
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CELT Adaptive Optics
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CELT modes of observation

• Seeing-limited (SL)
• Still requires wavefront information from
  guider star
• Likely to be 10-20 Zernike modes or equivalent
• Single conjugate AO (SCAO)
• Sufficient for observations longward of l 3.5
  mm
• Emphasizes low emissivity, possibly polarimetry
  (dust)
• Multiconjugate AO (MCAO)
• Wide FOV correction
• 2 arcmin diameter optimized for l 1-3.5 mm
• Extreme AO (EAO)
• NGS-based system targeting highest-contrast
  science
• Nearby extrasolar companion searches and study

Observing with CELT by band
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Optical design issues for CELT AO

• NGS SL mode
• Minimize number of surfaces
• NGS SCAO
• Maintain large science field of view
• Minimize emissivity
• LGS MCAO
• Provide for multiple conjugation locations
• Get relatively large field through a complex
  relay
• LGS elongation
• LGS fratercide
• Requires multiple NGS, as well
• NGS EAO
• Minimize scattered light (high spatial frequency
  errors)

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Seeing-limited (SL) observing mode

• Even SL mode requires aggressive active optics
• Primary focus mode
• Primary sensor error propagation
• Wind-induced vibrations in structure
• Regular telescope guider replaced with
  low-order wavefront sensor

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Single conjugate (SCAO) observing mode

• Natural guide star system
• Full sky coverage
• Based upon relatively loose wavefront error
  budget, which allows for large subaperture
  wavefront sensing and large isoplanatic angle
• Diffraction limited at 3.5 mm or longer
• Requires 500-1000 actuators
• Single adaptive element potentially an adaptive
  secondary
• Would prefer to minimize emmissivity, but
  resolution is the major science driver (NGST wins
  in sensitivity)

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Single conjugate (SCAO) observing mode
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SCAO issues

• Even for single-conjugate AO, many hard problems
  exist
• Segmented primary mirror
• Segment vibrations
• ACS error propagation of low order primary mirror
  modes
• Need more information than usual from guider
  camera (now a wavefront sensor)
• Deformable mirrors
• Adaptive secondary mirrors may be desirable
• Minimizes emissivity for IR observations
• Requires technology development
• Number of actuators, size, stroke, linearity,
  hysteresis, coatings,
• Control
• Incorporation of NGS data into control laws
• Computation cost
• Very large number of actuators require new
  algorithms
• Systematics
• Small diffraction width accentuates previously
  acceptable error

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Multiconjugate (MCAO) observing mode

• Natural guides stars do not provide sufficient
  sky coverage at near-IR wavelengths (1-2.5 mm)
• Artificial beacons are necessary (ref. Ed
  Kibblewhites talk from Thursday)
• Focal anisoplanatism (cone error)
• MS wavefront error (D/d0)5/3
• where d0 is linear function of beacon height and
  scales l6/5
• Mauna Kea model atmosphere
• d0, assuming 45 degree zenith angle,  
• d0(10 km, MK, 45 zen) 1.08m  
• d0(92 km, MK, 45 zen) 3.60m  
• which leads to, for 1 mm observing wavelength, D
  30 m,   
• sFA(10km) 2540 nm  
• sFA(92km) 931 nm  
• which are both obviously unacceptable wavefront
  errors.

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MCAO issues I

• Need multiple laser guide stars
• Due to focal anisoplanatism
• Required even for narrow field AO (i.e. single
  isoplanatic angle)
• All guide stars would like to operate in closed
  loop (i.e. enjoy AO correction)
• How much useful information, in any, is available
  from open-loop guide star information?
• Need multiple natural guide stars
• Tilt indeterminism of individual beacons leads to
  unsensed modes of turbulence
• Major contribution from focus and astigmatic
  atmospheric modes
• Ellerbroek 3 NGS are sufficient for 5 LGS
• Research area How does one exploit whatever
  wavefront information supplied by fortuitous NGSs

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MCAO issues II

• Laser beacon elongation
• Sodium layer has finite thickness (typ. 10 km)
• For Shack-Hartmann sensor, each subaperture sees
  elongated beacon
• Beam elongation (beacon length off-axis
  distance)/(height2)
• Possible solutions?
• Gating of wavefront sensor camera (natural for
  Rayleigh beacons)
• Membrane mirror for adaptive focus during pulse
  flight
• Elongated beacons still give good centroid
  estimates in one direction
• Is there really a problem?
• Laser beacon fratercide
• Multiple laser beacons tend to obscure each other
  with the Rayleigh scatter tail
• Worst effect for subapertures near beacon launch
  points
• Beacon launch from behind secondary always a
  problem

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MCAO issues III

• Many reflections
• Renewed emphasis on coatings
• Minimize emissivity and scattering
• Requires new calibration techniques
• Multiple wavefront sensors much each now be
  calibrated
• Above and beyond problem of lower acceptable rms
  wavefront error
• Complex software
• Multiple guide star acquisition

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Three MCAO geometries

• Offner relay
• Compact triad
• Crossed MCAO

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Offner Relay

• 11 relay
• Three concentric spherical surfaces
• Corrected field is determined by physical size of
  relay
• In other words, the corrected field (in mm) of a
  2 meter Offner is twice the dimension of a 1
  meter Offner
• Located beyond Nasmyth focus of an ELT, first two
  reflections of Offner occur at interesting
  conjugates
• Example
• ELT secondary is first conjugate
• First two reflections of Offner conjugate to 2 km
  and 8 km altitude
• Requires large, powered adaptive elements
• Exploit adaptive secondary technologies?

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CELT AO - 2amin Offner relayConjugates at 0km,
2km, 8km
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CELT - MCAO Offner relay (detail)
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CELT - MCAO Double Offner relay(Conjugates at
0, 2, 5, 8, 12 km)
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Pupil magnification

• Internal CELT pupils are limited by FOV
• Practical angles at any (current) adaptive device
  about 20 degrees incidence
• For isoplanatic angle of 20 asec, limit of pupil
  magnification 1/3600
• 8.33 mm pupil diameter
• Actuator spacing of 130 mm for 30m telescope (for
  4,000 Nact system)
• Selecting standard pupil diameter of 100 mm
• 20 asec FOV becomes lt 2 degrees
• 1mm AO technology is initially usable (Nact
  8,000)
• 200 mm MEMs development increases Nact to
  200,000

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Courtesy A. Meinel
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Courtesy A. Meinel
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Courtesy A. Meinel
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Extreme (EAO) observing mode

• Visible, high contrast observations
• Requires 10,000 100,000 phase actuators
• Will likely require amplitude correction
• Requires very high photon fluxes for sufficient
  SNR per subaperture
• Laser power required scales as l-18/5
• Hard
• Return to NGS only system, at least initially
• Used for nearby stars only
• Limitations to exosolar planet detection may be
  imposed by primary mirror
• Diffraction from segment gaps
• Segment vibrations

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No-laser atmospheric tomography (Raggozoni,
Gilmozzi)

• Large aperture telescopes sample an enormous
  volume of the Earths atmosphere
• Multiple NGSs could be used to determine 3-D
  structure of turbulence
• Multiple (perhaps fewer) DMs could correct a
  significant field
• For example, for 30m CELT, beam shear of 15m
  (arbitrary), yields 5 arcmin radius
• Requires N guide stars
• However, corrected field grows as D2
• Finding 3 NGSs of 13th mag within 5 arcmin (30m
  CELT) reasonable (P 50)
• Finding 5 within 8.5 amin (50m CELT) likely (P
  90)
• Exploit all useful NGSs within technical field
  with multiple pyramids in Foucault-like wavefront
  sensor (ref. Gary Chanans talk from Monday)

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CELT Structure
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CELT K structural schematic
Courtesy S. Medwadowski
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CELT MWD(millimetre dish) Concept
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CELT MWD Servicing HiCass
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CELT MWD concept
Concept by A. Meinel, D. Woody, and R.
Dekany Model by A. Meinel
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CELT MWD - Inboard Nasymth
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CELT MWD - Outboard Nasmyth
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CELT MWD - Servicing Secondaries
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ELT AO Technology Roadmap