AO concepts for extremely large telescopes by R8RVowI9

VIEWS: 6 PAGES: 54

									AO concepts for extremely large telescopes

                    Rich Dekany
                Jet Propulsion Laboratory



              Richard.Dekany@jpl.nasa.gov
                          Today’s show...

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

• Plot of largest optical/IR
  telescope size vs. time reveals
  exponential growth
    – Remarkable given various




                                        Log10 collecting area (meters2)
      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

                                                                          Courtesy J. Nelson
                                   1949
             Hale


                                   1990
                           HST
             Keck1


                                   1995
             Keck2
             MMT
             HET

   Large     Gemini (x2)
             VLT (x4)
             Magellan              2000
             ….others      SIRTF
 telescope   LBT (x2)
             GTC
  projects                         2005



1950-2020    CELT
                           NGST
                                   2010



                                   2015



                                   2020
     California Extremely Large Telescope
                    (CELT)
• Features                              • Baseline design
   – 30 meter class optical / IR            – Two-mirror Ritchey-Chretien
     observatory                            – F/1.5 primary mirror
   – Filled-aperture, fully pointable           • ~1080 hexagonal segments
   – Advanced adaptive optics                   • 0.9 m (shortest edge) segment
   – State-of-the-art instrument                  diameter
     complement                                 • Segments organized onto rafts
                                                  for handling
   – Cost ~ TBD
                                                • Non-interlocking edge
   – Completion date ~ TBD                        sensors
                                            – Final F/# ~ 15 (may shorten)
                                            – Nasmyth foci only
CELT Science
             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
• 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 Jupiter’s atmosphere
             – Comparable to Galileo images
    – Many others…
Courtesy M. Mountain
CELT Telescope
                               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
                           SCALING LAW DATA
                               OPTICAL
                               RADIO
                                OPTICAL ENDOSTRUCTURES


                                                         TELESCOPE DIAMETER (m )
                                                              4     10   20 30                                    100

   Large                                      2.4
                                              2.2
 telescope

               log TELESCOPE COST(M$,1999$)
                                              2.0                              SU B                                     100 M$
                                                                  HA LE                                    DS N
                                                                                        KE CK
                                              1.8
cost scaling                                  1.6
                                                             MMT-2
                                                                          VL
                                                                          T
                                                                                    (COL)
                                                                                 GE M
                                                                                                                   BONN


                                                                               (MA G)           ENDOSTRUCTURE
    laws                                      1.4          MMT-1
                                                                                                TELESCOPES




                                                                     S
                                                           NT T




                                                                  RE
                                              1.2                         HE T           SM T




                                                                TU
                                              1.0




                                                                                                     DIO
                                                              UC
                                                            TR
                                              0.8




                                                                                                  RA
                                                           OS
                                              0.6        EX  NANJIN G
                                              0.4
                                              0.2
                                                    0                             1.0                              2.0
                                                        log TELESCOPE DIAMETER (m)
                   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 management
    and 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
CELT Primary Mirror
                    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
CELT   Keck
             Optimal Segment Size (Nelson, Chanan)
Factors favoring many small segments:

          • Lower mass, easier to support
          • Lower material costs
          • Easier to fabricate:
                                                    C22   = astigmatic coefficient
                                  2
                                 a R   2            a     = hexagon side length
                        C 22                       R     = off axis distance
                                  k3                k     = primary radius of curvature

Factors 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                  84       for Nseg = 36
                no. edges  Nseg =         3168       for Nseg = 1098
    Keck Stressed Mirror Polishing Set-up



Bending Levers                Polishing Lap
                 Blank                          Weights




                         Back Support



                                Optical Polishing Table
                                title
Proposed CELT Stressed Mirror Polishing Set-up

                     posts glued to
     force devices    blank edge
         (12)




    blank
               polished
                surface



                          Arrows indicate force direction and magnitude
                             required to create / remove astigmatism
             Keck Sensor Geometry

                       R = 35 m


        Mirror Segment             7.5 cm


               Sensor Mount
               Sensor Body
Conducting             Sensor Paddle
Surfaces
                2 mm
                          L
Proposed CELT Sensor Geometry
                title




     Non-Interlocking Sensors
CELT Adaptive Optics
             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
        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)
      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
                    Nact & Sky coverage vs. wavlength
                         (D=30m, 20 e- per subap)


100000                                           10




10000                                            1
                                                         Nact for const fitting Strehl 0.8
                                                         Nact for const fitting Strehl 0.5
                                                         Nact for const fitting Strehl 0.2
 1000                                            0.1     P(N>0) in isoplanatic patch
                                                         Isoplanatic Angle (asec)


  100                                            0.01




   10                                            0.001
         0.1            1                   10
               Wavelength (microns)
  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)
Single conjugate (SCAO) observing mode
                              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
   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 Kibblewhite’s 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.
                         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 NGS’s
                           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
                      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
           Three MCAO geometries

• Offner relay
• Compact triad
• Crossed MCAO
                          Offner Relay
• 1:1 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?
CELT AO - 2amin Offner relay
   Conjugates at “0”km, 2km, 8km
CELT - MCAO Offner relay (detail)
CELT - MCAO Double Offner relay
    (Conjugates at “0”, 2, 5, 8, 12 km)
                    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 < 2 degrees
       • 1mm AO technology is initially usable (Nact ~ 8,000)
       • 200 mm MEMs development increases Nact to ~ 200,000
Courtesy A. Meinel
Courtesy A. Meinel
Courtesy A. Meinel
         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
         No-laser atmospheric tomography
                            (Raggozoni, Gilmozzi)


• Large aperture telescopes sample an enormous volume of the Earth’s
  atmosphere
• Multiple NGS’s 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 NGS’s 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 NGS’s within technical field with multiple pyramids in
      Foucault-like wavefront sensor (ref. Gary Chanan’s talk from Monday)
CELT Structure
CELT – K structural schematic




                     Courtesy S. Medwadowski
 CELT – MWD
(millimetre dish)
    Concept
CELT MWD – Servicing HiCass
CELT MWD concept




            Concept by A. Meinel, D. Woody, and R. Dekany
                                       Model by A. Meinel
 CELT MWD -
Inboard Nasymth
CELT MWD - Outboard Nasmyth
CELT MWD - Servicing Secondaries
ELT AO Technology Roadmap

								
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