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The Challenge - Multiple LGS

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The Challenge - Multiple LGS Powered By Docstoc
					 Can we afford to build an extremely
large groundbased diffraction limited
        optical/IR telescope?

             Jim Oschmann
             Francois Rigaut
              Mike Sheehan
              Larry Stepp
             Matt Mountain

          Gemini Observatory            1
 Can we afford to build an extremely
large groundbased diffraction limited
        optical/IR telescope?

       Or can we afford ~ $1,000M

             Probably yes...



                                        2
       Framework for a credible
      Extremely Large/Maximum
      Aperture Telescope Concept

    Science Case           Gallagher et al, Strom et al

           An adaptive optics
               solution
                                     Rigaut et al


Mountain et al    A telescope concept    
                            A viable instrument
   Ramsay Howat et al              model             
                                                           3
Spectroscopic Imaging at 10 milli-arcsecond
                resolution
      - using NGST as “finder scope”
                          Simulated NGST K band
                             image
                          • Blue for z = 0 - 3
                          • Green for z = 3 - 5
                          • Red for z = 5 - 10
                           = 0.1

                               l

                                     2K x 2K
                                       IFU
                                   0.005” pixels

                                                   4
     48 arcseconds
Modeled characteristics of 20m and 50m telescope
      Assumed point source size (mas)

      20M        1.2mm 1.6mm 2.2mm 3.8mm 4.9mm 12mm 20mm
      (mas)       20    20    26   41    58       142    240

      50M        1.2mm 1.6mm 2.2mm 3.8mm 4.9mm 12mm 20mm
      (mas)      10    10    10    17    23       57     94

        h        70%   70%   50%   50%   50%      50%        50%

      Assumed detector characteristics

       1mm < l < 5.5mm                    5.5mm < l < 25mm

       Id         Nr    qe                 Id           Nr         qe

      0.02 e/s    4e   80%               10 e/s     30e        40%

      (Gillett & Mountain, 1998)                                        5
                  Relative Gain of groundbased 20m and 50m
                         telescopes compared to NGST
                           Imaging                           Velocities ~30km/s
                  1                  10                  1                  10
           100                              100
                                                   100                                 100
                                  50M R=5                               50m R=10,000




                                                                                                  Groundbased
                                  20m R=5                               20m R=10,000




                                                                                                  advantage
            10                              10
                                                    10                                 10




              1                             1
                                                     1                                 1
S/N Gain




                                                                                                     NGST advantage
            0.1                             0.1
                                                   0.1                                 0.1




           0.01                             0.01
                                                  0.01                                 0.01




           1E-3                             1E-3
                                                1E-3                                   1E-3
                  1                 10                   1                  10
                                                                                              6
                      W avelength ( m m )                    W avelength ( m m )
                An Adaptive Optics Solution
                                                                         The Challenge - Multiple Laser Beacons
                                                                             - still a lot of technologies to develop
                                                                    *    *     *      *    *
       p erfo is multiconjugate?
     AOWhatrm ance o n a 50m                                                                    SRFA ~ 0.75 requires NBeacons

            Telesco p e
              (Rigaut, this workshop)                                                                  1.2mm
                                                                                                       1.6mm
                                                                                                                      75
                                                                                                                      40
           Turbulent laye r 1                                                                          2.2mm          20
                      5k actuator AOS on 50-m (Median Seeing)                DM 1
                                                                                                       3.8mm          5
              1                                                                                        4.9mm          3
            0.9                                                               1.2 micron               12.0mm         <=1
            0.8
            0.7
                                                                              1.6 micron               20.0mm         <=1
            0.6                                                               2.2 micron
 Str ehl




            0.5                                                               3.8 micron
            0.4
           Turbulent laye r 2                                                 4.9 2
                                                                             DMmicron
            0.3
            0.2                                                               12 micron
            0.1                                                               20 micron
              0
                        10        20
                0 Off axis ray corrected   30     40        50      60
                      Field Angle constrained to small field of view
   Ø Diffraction limited imaging(arcsec)
Chun, 1998                                  On axis ray corrected                                                       7
                                                                                           19
        New Directions for Adaptive Optics
    ~ arcminute corrected FOV‟s possible (Rigaut et al)
                          No correction (AO off)
                           MCAO on
• Numerical
  simulations
   – 5 guide stars & 5
      Wavefront           Optical Performance - Strehl Ratio at 500nm
      sensors                       across a 20” x 20” FOV
                                       (Ellerbroek,1994)
   – 2 mirrors
   – 8 turbulence              Multiconjugate Adaptive Optics
      layers                 On Axis   Edge FOV Corner FOV
   – 40‟‟ Field of view
                               0.942        0.953         0.955
   – J band
• Fully corrected
  PSF across full field
  of view
                                                                        8
  Instrumentation -- the next constraint?
         (Ramsay Howatt et al)

                              R = 8,000 across J, H & K
          2K x 2K
            IFU
        0.005” pixels
                        l

                                        4.2107 109
                                      6.7 X x Pixels




                                                           1.2 m
         10 arcsec
                                      18.5 mm pixels
Lets not assume diffraction
limited instruments for
30m ~ 100m telescopes will be small
                                          1.2 m        9
                                 The next step ?             50m telescope




A 400 year legacy of groundbased telescopes
                       1400
Cumulative Area (m )
2




                        900



                        400



                          0
                       -100
                          1600   1700   1800   1900   2000         10
                                        Year
                  Technology has made telescopes
                 far more capable, and affordable
                                                         The Cost per Square Meter of Telescope Collecting Area
                        70000
                       1400                 400 years of inflation
Cumulative Area (m )



                                                         0
                                             10
 2




                        60000
                                                         -1
                                                       10
                        50000
                                     2


                        900
                                     Relative Cost/m
                                                         -2
                        40000                          10
    UK CPI




                                                         -3
                        30000                          10
                                                                  x 1000
                                                         -4
                        20000                          10
                        400
                        10000                            -5
                                                       10

                            0                            -6
                          0                            10
                       -100   1600                     1700     1800    1900     2000
                                                              1600     1700      1800      1900      2000
                          1600                           1700 Year      1800            1900           2000
                                                                                Year
                                                                       Year                                 11
                                        Cassegrain
                                        Instrument
                   Optical Design




                                             #2
• Requirements
  – 50m aperture




                                        Adaptive
                                         Optics
              ~ 3m




                                          Unit
  – Science field of view 0.5 - 1.0
   50m
    arcminutes
  – Useable field of view 1.0 - 2.0
    arcminutes (for AO tomography)




                                        Cassegrain
                  2m diameter




                                        Instrument
  – Minimize number of elements (IR




                                             #1
    performance)
  – Aim for structural compactness
  – KISS        F/20 Cassegrain focus

F/1 parabola M1, 2m diameter M2
                                                 12
        Optical Performance
1 arcminute FOV (Science Field)
                            30                       60
                0 arcsec




       0 arcsec.                  30 arcsec.
                                  l/10

     0 arcsec                    30 arcsec
                    60 arcsec.

                                  Guide star FOV
                                     rms wavefront error
                                     1 micron wavelength   13
         Primary Mirror Approach
F/1 Segmented Parabola
                                                 The volume of glass in
                                                 a 50-mm thick 8-meter
                                                 segment is 2.5 cubic meters.
                                                 This volume is equivalent to
                                                 a stack of 1.5-meter diameter
                        50m                      boules 1.4 meters high.



                                        Actively controlled polishing
Segment testing (no null lenses) The sag of an 8-meter segment is only 80 mm

                                                   Testing
                 ~25m
                                                Ion Figuring
                                                                        14
                                                Final Testing
       Primary Mirror Support

   To reduce mass, reduce mirror substrate thickness
    ~ 50mm (1/4 of Gemini, ESO-VLT)

   Individual segments still have to be supported against
    self weight




        Gravitational print through requires between 120 - 450
        support points for a 20 cm thick meniscus
                                                             15
        Primary Mirror Support -
               continued
• As self weight
  deflection a D4/t2,
   ~8m diameter, 50mm
   segment will need ~
   1800 support points
• How many active
  support points do we
  need to correct
  deformations due to
  wind and thermal
  gradients?

• Estimate 1 in 6,
  ~ 300/segment which implies
  > 10,000 actuators to
  actively support a 50m mirror    16
Does maintaining 10,000 actuators challenge
      the Quality Control Engineers?
 • What Mean Time Between Failures (MTBF) does this
   require?
    – Assume 95% up-time, over 356 x 12 hour nights
    – Assume unacceptable performance will occur when 5% of
      actuators fail
    – Assume it takes 1 hour to replace actuator, and that we can
      service 8 actuators a day, over 250 maintenance days
    – Therefore we can replace/service 2,000 actuators/year
 • MTBF required is 380,000 hours
 • Required service life of each actuators, assuming
   maintenance is 5 years

                                                                    17
Challenges for the Structural Engineers ...
Telescope Optical Structure Requirements:
• 50m surface must be held ~ l/10 against gravitational and wind loads
• Relative pointing and tracking ~ 3 arcseconds rms
   •   Absolute pointing/tracking provided by Star-tracker
   •    Precision guiding/off-setting controlled by M4 and A&G/AO system
• “Clean” top-end for IR emissivity, but rigid enough to launch 5 laser beacons

• Challenges
• 20mm mirror substrate still weighs ~ 110 kg/m2
  (c.f ~ 75 kg/m2 for Gemini/Zeiss M2)
• Mirror segments + cells could weigh 5.5 x 45 + 200 =
  450 tonnes
• Wind…………..
• 10 m/s across 50m a lot of energy at ~ 0.2 Hz                        18
                 Resonant Frequencies of Large Telescopes
                               Parabolic Reflector
                                Antenna Systems
                         Optics Systems (Laser/Infrared)
                        Lowest Servo Resonant Frequency
Frequency (Hz)




                                                                 2Hz

                               Telescope Aperture     50m
                                                            19
Conceptual Design for an F/1 50m
     Optical/IR Telescope




                                   20
      Optical/Mechanical concept
                                            Three levels of figure
                                            control:
    Mirror-to-cell actuators
                                             • Each mirror segment
           Integrated mirror/cell segment
                                               is controlled within
                                               an individual cell
                                             • Each cell is then
                 Large stroke actuators         controlled with respect
                                                to the primary mirror
                                                support structure
                                            • The support structure
                                              may have to use “smart
Mirror support truss                          structure” technology
with smart structure
                                              to maintain sufficient
elements/active damping
as needed
                                              shape and/or damping
                                              for slewing/tracking
                                                              21
Concept Summary
            Optical support structure
            uses at least three levels
            of active control

             Collimated beam allows
             M3 & M4 to be tested
             independently and
             allows AO/instrument
             structure to be rigidly
             coupled to F/20 focus
             - insensitive to translation
               or rotation relative
               to 50m structure
             M2 easy to make/test
             - may need a little more
               rigidity….
                                22
      An Enclosure for 50m -- “how big?”



75m                   75m                        30 degrees
      150m                                                       150m

 • Restrict observing range to airmasses < 2.0
 • “Astro-dome” approach
 • Heretical proposition #1 - excavate
       – significantly lowers enclosure cost
       – further shields telescope from wind
       – reliant on AO to correct boundary layer
 • Heretical proposition #2 - perhaps the wind characteristics of
      a site are now more important than the seeing characteristics   23
Framework for a credible Extremely Large/Maximum
           Aperture Telescope Concept



       Science Case
            An adaptive optics
                solution

                   A telescope concept

                          A viable instrument
                                 model

                                                24
Image of a 21st Century Ground-Based Observatory
                    -- 50m Class




                                              25
26
               How do we cost a 50m?

                   “What can it cost?”
                                                     (1999)
50m Telescope                               costs (1997$)
                                                        )
   Primary mirror assembly                 $622M $522
   Telescope structure & components        $190M
                                                  Scaled costs
   Secondary mirror assembly               $11M
   Mauna Kea Site                          $78M
   Enclosures                               $70M
   Controls, software & communications      $26M
   Facility instrumentation (A&G, AO)       $35M Constrained
                                                    costs
 Coating & cleaning facilities               $9M
 Handling equipment                          $5M
 Project office                             $40M
• Contingency                                $100M
                 Total            $1,086M
                                                                 27
    S (Keck + Gemini + ESO-VLT + Subaru) = $1,560M
            How do we cost a 50m?
              Risk assessment
• Adaptive Optics
   – multiple-conjugate AO needs to be demonstrated
   – deformable mirror technology needs to expanded for 50m ( x
     10 - 20 more actuators
• How do we make a “light-weight”, 4 - 8m aspheric
  segment mounted in its own active cell and can we
  afford 45 - 180 of them?
• How much dynamic range do we need to control cell-
  segment to cell-segment alignment ?
       Will “smart”, and/or active damping systems have to be
        used telescope
       evaluate by analysis and test.

 Composites or Steel?
                                                                  28
       Risk assessment - continued
 Telescope Structure and wind loading
        We need to characterize this loading in a way that is relatively easy to use
         in finite element analysis. This is easy, but mathematically intensive.
         Basically for each node that gets a wind force, a full vector of force cross
         spectra is generated, therefore the force matrix is a full matrix with an
         order equal to the number of forces (10‟s of thousands).

 Enclosure concept (do we need one)?
    What concept can we afford both in terms of dollars/euros and
     environmental impact (note Heretical Proposition #2)


 WE NEED A TECHNOLOGY TEST-BED
    a 10m - 20m “new technology telescope”
    this is probably to only way to establish a credible cost for a
     50m - 100m diffraction limited optical/IR groundbased
     telescope                                                    29
 “Supposing a couldn‟t
“Supposing wetree fell
 down a 50 or 100m
afford Pooh, when we
 were when we could
Pooh, underneath it?”
have been doing
 “Supposing it didn‟t,”
something more
 said Pooh
„useful `” after careful
 thought.
“Supposing we could,”
     The House at Pooh Corner
said Pooh after careful
thought.

     With apologies to
The House at Pooh Corner




                       30

				
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