Optical Fabrication for Next Generation Optical Telescopes; Terrestrial and Space

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Optical Fabricationfor Next Generation Optical
Telescopes Terrestrial and Space
Robert E. Parks Optical Perspectives Group,
LLC Tucson, AZ September, 2002
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Background

• Difficult to discuss all of optical fabrication
  in one hour
•  
• Assume you will be involved in NG telescope
  design and fab
•  
• Outline the problems and decisions relative to
  fabrication
•  
• Suggest methods of dealing with fabrication
  issues
•  
• Some methods never used before, designed to
  provoke thought
•  
• No solutions, but places to begin thinking
•  
• Some testing too fab and test intimately
  related

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Terrestrial and Space

• Many similar problems yet some important
  differences
•  
• Emphasis on terrestrial but will point out
  special problems
•  
• Emphasis on many possible approaches
  nothing is right or wrong
•  
• Choices influenced by end use, flexibility,
  budget, facilities,
•  
• talents of project team and project charisma
•  
• Best choices optimize resources to achieve a
  particular goal
•  

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What will the NG telescope look like?

• Monoliths have reached a practical upper
  limit at 8 m
•  
• NGT will be segmented applies to space
  also, deployable
•  
• Secondaries most likely will be monoliths
•  
• Segments will be solid or sandwich
  construction no castings
•  
• Segment and support mass must be minimized
•  
• Segments will be a glassy material
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• Unobstructed aperture? Mechanical and
  optical advantages

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Why glassy material?

• Can be polished to correct shape and smoothness
•  
• Temporally stable, very homogeneous, low
  CTE, no humidity
•  
• Almost perfectly elastic easy FE modeling
  of deflections
•  
• Relatively inexpensive and lightweight
  density modulus of Al
•  
• Easily inspected for impurities and strain
  because it is transparent
•  
• Easy to see if damaged it breaks or returns
  to original shape
•  
• Negative low thermal conductivity need
  thin cross section

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Primary mirror construction

• 3 layers reflecting film, glass substrate,
  support structure
•  
• Film is the mirror high reflectivity over
  broad wavelength band
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• Substrate supports film, gives it smoothness
  and HSF shape,
•  
• Substrate may be considered rigid depending on
  size
•  
• Structure actively controls rigid body
  motion of substrate
•  
• Controls shape of array of segments
•  
• May control low spatial frequency shape of
  substrate

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Segment outline

• Assume a close packed, circular array
•  
• 2 logical choices trapezoids or hexes
•  
• Trapezoids all same shape and figure in
  same ring good
•  
• each ring different shape and acute corners -
  undesirable
•  
• Hexes all same shape, close to circular
  outline good
•  
• many different figures that are angle dependent
  not good
•  
• Difficult choice but probably hexes best for
  large terrestrial

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What is the topography of the segments?

• In general. off-axis conics, hyperbolas, very
  nearly parabolas
•  
• Hard to shape because curvature changes with
  aperture radius
•  
• Spheres are easy, constant curvature, lap is
  rigid, hits highs
•  
• Rr(r) Rv 1 (r/Rv)2 3/2 Rv 1
  (1/4f)2 3/2 (radial)
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• Rt(r) Rv 1 (r/Rv)2 1/2 (tangential
  and parabola)
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• Therefore, segments are largely astigmatic
  or potato chip
•  
• relative to the nearest spherical surface

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Aspheric departure from a sphere
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A harder look at segment topography

• Sag of a parabola is zp r2/2Rv
•  
• Sag of a sphere is zs Rv ( R2 -
  r2)1/2
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• Delta sag, ? ? r4/8Rv3
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• With monolith, absolute Rv not very
  important, hardware issue
• If segment has wrong Rv it is a
  figure error

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Geometry of off-axis segment departure
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Transformation of coordinate system
Spherical aberration
Coma
Focus
Astigmatism
Tilt
Piston
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Off-axis segment topography
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Segment blank fabrication

• Glass is shaped by disintegration diamond
  wheel grinding
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• Hot form to near net shape to reduce
  grinding costs
•  
• Handling fixtures, lifting equipment and
  storage space
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• Grinding introduces surface stresses cause
  unstable deformation
•  
• Etch or polish non-optical surfaces to remove
  surface stress
• Grinding produces high local forces that
  must be resisted
•  
• All edges need bevels (chamfers) to prevent
  damage
•  
• Different base radius as function of radial
  location
• Radius must be known to an absolute standard

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Aspheric figuring

• Dwell time computer controlled polishing
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• Bend and polish developed for and used on
  Keck
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• Active stressed lap developed at U of AZ
  Mirror Lab
•  
• Ion figuring for final stages of figuring
•   Bend and polish on a continuous polishing
  machine
• RAP
• Mask and etch

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Dwell time computer controlled polishing

• Extension of traditional optician craftsman
  technique
•  
• Sit longer on highest locations using sub
  diameter tools
•  
• Process does not converge well repeated
  test and polish cycles
•  
• Problems at edges, need small tools to cope
  with edges
•  
• Brute force method, not as deterministic as
  it seems
•  
• Surface stresses are part of problem going from
  grind to polish
• Real problem where absolute radius must be held

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Bend and Polish

• Bend segment to reverse of desired figure
• Grind and polish spherical, then release bending
  forces
• Making aspheres now as easy as making spheres
• Smooth figure right to edge of segment
• Bending procedure easily modeled
• Localized edge effects due to forces and moments
• New stresses when segments cut to hexes
• Final figuring by ion polishing

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Actively stressed lap

• Extension of dwell time and bend and polish
• Uses bend and polish math, moments to control lap
  shape
• Uses dwell time to remove high areas
• Because lap always fits asphere, large lap can be
  used
• So far used just on rotationally symmetric
  mirrors
• Produces smooth surface and good edges
• Can be used be used with off-axis segments with
  azimuthal segment orientation constraint on lap
  shape
• Some final localized figuring by hand

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Ion figuring

• Computer controlled dwell time material removal
  method
• Ions in a vacuum remove glass by bombardment
• Non-contact material removal method
• Good deterministic method for small material
  removal
• Five to ten times improvement in figure per pass
• Generally one pass sufficient
• Too slow to introduce aspheric figure
• Too slow to polish from a grind
• Cost effective for what it can do

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Bend and polish on a CP machine

• Similar to bend and polish but several segments
  at once
• Bend segment, place face down on annular
  spherical lap
• Lap kept spherical by a conditioning tool
• Promises to be cost effective
• Concept used successfully on small, precise
  spherical optics
• Would require new bending jig concept
• Would require method of changing lap radius
• Large capital investment for an unproven method

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Reactive Atom Plasma (RAP)

• Ambient pressure reactive gas plasma removal
  process
• Non-contact material removal method
• Wide range of removal rates 500 um to 0.1
  nm/min
• Non-linear with distance to glass so tends to
  smooth
• Can polish from ground state
• Used as a CCP with dwell time and reactive gas
  concentration
• Diameter of active removal function easily
  changed
• In early stages of development by a private firm
• Patented by RAPT Industries, Inc.

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Mask and etch

• Conventionally polish blanks to correct radius
• Make masks for 1 um contour levels
• Mask lowest point on final mirror
• Etch in ammonium bi-fluoride to remove 1 um
• Apply mask for next lowest level and etch again
• Repeat until all contour levels complete
• Smooth level boundaries with conventional
  flexible lap
• Polished surface not degraded by etching
• Needs development, worked on small sample

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Issues with grinding and polishing methods

• Method may be limited by blank structure
• All deterministic methods leave residual scallop
  of the dimension of the small spatial scale tool
  used
• May need brief conventional polishing to smooth
  ripple
• Contact methods distort surface and roll edges
  and corners
• Non-contact methods do not inherently smooth
• May need a combination of methods to reach final
  figure
• Process control will be necessary for consistent
  results
• More segments make more methods feasible
• Incredible computing power available to model
  methods

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Conclusions/predictions

• Space optics harder to make, less options for fab
  test
• For earth based bend and CP polish as first step
• Then a non-contact method for higher order
  correction
• Possibly conventional flex or stressed lap for
  smoothing
• Need quality assurance plan from start one error
  is 1000
• Do experiments before committing to a fab plan

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Segment bent in bending fixture
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Actively stressed lap schematic picture
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Large continuous polisher
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Reactive Atom Plasma (RAP) processing
Non-contact shaping/polishing damage
removal Large range in removal rates 500 mm/min
for SiO2 100 mm/min for SiC as low as 0.1
nm/min Deterministic Atmospheric process no
vacuum chamber Large range of tool sizes
RAP torch in operation
Polishes SiO2 to 0.18 nm
Gaussian tool shape
Nanometer-scale corrections
Presently being developed for large optics
fabrication
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Grinding in segment topography
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Components of segment aspheric departure
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Large Optical Generator
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Aspheric departure to scale
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Variation of radius of curvature with aperture
radius
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Model bending fixture
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Bending fixture moments
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Deterministic surface ripple