Transcript Document

Optical Fabrication
for Next Generation Optical Telescopes;
Terrestrial and Space
Robert E. Parks
Optical Perspectives Group, LLC
Tucson, AZ
September, 2002
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
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
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
Unobstructed aperture? Mechanical and optical advantages
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
Primary mirror construction
3 layers; reflecting film, glass substrate, support structure
Film is the mirror; high reflectivity over broad wavelength band
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
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*
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)
Rt(r) = Rv[ 1 + (r/Rv)2 ]1/2 (tangential and parabola)
Therefore, segments are largely astigmatic or potato chip
relative to the nearest spherical surface
Aspheric departure from a sphere
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
Delta sag,   r4/8Rv3
With monolith, absolute Rv not very important, hardware issue
If segment has wrong Rv it is a figure error
Geometry of off-axis segment departure
r
a
Rv
h
sphere
r
parabola
dz
Transformation of coordinate system
z 
b 
0
4
b22 
b 
1
1
cv3 a 4

ax  h  ay  
8
cv3
b 
Spherical aberration
48
cv3 a 2 h 2
b 
4
cv3 3ah 3  2a 3 h 
3
Tilt
1
3
0
2
Astigmatism
2
" 2
2
"
cv3 a 3 h
Coma
6
cv3 4a 2 h 2  a 4 
Focus
16
cv3 3h 4  6a 2 h 2  a 4  Piston
b 
0
0
24
Off-axis segment topography
Segment blank fabrication
Glass is shaped by disintegration – diamond wheel grinding
Hot form to near net shape to reduce grinding costs
Handling fixtures, lifting equipment and storage space
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
Aspheric figuring
Dwell time computer controlled polishing
Bend and polish – developed for and used on Keck
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
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
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
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
Ion figuring
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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
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
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.
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
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
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
Segment bent in bending fixture
Actively stressed lap schematic picture
Large continuous polisher
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
RAP torch in operation
Polishes SiO2 to 0.18 nm
Gaussian tool shape
Nanometer-scale corrections
Deterministic
Atmospheric process
no vacuum chamber
Large range of tool sizes
Presently being developed for large optics fabrication
Grinding in segment topography
Components of segment aspheric departure
a
a 0,0= 0.000
Tilt removed
a 0,0= 0.000
a -1,1= 0.000
a -1,1= 0.000
a 1,1=1046.0
a 1,1= 0.000
a -2,2= 0.000
a -2,2= 0.000
a 0,2=18.000
a 0,2=18.000
a 2,2=18.000
a 2,2=18.000
a -3,3= 0.000
a -3,3= 0.000
a -1,3= 0.000
a -1,3= 0.000
a 1,3= 0.410
a 1,3= 0.410
a 3,3= 0.000
a 3,3= 0.000
Tilt and focus removed
a 0,0= 0.000
Tilt, focus & astig. removed
a 0,0= 0.000
a -1,1= 0.000
a -1,1= 0.000
a 1,1= 0.000
a 1,1= 0.000
a -2,2= 0.000
a -2,2= 0.000
a 0,2= 0.000
a 0,2= 0.000
a 2,2=18.000
a 2,2= 0.000
a -3,3= 0.000
a -3,3= 0.000
a -1,3= 0.000
a -1,3= 0.000
a 1,3= 0.410
a 1,3= 0.410
a 3,3= 0.000
a 3,3= 0.000
Large Optical Generator
Aspheric departure to scale
TIlt removed
a 0,0= 0.000
Tilt and focus removed
a 0,0= 0.000
a 1,-1= 0.00
a 1,-1= 0.00
a 1,1= 0.000
a 1,1= 0.000
a 2,-2= 0.00
a 2,-2= 0.00
a 2,0=18.000
a 2,0= 0.000
a 2,2=18.000
a 2,2=18.000
a 3,-3= 0.00
a 3,-3= 0.00
a 3,-1= 0.00
a 3,-1= 0.00
a 3,1= 0.410
a 3,1= 0.410
a 3,3= 0.000
a 3,3= 0.000
Tilt, focus and astig. removed
a 0,0= 0.000
a 1,-1= 0.00
a 1,1= 0.000
a 2,-2= 0.00
a 2,0= 0.000
a 2,2= 0.000
a 3,-3= 0.00
a 3,-1= 0.00
a 3,1= 0.410
a 3,3= 0.000
Variation of radius of curvature with aperture radius
Model bending fixture
Bending fixture moments
Deterministic surface ripple