Optical Fabrication for Next Generation Optical Telescopes; Terrestrial and Space

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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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*

8. 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

9. Aspheric departure from a sphere

10. 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

11. Geometry of off-axis segment departure

12. Transformation of coordinate system Spherical aberration Coma Focus Astigmatism Tilt Piston

13. Off-axis segment topography

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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.

22. 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

23. 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

24. 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

25. Segment bent in bending fixture

26. Actively stressed lap schematic picture

27. Large continuous polisher

28. 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

29. Grinding in segment topography

30. Components of segment aspheric departure

31. Large Optical Generator

32. Aspheric departure to scale

33. Variation of radius of curvature with aperture radius

34. Model bending fixture

35. Bending fixture moments

36. Deterministic surface ripple