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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

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optical fabrication for next generation optical telescopes terrestrial and space
Optical Fabricationfor Next Generation Optical Telescopes; Terrestrial and Space

Robert E. Parks

Optical Perspectives Group, LLC

Tucson, AZ

September, 2002


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
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
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
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
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
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
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

a harder look at segment topography
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

transformation of coordinate system
Transformation of coordinate system

Spherical aberration






segment blank fabrication
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
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


Mask and etch

dwell time computer controlled polishing
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 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
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
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
bend and polish on a cp machine
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
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
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
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

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


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