Effects of Frit Expansion on Mirror Optical Surfaces

1
Effects of Frit Expansion on Mirror Optical
Surfaces

• Robert Besuner
• 16 September 2004

2
Background

• SNAP PM, SM, and TM are currently envisioned to
  be closed-back, frit-bonded ULE.
• Frit is essentially a bonding agent between face
  sheets and core.
• Frit forms a fillet between parts.
• Exact properties of frit are proprietary, but it
  is known that in air, the frit itself expands
  irreversibly.
• Magnitude of this expansion is being sought...for
  this analysis a uniform bulk expansion of 1ppm on
  linear dimensions is applied.
• Elastic properties are assumed the same as ULE.
• Expansion of frit imposes strain on core and face
  sheets, causing some distortion of optical
  surface.
• If frit-imposed distortions are too high, may
  need to seek alternatives.

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Finite Element Analysis on SNAP Mirror Cells

• One ppm strain imposed on frit only.
• Frit assumed to have same elastic properties as
  ULE.
• Frit shape assumed a 45 degree fillet, 1mm x 1mm
  or 2mm x 2mm.
• Both of these conservatively have more frit matl
  than Corning predicts/observes, see following
  slide.
• Visual inspection of parts _at_ ITT (formerly EK)
  showed frit size and shape is pretty irregular,
  even on a continuous bead.
• In no instance was there as much as 2mm of fillet
  leg.
• Simpler to model 45 degree fillet.
• Assumption is a mirror cell in an infinite array
  of cells.
• No edge effects.
• Symmetries of hex and isogrid cells are taken
  advantage of to simplify models.
• Configuration variations
• 1mm or 2mm fillets
• All cells have identical fillets vs. One cell has
  finite fillet and adjacent cell has no fillet.

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Frit Profiles
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Primary Mirror Unit Cell
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Secondary Mirror Unit Cell
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Tertiary Mirror Unit Cell
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PM Distortion Result (All cells 1mm x 1mm frit
fillets)
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PM Distortion Result (All cells 1mm x 1mm frit
fillets)
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PM Distortion Result (1mm x 1mm fillet next to no
fillet)
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PM Distortion Result (1mm x 1mm fillet next to no
fillet)
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TM Distortion Result (All cells 1mm x 1mm frit
fillets)
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TM Distortion Result (All cells 1mm x 1mm frit
fillets)
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TM Distortion Result (1mm x 1mm fillet next to no
fillet)
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TM Distortion Result (1mm x 1mm fillet next to no
fillet)
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Tabular Distortion Results
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Conclusion

• It all depends on what the actual expected frit
  expansion is.
• There are definite distortions at mid-spatial
  frequencies.
• Distortions will vary linearly with frit
  expansion value.
• Alternatives
• Low-Temperature Fusion (no frit).
• Bonded Zerodur (new process, no heritage).
• Open-back Zerodur (mass is higher).
• Open-back Silicon Carbide (bulk thermal
  expansion).