Dark Energy Camera

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Finite element analysis of DECam May 11 C1
corrector lens gravity and thermal load cases
Bruce C. Bigelow, Physics Department, University
of Michigan 6-7 July 2005
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FEA of May 11 C1 element

• This talk
• Conceptual design for an athermal elastomeric
  lens mount (RTV)
• Description of objectives, model, and load cases
• Deflection and stress results

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FEA of May 11 C1 element

• Objectives
• Determine deflections of C1 under gravity loads
  for RTV mount
• Determine stresses in C1, elastomer, and cell
  for -20C temp swing
• Models
• 1/2 geometry model
• 3mm RTV elastomeric bond (radial and axial
  directions)
• Invar cell, FS lens
• Assume a perfectly rigid barrel
• Load cases
• gravity along optical axis
• gravity across optical axis
• steady-state temperature change of -20C

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FEA of May 11 C1 element
3d solid (brick) elements
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FEA of May 11 C1 element
Detail of elastomer bond line
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FEA of May 11 C1 element
Symmetry boundary conditions and constraints
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FEA of May 11 C1 element
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FEA of May 11 C1 element
Gravity across optical axis, deflections across
optical axis in meters (2 microns at center)
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FEA of May 11 C1 element
Gravity across optical axis, deflections along
optical axis, in meters (/- 2 microns)
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FEA of May 11 C1 element
Gravity along optical axis, deflections along
optical axis in meters (7.6 microns)
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FEA of May 11 C1 element
-20C temp change, max. stresses in Pa (1450 PSI
max)
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FEA of May 11 C1 element
Stresses in cell in Pa (1450 PSI max)
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FEA of May 11 C1 element
Stresses in RTV in Pa (12 PSI max)
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FEA of May 11 C1 element
Stresses in lens in Pa (217 PSI)
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FEA of May 11 C1 element

• Conclusions
• Gravity deflections seen here are probably
  negligible
• Thermal stresses are very low relative to yield
  for all three materials
• The RTV mount provides stiff yet thermally
  compensating support
• This FEA model is readily adapted to other
  geometry and materials
• Although 90mm central thickness of C1 appears to
  be OK for mounting and thermal loads, it will
  very difficult to fabricate and test. The RTV
  mount can easily accommodate a thicker lens.