FOCAL PLANE MOUNTS

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FOCAL PLANE MOUNTS
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FEA Model

• Description of the Model
• Things learned along the way
• I received much help from
• Mike Sholl and Bobby Besuner -Thanks
• Most recent results
• Thermal Loads
• Gravity Loads
• Influence of the Optics Bench
• Rigid Optics Bench Required

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Description of the Model

• Shield (not modeled)
• Shell elements,
• supported from flange on the shield
• Optics Bench (not modeled)
• Polyimide Carbon Fiber
• 2 mm thick
• Bipods (Strut, Flexure, Foot)
• Keck Flexure solid elements
• Strut Shell elements
• All components Titanium
• Details on size Next Page
• Focal Plane
• Solid elements, Molybdenum
• FP 50 mm thick, Radius - 304 mm (red),
• Edge flange 25 mm thick, (orange varies 32mm
  to 50 mm)
• Mounts attached to flange, at mid-plane of Focal
  Plane

Boundary conditions Symmetry plane, Clamped in
XYZ where bipod attaches to the optics
bench Thermal load everything -130 C, optics
bench room Gravity loading
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Description of the Bipods
3 mm chamfer at base of flexure Flexure Nesting
50 Overall Blade Length including chamfer, 12.7
mm Keck boss OD 32 mm, Keck boss ID 16 mm, 25 mm
long Strut OD 19 mm, ½ mm wall, 76 mm long 90
degree opening angle, all Titanium
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Things learned along the way-I

• Thermal Loading, Flexures nested 75
• 35 micron tilt in the focal plane
• Cause
• Tangential flexures off bipod strut center
• ½ mm

Tangential Flexure
Deflection normal to the focal plane
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Things learned along the way-II
Gravity Loading Blades Nested 75 Gravity
loading causes both a Shear stress and a
Tensile stress Flexure opens up Max Flexure
Stress 135 MPa Focal plane sag in this case
100 microns
Exaggerated distortion Flex OD 32 mm, ID 16 mm
Boss axial length 19 mm Blade thickness 1 mm
Blade axial length 11 mm Blade offset 3 mm (8
mm nested)
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Most Recent Results -Model
3 mm chamfer at base of flexure Flexure Nesting
50 Overall Blade Length, 12.7 mm Keck boss OD 32
mm, Keck boss ID 16 mm, 25 mm overall
length Strut OD 19 mm, ½ mm wall, 76 mm long 90
degree opening angle, all Titanium
Focal plane and Focal Plane Bipods modeled Upper
part of bipod feet clamped in X,Y,Z
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Most Recent Results ThermalAll Features _at_-130
C, Stresses
Flexure Stress 57 MPa, (SF 14x) Strut stress 0.3
Mpa
Exaggerated Distortion Max Stress at base of
clamped foot Also high between Titanium base and
Focal Plane 137 MPa
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Most Recent Results ThermalAll Features _at_-130
C, Deflections
Flexure Distortion Magnitude (something goofy in
the model? -Strut shrinks 200 um) Radial
distortion of FP as expected, 560 microns,
685 mm diameter
Focal plane axial shift 380 microns Expected
121mm9.2e-6150C 167 microns
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Most Recent Results Gravity
Blades Nested 50 Max flexure Stress 75 MPa (SF
10x) Stress in strut 14 Mpa Note Some Flexure
Blades are not aligned with gravity
Exaggerated Distortion (normal to FP) Max sag,
60 microns Out of plane, 22 microns
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More Results 2 mm thick Optics Bench
Gravity only, Deflections plotted Max sag, 3.7
mm Max Stress 100 MPa
Thermal Loading, Deflections Plotted Optics Bench
Warm- All Else -130 C Max sag, .5 mm Max Stress
340 Mpa Cold Ti and warm Carbon Fiber
Bench Note Asymmetric distortion due to
asymmetric Bench shape
Optics Bench needs to be 100x stiffer Equivalent
to 20 mm thick
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Conclusions
Flexures Nested 75 Large Focal Plane
displacements High Stresses Flexures Nested 50
and with chamfer Safety factor of 10x on yield
stresses Focal plane displacements about 60
microns sag, 22 microns out of plane 2 mm thick
Optics Bench Focal plane displacement with
gravity loading 4 mm Optics Bench needs to be
about 100x stiffer or equivalent to 20 mm thick