# Presentation: 929750

## FOCAL PLANE MOUNTS

## 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

## 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

## 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, 1⁄2 mm wall, 76 mm long
- 90 degree opening angle, all Titanium

## 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
-       ~ 1⁄2 mm

- Tangential Flexure

- Deflection normal to
- the focal plane

## 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)

## Most Recent Results -Model

- Focal plane and Focal Plane Bipods modeled
- Upper part of bipod feet clamped in X,Y,Z

- 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, 1⁄2 mm wall, 76 mm long
- 90 degree opening angle, all Titanium

## Most Recent Results –ThermalAll Features @-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

## Most Recent Results –ThermalAll Features @-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
- 121mm*9.2e-6*150C=
- 167 microns

## 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

## 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

## 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