An alternative spectrograph mount

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An alternative spectrograph mount

• Bruce C. Bigelow
• University of Michigan
• Department of Physics
• 5/14/04

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Spectrograph mount

• Objectives
• Design a high-performance spectrograph mount to
  FP
• high stiffness / high first resonance
• low launch stresses, small deflections
• simple, clear interface to focal plane (ICD)
• ease installation and removal of spectro.
• minimize mass, part count
• Performance goals (from Besuner, 4/29/04)
• first resonance above 100 Hz
• material failure (yield) above 40g
• acceptable focal plane distortion for DT 160K

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Spectrograph mount

• Design features
• Truss structure, Invar tubes
• 21 Truss geometry
• Horizontal translations, no tilts
• Attaches to common focal plane mounting points
• Essentially no loads carried by focal plane
  assembly
• Simple interface to spectrograph
• 3 discrete support points, or round flange
• Supports spectrograph load near center of mass
• minimizes moment loads
• Simple interface to FP (mount points, cylindrical
  volumes)
• Spectrograph and mount easily separate from FP or
  each other

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Spectro mount on FP
21 truss geometry inner to outer triangle
dimensions
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Spectro mount on FP
FP assembly with spectro structure attached
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Spectro installed
Ease of access to detector connections
FP assembly with spectrograph included (note
redundant str.)
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Spectrograph mount

• FE Analyses
• General
• Spectro mass modeled as 10Kg Invar cylinder
• Invar spectrograph mounting truss
• 7075 Alum. focal plane baseplate (extreme case)
• Static
• Strut geometry 25 mm OD, 2 mm wall thickness
• 40 G transverse acceleration (launch
  orientation)
• Dynamic
• First 10 mode shapes, frequencies
• Thermal
• Delta T -160K
• Stresses and deflections

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Static FEA
Deflections Ay 40G, Dy 137 microns (0.005)
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Static FEA
Stress Ay 40G, s 23.7 MPa (3436 Psi) (250
MPa yield)
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Dynamic FEA

• First 6 freq
• 413 Hz
• 415 Hz
• 416 Hz
• 470 Hz
• 478 Hz
• 490 Hz

First resonance 413 Hz, transverse mode
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First resonance (.avi)
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Thermal FEA
Elements Purple Al 7075, Red, Blue - Invar
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Thermal FEA
Axial deflection for -160 K 940 microns
(0.037), due to Al baseplate (dl 50 microns
for 0.25m of Invar). Much less for MZT
baseplate, can be thermally compensated with
strut materials.
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Thermal FEA
Stress for -160 K 67.4 MPa (9773 Psi) (worst
case with Al baseplate, simple model)
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Spectrograph mount

• Conclusions
• Truss structure meets all design objectives
• high stiffness, high resonance
• low stresses
• simple, clear interface to FP
• easy installation, removal, and access to FP
• minimum part count, low mass
• Truss structure meets all performance goals
• first resonance above 400 Hz (100 Hz goal)
• material failure safety factor of 10 at 40g
• minimal or no distortion of focal plane
  (independent)
• Truss is a viable alternative to flexure mount