An alternative spectrograph mountAn alternative spectrograph mount

Bruce C. Bigelow

University of Michigan

Department of Physics

5/14/04

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Spectrograph mountSpectrograph 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 mountSpectrograph mount

Design features: Truss structure, Invar tubes 2:1 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 FPSpectro mount on FP

2:1 truss geometry – inner to outer triangle dimensions

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Spectro mount on FPSpectro mount on FP

FP assembly with spectro structure attached

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Spectro installedSpectro installed

Ease of access to detector connectionsFP assembly with spectrograph included (note redundant str.)

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Spectrograph mountSpectrograph 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 FEAStatic FEA

Deflections: Ay = 40G, Dy = 137 microns (0.005”)

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Static FEAStatic FEA

Stress: Ay = 40G, s = 23.7 MPa (3436 Psi) (250 MPa yield)

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Dynamic FEADynamic FEA

First resonance = 413 Hz, transverse mode

First 6 freq:

1. 413 Hz

2. 415 Hz

3. 416 Hz

4. 470 Hz

5. 478 Hz

6. 490 Hz

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First resonance (.avi)First resonance (.avi)

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Thermal FEAThermal FEA

Elements: Purple – Al 7075, Red, Blue - Invar

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Thermal FEAThermal 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 FEAThermal FEA

Stress for -160 K = 67.4 MPa (9773 Psi) (worst case with Al baseplate, simple model)

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Spectrograph mountSpectrograph 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