An alternative spectrograph mount Bruce C. Bigelow University of Michigan Department of Physics 5/14/04

2 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 D T = 160K

3 Spectrograph 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

4 Spectro mount on FP 2:1 truss geometry – inner to outer triangle dimensions

5 Spectro mount on FP FP assembly with spectro structure attached

6 Spectro installed Ease of access to detector connections FP assembly with spectrograph included (note redundant str.)

7 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

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

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

10 Dynamic FEA First resonance = 413 Hz, transverse mode First 6 freq: Hz Hz Hz Hz Hz Hz

11 First resonance (.avi)

12 Thermal FEA Elements: Purple – Al 7075, Red, Blue - Invar

13 Thermal FEA Axial deflection for -160 K = 940 microns (0.037”), due to Al baseplate (d l = 50 microns for 0.25m of Invar). Much less for MZT baseplate, can be thermally compensated with strut materials.

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

15 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