The document describes an accelerated life test for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) fiber positioning system. A hexapod test apparatus was designed to replicate the dynamic motion of the fiber segments over a 5-year projected lifespan in a compressed 2-3 month period. Key design parameters included the hexapod travel limits, actuator speed and force capabilities, and custom end joints to handle misalignments. The test aims to continuously measure changes in focal ratio for individual fibers to evaluate the optical performance over repeated flexing cycles.

1. IFU Accelerated Life Test
(Overview)
Ian Soukup
Center for Electromechanics

2. Hobby Eberly Telescope
One of the World’s largest optical telescopes
• 9.2 meter Effective Aperture
• 78 square meter Primary Mirror
Tracking is achieved by moving the tracker
instrument package instead of moving the
entire telescope

3. HET Upgrade for VIRUS
(HETDEX)
The immense light-gathering capability of
Focal Surface
HET will allow HETDEX to build the
largest map of the universe ever Strain Relief 1
(SR1)
produced.
IFU Bundle
Strain Relief 2
(SR2)
• Up to 192 spectrographs (VIRUS)
Strain Relief 3
(SR3)
• As many as 43,008 optical fibers with
a 21 meter average length
VIRUS
Fiber System Layout and Mechanical Coupling
Fiber Segment MCP Beginning MCP End Behavior Length (m)
1 Focal Surface Strain Relief 1 Static <1
2 Strain Relief 1 Strain Relief 2 Dynamic 7
3 Strain Relief 2 Strain Relief 3 Dynamic 9
4 Strain Relief 3 VIRUS Cabinets Static 4

4. Design Concerns
• Optical performance of the fibers is
unknown after repeated cycles of
extending, contracting and flexing
• Fiber quality and optical performance
are dictated by:
– Transmission
• Ratio of light exiting to light entering
– Focal Ratio Degradation (FRD)
• Gradual decline in the F-ratio as light Figure: Far field images of the input and
passes from input end to output end
output spots. FRD has scattered
(scattering of light at exit)
the light both outwards and into the
central obscuration (output spot -
• Factors that lead to FRD right)
– Specific to User Control
• Mechanical Deformation (Macro / Micro
Bending)
• Localized stresses or stress points
• Size and complexity of the fiber system
– Specific to Manufacture Control for HETDEX is much greater than
• Random Irregularities
previous fiber systems deployed
• Fiber End Preparation
• Materials

5. Accelerated Life Test
• Test Objective:
– Provide a life cycle test simulating the HETDEX fiber system behavior, while continuously measuring relative
changes in the output focal ratio of individual fibers over the duration of the test
• Design Constraints: Item Constraint Driver Constraint Description
Deploy HETDEX strain reliefs at their proposed
1 Defines Routing (Volume)
relative spatial orientation
2 Operate Test within Task B Defines Volume
Replicate full range of telescope fiber movement
3 Defines Motion (Volume)
between Strain Relief 1 & 2
4 Restrain 48 IFU Conduits Defines Force
Perform 5 year projected life cycle test within 2 to
5 Defines Rate
3 months
200 mm X 200 mm X 700 mm
6 Provide space for optical testing instruments
optical test bench at input and exit
7 Provide test bed for hexapod control system Defines Parallel Kinematics
• Design Concepts – (1) Rotary Fiber Tester (2) Cart / Cable Fiber Tester (3) Hexapod Fiber Test Apparatus

6. Concept 1

7. Concept 2

8. Concept 3

9. Design Parameters
• Hexapod Travel:
– Calculated using geometric constraints imposed by HETDEX Tracker Design
– Explicitly replicates the fiber system movement of segment 2 throughout its operational limits
• Z = Max: 116.33 mm Min: 0 mm • θx/θy = Max: 9 deg Min: -9 deg
• X/Y = Max: 391.99 mm Min: -391.99 mm • ρ = Max: 21 deg Min: -21 deg
• Actuator Travel:
– Determined by positioning the SolidWorks model at over 1500 discrete locations within its
travel range and measuring the displacement of individual components
• Actuator Travel: 496.08 mm • Upper U-Joint Rotation: 11 degrees
• Upper U-Joint Misalignment: 44 degrees • Lower U-Joint Misalignment: 31 degrees
ρ
Z
θy Y
θx X

10. Design Parameters
• Actuator Speed:
– 21.9 mm/s Calculated at Tracker Slew Speed w/ Rho
– Test Duration for 5 year life cycle representation at 21.9
mm/s
• 360 hrs (15 – 24 hr days)
• Actuator Force: (worse case travel condition with fully
weighted IFUs)
– 10.4 kN – Calculated using SolidWorks Simulation
• End Joint Design
– 2 DOF and 3 DOF joints are required between the actuators and end plates
– Custom designed end joints were needed to handle the large misalignment and axial loads

11. Optical Bench Axis
• Designed to move the fiber test bundle so that it approximates the dynamic behavior
exhibited by fiber segment 3 of the HETDEX fiber system.
• Optical Bench Components:
– Custom Wire Rope Drum
– AKM-53G servo drive motor w/ Nema True Gear Box
– Drum Mount Platform (Stationary)
– Linear Bearing Platform (Stationary)
– Optics Platform
1.2 m

12. IFU Hexapod

13. Conduit Routing to Strain Relief 2

14. IFU Test Strain Relief 2

15. Lower Axis

16. Fig. 9

17. IFU Test Strain Relief 1

18. Fig. 2