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Design of the Fiber Optic Support System and Fiber Bundle Accelerated Life Test for VIRUS

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Design of the Fiber Optic Support System and Fiber Bundle Accelerated Life Test for VIRUS Design of the fiber optic support system and fiber bundle accelerated life test for VIRUS Ian M. Soukup†a, Joseph H. Benoa, Richard J. Hayesa, James T. Heislera, Jason R. Mocka, Nicholas T. Mollisona, John M. Goodb, Gary J. Hillb, Brian L. Vattiatb, Jeremy D. Murphyc, Seth C. Andersond, Svend M. Bauere, Andreas Kelze, Martin M. Rothe, and Eric P. Fahrentholdf aThe University of Texas Center for Electromechanics, 1 University Station R7000, Austin, Texas USA 78712 bThe University of Texas McDonald Observatory, 1 University Station C1402, Austin, Texas, USA 78712-0259 cThe University of Texas Department of Astronomy, 1 University Station C1400, Austin, Texas, USA 78712-0259 d904 W 22nd 1/2 St., Apt 214, Austin, Texas, USA 78712-0259 eAstrophysikalisches Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany fThe University of Texas Department of Mechanical Engineering, ETC 4.136, 1 University Station C2200, Austin, Texas, USA 78712-0292 ABSTRACT The quantity and length of optical fibers required for the Hobby-Eberly Telescope* Dark Energy eXperiment (HETDEX) create unique fiber handling challenges. For HETDEX‡, at least 33,600 fibers will transmit light from the focal surface of the telescope to an array of spectrographs making up the Visible Integral-Field Replicable Unit Spectrograph (VIRUS). Up to 96 Integral Field Unit (IFU) bundles, each containing 448 fibers, hang suspended from the telescope's moving tracker located more than 15 meters above the VIRUS instruments. A specialized mechanical system is being developed to support fiber optic assemblies onboard the telescope. The discrete behavior of 448 fibers within a conduit is also of primary concern. A life cycle test must be conducted to study fiber behavior and measure Focal Ratio Degradation (FRD) as a function of time. This paper focuses on the technical requirements and design of the HETDEX fiber optic support system, the electro-mechanical test apparatus for accelerated life testing of optical fiber assemblies. Results generated from the test will be of great interest to designers of robotic fiber handling systems for major telescopes. There is concern that friction, localized contact, entanglement, and excessive tension will be present within each IFU conduit and contribute to FRD. The test apparatus design utilizes six linear actuators to replicate the movement of the telescope over 65,000 accelerated cycles, simulating five years of actual operation. Keywords: Center for Electromechanics, CEM, Hobby-Eberly Telescope, HET, HETDEX, University of Texas, McDonald Observatory, fiber optics, Integral Field Unit, IFU, Focal Ratio Degradation, FRD, VIRUS, accelerated life test * The Hobby-Eberly Telescope is operated by McDonald Observatory on behalf of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximillians-Universität München,and Georg-August- Universität, Göttingen † R.S.: E-mail: [email protected] ‡ http://hetdex.org/ 1. INTRODUCTION The Hobby Eberly Telescope, designed to gather large amounts of light for spectroscopy, is undergoing a transformation to begin the search for "Dark Energy". Dark energy is a term used to describe unknown forces causing the acceleration of cosmic expansion1,2. To help define the evolution and significant properties of dark energy, the Hobby Eberly Telescope at the McDonald Observatory (MDO) was chosen to survey and map the locations of other galaxies in space. The experiment, which is estimated to be completed over 100 nights of viewing, is now referred to as HETDEX3. The fabrication and installation of 192 spectrographs called VIRUS4,5 is the foundation of the telescope upgrade3. The Ground-based and Airborne Instrumentation for Astronomy III, edited by Ian S. McLean, Suzanne K. Ramsay, Hideki Takami, Proc. of SPIE Vol. 7735, 77354W · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.856968 Proc. of SPIE Vol. 7735 77354W-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 08/11/2014 Terms of Use: http://spiedl.org/terms VIRUS instruments are feed by as many as 43,008 optical fibers that initiate at the focal surface and are mechanically coupled and guided by strain reliefs in route to VIRUS6; the strain reliefs define the fiber optic support system. The fiber optic support system is instrumental in the successful operation of the HETDEX survey. This system has three primary objectives: efficiently guide fibers creating the shortest route to VIRUS, prevent fiber stress that could result in FRD, and integrate to telescope components without adding to light obscuration. Fiber management is important because studies have concluded that mechanical deformation of optical fibers can result in FRD7-10. Predicting the fiber behavior for HETDEX is very difficult due the size and complexity of the fiber system. Long fiber runs, large motion envelopes and thousands of optical fibers will likely complicate the mechanical behavior of the fibers in ways that require better understanding before full implementation on the telescope. A life cycle test will be conducted to study fiber behavior and measure focal ratio degradation as a function of time. The life cycle test will guide the detailed design of the fiber optic support system as well as provide results illustrating the effects of accelerated load histories on performance of optical fibers. 2. FIBER OPTIC SUPPORT SYSTEM 2.1 System overview There are three points on the telescope that structurally support the IFUs. The locations of the IFU end terminations and these three points define the HETDEX optical fiber system routing from its input at the focal surface to its output at the VIRUS cabinets. The three points of mechanical coupling and structural support are referred to as strain reliefs and are identified from input to output as Strain Relief 1 (SR1), Strain Relief 2 (SR2), and Strain Relief 3 (SR3), Figure 1. Figure 1. The HETDEX optical fiber system is illustrated above. Strain Relief 1, Strain Relief 2, and Strain Relief 3 are colored light blue. The fiber bundles routed from the focal surface to VIRUS are colored dark blue. The HETDEX optical fiber system path consists of two runs that hang from the tracker. Each run contains up to 48 IFUs that retain a total of 21,504 optical fibers6. An IFU consists of 448 optical fibers, spiral wound aluminum conduit, and a PolyVinyl Carbonate (PVC) jacket11. Internally, Kevlar sheathing protects the fibers by reducing friction and preventing abrasion between the fibers and the aluminum conduit. The sheathing is also being used to determine the overall length of the conduit since it is dimensionally more stable along its length than the conduit. Fiber core diameter is 266 microns resulting in a 30% conduit fill factor, or the ratio of total fiber cross section area to conduit cross section area6. The Proc. of SPIE Vol. 7735 77354W-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 08/11/2014 Terms of Use: http://spiedl.org/terms unoccupied volume within the conduit permits differences in relative length and bend radius between conduit and fiber as each flexes during HET operation. The conduit outside diameter is 17.2 mm with a mass per unit length of 300 g/m11. The fibers within each conduit have a mass per unit length of 80 g/m11. The average IFU length is approximately 21 m to accommodate telescope movement and the distance between the focal surface and virus cabinets. Figure 2. The image on the left is a sketch of the IFU Focal Surface Mount plate. The fiber input head and output region are displayed in the pictures to the right. The optical fibers are coupled to the conduit at the input end through a component called the "hub". At the exit end the optical fibers exit the conduit and are terminated at the VIRUS cabinets that are located on each side of the telescope structure. Between the input and exit ends the fibers are shielded by the conduit, but are not coupled to the conduit. Three strain reliefs positioned between the IFU input and exit provide mechanical coupling to support and control the IFU conduit. The IFU's have been designed and fabricated in partnership with Astrophysikalisches Institut Potsdam (AIP). Detailed information regarding the IFUs can be located in the "Production and performance of replicable integral field units for VIRUS", reference 11. For fiber management, preventing tight bend radii is important to prevent FRD and ultimately; fiber damage. The fiber optic support system will allow no less than a bend radius of 150 mm during operation. The conduit will permit a minimum bend radius of 100 mm during installation and handling while the individual fibers have been designed to handle bend radii less than 100 mm without jeopardizing optical performance. The optical fiber system routing is summarized in Table 1. The fiber run is split into segments between each Mechanical Coupling Point (MCP) for reference throughout the paper. The behavior of fibers in each segment is listed; "static" implies that there is no relative movement between the points on either end of that segment while "dynamic" means that relative movement between the coupling points exists. Table 1. 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 2.2 Strain Relief 1 Strain Relief 1 attaches to the top of the telescope and securely clamps and guides the conduit out away from the surrounding telescope structure and equipment. SR1 isolates the input end of the fibers at the focal surface from motions of the tracker and Prime Focus Instrument Package (PFIP). This structure carries the weight of the IFUs, fibers and conduit, between the focal surface and strain relief 2, as well as a majority of the weight of the fiber from Strain Relief 2 to Strain Relief 3.
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