Ian M. Soukup

Current status of the Hobby-Eberly Telescope wide-field upgrade

The Hobby-Eberly Telescope (HET) is an innovative large telescope of 9.2 meter aperture, located in West Texas at the McDonald Observatory (MDO). The HET operates with a fixed segmented primary and has a tracker which moves the four-mirror corrector and prime focus instrument package to track the sidereal and non-sidereal motions of objects. A major upgrade of the HET is in progress that will increase the pupil size to 10 meters and the field of view to 22′ by replacing the corrector, tracker and prime focus instrument package. In addition to supporting the existing suite of instruments, this wide field upgrade will feed a revolutionary new integral field spectrograph called VIRUS, in support of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEXχ). This paper discusses the current status of this upgrade.

Design and development of a long-travel positioning actuator and tandem constant force actuator safety system for the Hobby Eberly Telescope wide-field upgrade

The Wide Field Upgrade presents a five-fold increase in mass for the Hobby-Eberly Telescopes* tracker system. The design of the Hobby-Eberly Telescope places the Prime Focus Instrument Package (PFIP) at a thirty-five degree angle from horizontal. The PFIP and its associated hardware have historically been positioned along this uphill axis (referred to as the telescopes Y-axis) by a single screw-type actuator. Several factors, including increased payload mass and design for minimal light obscuration, have led to the design of a new and novel configuration for the Y-axis screw-drive as part of the tracker system upgrade. Typical screw-drive designs in this load and travel class (approximately 50 kilonewtons traveling a distance of 4 meters) utilize a stationary screw with the payload translating with the moving nut component. The new configuration employs a stationary nut and translating roller screw affixed to the moving payload, resulting in a unique drive system design. Additionally, a second cable-actuated servo drive (adapted from a system currently in use on the Southern African Large Telescope) will operate in tandem with the screw-drive in order to significantly improve telescope safety through the presence of redundant load-bearing systems. Details of the mechanical design, analysis, and topology of each servo drive system are presented in this paper, along with discussion of the issues such a configuration presents in the areas of controls, operational and failure modes, and positioning accuracy. Findings and results from investigations of alternative telescope safety systems, including deformable crash barriers, are also included.

Design, testing, and installation of a high-precision hexapod for the Hobby-Eberly Telescope dark energy experiment (HETDEX)

Engineers from The University of Texas at Austin Center for Electromechanics and McDonald Observatory have designed, built, and laboratory tested a high payload capacity, precision hexapod for use on the Hobby-Eberly telescope as part of the HETDEX Wide Field Upgrade (WFU). The hexapod supports the 4200 kg payload which includes the wide field corrector, support structure, and other optical/electronic components. This paper provides a recap of the hexapod actuator mechanical and electrical design including a discussion on the methods used to help determine the actuator travel to prevent the hexapod payload from hitting any adjacent, stationary hardware. The paper describes in detail the tooling and methods used to assemble the full hexapod, including many of the structures and components which are supported on the upper hexapod frame. Additionally, details are provided on the installation of the hexapod onto the new tracker bridge, including design decisions that were made to accommodate the lift capacity of the Hobby- Eberly Telescope dome crane. Laboratory testing results will be presented verifying that the performance goals for the hexapod, including positioning, actuator travel, and speeds have all been achieved. This paper may be of interest to mechanical and electrical engineers responsible for the design and operations of precision hardware on large, ground based telescopes. In summary, the hexapod development cycle from the initial hexapod actuator performance requirements and design, to the deployment and testing on the newly designed HET tracker system is all discussed, including lessons learned through the process.

Design of the fiber optic support system and fiber bundle accelerated life test for VIRUS

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 telescopes 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.

The influence of motion and stress on optical fibers

We report on extensive testing carried out on the optical fibers for the VIRUS instrument. The primary result of this work explores how 10+ years of simulated wear on a VIRUS fiber bundle affects both transmission and focal ratio degradation (FRD) of the optical fibers. During the accelerated lifetime tests we continuously monitored the fibers for signs of FRD. We find that transient FRD events were common during the portions of the tests when motion was at telescope slew rates, but dropped to negligible levels during rates of motion typical for science observation. Tests of fiber transmission and FRD conducted both before and after the lifetime tests reveal that while transmission values do not change over the 10+ years of simulated wear, a clear increase in FRD is seen in all 18 fibers tested. This increase in FRD is likely due to microfractures that develop over time from repeated flexure of the fiber bundle, and stands in contrast to the transient FRD events that stem from localized stress and subsequent modal diffusion of light within the fibers. There was no measurable wavelength dependence on the increase in FRD over 350 nm to 600 nm. We also report on bend radius tests conducted on individual fibers and find the 266 μm VIRUS fibers to be immune to bending-induced FRD at bend radii of R 10 cm. Below this bend radius FRD increases slightly with decreasing radius. Lastly, we give details of a degradation seen in the fiber bundle currently deployed on the Mitchell Spectrograph (formally VIRUS-P) at McDonald Observatory. The degradation is shown to be caused by a localized shear in a select number of optical fibers that leads to an explosive form of FRD. In a few fibers, the overall transmission loss through the instrument can exceed 80%. These results are important for the VIRUS instrument, and for both current and proposed instruments that make use of optical fibers, particularly when the fibers are in continual motion during an observation, or experience repeated mechanical stress during their deployment.≥

HETDEX tracker control system design and implementation

To enable the Hobby-Eberly Telescope Dark Energy Experiment, The University of Texas at Austin Center for Electromechanics and McDonald Observatory developed a precision tracker and control system – an 18,000 kg robot to position a 3,100 kg payload within 10 microns of a desired dynamic track. Performance requirements to meet science needs and safety requirements that emerged from detailed Failure Modes and Effects Analysis resulted in a system of 13 precision controlled actuators and 100 additional analog and digital devices (primarily sensors and safety limit switches). Due to this complexity, demanding accuracy requirements, and stringent safety requirements, two independent control systems were developed. First, a versatile and easily configurable centralized control system that links with modeling and simulation tools during the hardware and software design process was deemed essential for normal operation including motion control. A second, parallel, control system, the Hardware Fault Controller (HFC) provides independent monitoring and fault control through a dedicated microcontroller to force a safe, controlled shutdown of the entire system in the event a fault is detected. Motion controls were developed in a Matlab-Simulink simulation environment, and coupled with dSPACE controller hardware. The dSPACE real-time operating system collects sensor information; motor commands are transmitted over a PROFIBUS network to servo amplifiers and drive motor status is received over the same network. To interface the dSPACE controller directly to absolute Heidenhain sensors with EnDat 2.2 protocol, a custom communication board was developed. This paper covers details of operational control software, the HFC, algorithms, tuning, debugging, testing, and lessons learned.

Use of failure modes and effects analysis in design of the tracker system for the HET wide-field upgrade

In support of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), the Center for Electromechanics at The University of Texas at Austin was tasked with developing the new Tracker and control system to support the HETDEX Wide-Field Upgrade. The tracker carries the 3,100 kg Prime Focus Instrument Package and Wide Field Corrector approximately 13 m above the 10 m diameter primary mirror. Its safe and reliable operation by a sophisticated control system, over a 20 year life time is a paramount requirement for the project. To account for all potential failures and potential hazards, to both the equipment and personnel involved, an extensive Failure Modes and Effects Analysis (FMEA) was completed early in the project. This task required participation of all the stakeholders over a multi-day meeting with numerous follow up exchanges. The event drove a number of significant design decisions and requirements that might not have been identified this early in the project without this process. The result is a system that has multiple layers of active and passive safety systems to protect the tens of millions of dollars of hardware involved and the people who operate it. This paper will describe the background of the FMEA process, how it was utilized on HETDEX, the critical outcomes, how the required safety systems were implemented, and how they have worked in operation. It should be of interest to engineers, designers, and managers engaging in complex multi-disciplinary and parallel engineering projects that involve automated hardware and control systems with potentially hazardous operating scenarios.

Testing, characterization, and control of a multi-axis, high precision drive system for the Hobby-Eberly Telescope Wide Field Upgrade

A multi-axis, high precision drive system has been designed and developed for the Wide Field Upgrade to the Hobby- Eberly Telescope at McDonald Observatory. Design, performance and controls details will be of interest to designers of large scale, high precision robotic motion devices. The drive system positions the 20-ton star tracker to a precision of less than 5 microns along each axis and is capable of 4 meters of X/Y travel, 0.3 meters of hexapod actuator travel, and 46 degrees of rho rotation. The positioning accuracy of the new drive system is achieved through the use of highprecision drive hardware in addition to a meticulously tuned high-precision controller. A comprehensive understanding of the drive structure, disturbances, and drive behavior was necessary to develop the high-precision controller. Thorough testing has characterized manufacture defects, structural deflections, sensor error, and other parametric uncertainty. Positioning control through predictive algorithms that analytically compensate for measured disturbances has been developed as a result of drive testing and characterization. The drive structure and drive dynamics are described as well as key results discovered from testing and modeling. Controller techniques and development of the predictive algorithms are discussed. Performance results are included, illustrating recent performance of several axes of the drive system. This paper describes testing that occurred at the Center for Electromechanics in Austin Texas.

Design and analysis of the tracker bridge for the Hobby-Eberly Telescope* wide field upgrade

A large structural weldment has been designed to serve as the new star tracker bridge for the Wide Field Upgrade to the Hobby-Eberly Telescope at McDonald Observatory in support of the Hobby-Eberly Telescope Dark Energy Experiment‡. The modeling approach, analysis techniques and design details will be of interest to designers of large structures where stiffness is the primary design driver. The design includes detailed structural analysis using finite element models to maximize natural frequency response and limit deflections and light obscuration. Considerable fabrication challenges are overcome to allow integration of precision hardware required for positioning the corrector optics to a precision of less than 5 microns along the 4-meter travel range. Detailed descriptions of the bridge geometry, analysis results and challenging fabrication issues are discussed.

Wind loading analysis and strategy for deflection reduction on HET wide field upgrade

Wind loading can be a detrimental source of vibration and deflection for any large terrestrial optical telescope. The Hobby-Eberly Telescope* (HET) in the Davis Mountains of West Texas is undergoing a Wide Field Upgrade (WFU) in support of the Dark Energy Experiment‡ (HETDEX) that will greatly increase the size of the instrumentation subjected to operating wind speeds of up to 20.1 m/s (45 mph). A non-trivial consideration for this telescope (or others) is to quantify the wind loads and resulting deflections of telescope structures induced under normal operating conditions so that appropriate design changes can be made. A quasi-static computational fluid dynamics (CFD) model was generated using wind speeds collected on-site as inputs to characterize dynamic wind forces on telescope structures under various conditions. The CFD model was refined until predicted wind speed and direction inside the dome agreed with experimental data. The dynamic wind forces were then used in static loading analysis to determine maximum deflections under typical operating conditions. This approach also allows for exploration of operating parameters without impact to the observation schedule of the telescope. With optimum combinations of parameters (i.e. dome orientation, tracker position, and louver deployment), deflections due to current wind conditions can be significantly reduced. Furthermore, the upper limit for operating wind speed could be increased, provided these parameters are monitored closely. This translates into increased image quality and observing time.