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.
The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX )at the University of Texas McDonald
Observatory will deploy the Visible Integral-Field Replicable Unit Spectrograph (VIRUS) to survey large areas of
sky. VIRUS consists of up to 192 spectrographs deployed as 96 units. VIRUS units are fiber-fed and are housed in
four enclosures making up the VIRUS Support Structure (VSS). Initial design studies established an optimal array
size and an upper and lower bound on their placement relative to the existing telescope structure. Tradeoffs
considering IFU (optical fiber) length, support structure mass and ease of maintenance have resulted in placement of
four 3 × 8 arrays of spectrograph pairs, about mid-point in elevation relative to the fixed HET structure. Because of
the desire to minimize impact on the modal performance of the HET, the VSS is required to be an independent, selfsupporting
structure and will only be coupled at the base of the telescope. Analysis shows that it is possible to
utilize the existing azimuth drives of the telescope, through this coupling, which will greatly simplify the design and
reduce cost. Each array is contained in an insulated enclosure that will control thermal load by means of heat
exchangers and use of facility coolant supply. Access for installation and maintenance on the top, front, and rear of
the enclosures must be provided. The design and analysis presented in this paper must provide an optimum balance
in meeting the stringent requirements for science and facility constraints such as cost, weight, access, and safety.
The Wide Field Upgrade presents a five-fold increase in mass for the Hobby-Eberly Telescope's* 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 telescope's 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.
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 (HETDEX). This paper discusses the current status of this
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.
The engineering and design of systems as complex as the Hobby-Eberly Telescope's* new tracker require that multiple
tasks be executed in parallel and overlapping efforts. When the design of individual subsystems is distributed among
multiple organizations, teams, and individuals, challenges can arise with respect to managing design productivity and
coordinating successful collaborative exchanges. This paper focuses on design management issues and current practices
for the tracker design portion of the Hobby-Eberly Telescope Wide Field Upgrade project. The scope of the tracker
upgrade requires engineering contributions and input from numerous fields including optics, instrumentation, electromechanics,
software controls engineering, and site-operations. Successful system-level integration of tracker subsystems
and interfaces is critical to the telescope's ultimate performance in astronomical observation. Software and process
controls for design information and workflow management have been implemented to assist the collaborative transfer of
tracker design data. The tracker system architecture and selection of subsystem interfaces has also proven to be a
determining factor in design task formulation and team communication needs. Interface controls and requirements
change controls will be discussed, and critical team interactions are recounted (a group-participation Failure Modes and
Effects Analysis [FMEA] is one of special interest). This paper will be of interest to engineers, designers, and managers
engaging in multi-disciplinary and parallel engineering projects that require coordination among multiple individuals,
teams, and organizations.
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.
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of a baseline build of 150 identical
spectrographs (arrayed as 75 units, each with a pair of spectrographs) fed by 33,600 fibers, each 1.5 arcsec diameter,
deployed over the 22 arcminute field of the upgraded 10 m Hobby-Eberly Telescope (HET). The goal is to deploy 96
units. VIRUS has a fixed bandpass of 350-550 nm and resolving power R~700. VIRUS is the first example of
industrial-scale replication applied to optical astronomy and is capable of spectral surveys of large areas of sky. The
method of industrial replication, in which a relatively simple, inexpensive, unit spectrograph is copied in large numbers,
offers significant savings of engineering effort, cost, and schedule when compared to traditional instruments.
The main motivator for VIRUS is to map the evolution of dark energy for the Hobby-Eberly Telescope Dark Energy
Experiment (HETDEX+) using 0.8M Lyman-α emitting galaxies as tracers. The full VIRUS array is due to be deployed
in late 2011 and will provide a powerful new facility instrument for the HET, well suited to the survey niche of the
telescope. VIRUS and HET will open up wide field surveys of the emission-line universe for the first time. We present
the design, cost, and current status of VIRUS as it enters production, and review performance results from the VIRUS
prototype. We also present lessons learned from our experience designing for volume production and look forward to
the application of the VIRUS concept on future extremely large telescopes (ELTs).
The Hobby-Eberly Telescope Dark Energy eXperiment [HETDEX] will employ over 43,000 optical fibers to feed light
to 192 Visible Integral-Field Replicable Unit Spectrographs [VIRUS]. Each VIRUS instrument is fed by 224 fibers. To
reduce cost, the spectrographs are combined into pairs; thus, two bundles of 224 fibers are combined into a single
Integral Field Unit [IFU] of 448 fibers. On the input end the fibers are arranged in a square 'dense-pack' array at the
HET focal surface. At the output end the IFU terminates in two separate linear arrays which provide entry slits for each
spectrometer unit. The IFU lengths must be kept to an absolute minimum to mitigate losses; however, consideration of
overall project cost and duration of the science mission have resulted in the generation of two competing concepts.
Multiple axes of motion are imposed on the IFUs as they span the shortest distance from the focal surface to each
VIRUS unit. Arranging and supporting 96 IFUs, that have a total mass over 450 kg, in a manner that is compatible with
these complex translations, together with the management of accompanying forces on the tracking mechanism of the
HET, presents a significant technical challenge, which is further compounded by wind buffeting. The longer IFU
concept is favored due to overall project cost, but requires tests to assure that the fibers can withstand forces associated
with a height differential of 16.25 meters without FRD losses or breakage.
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