The preliminary concept development phase of the Southern African Large Telescope (SALT) mini-tracker (MT) project was completed. The addition of up to four MTs to the telescope would in effect create multiple four-to-six-meter class telescopes using SALT’s 10-m diameter primary mirror. Each MT would be able to provide spectroscopic follow up for current and future large astronomical surveys (e.g. MeerKAT, eROSITA, Gaia, LSST, SKA, etc.). This phase included development of a novel optical design for the spherical aberration corrector, preliminary mechanical design for the telescope interface and the MTs themselves, and simulation tools to calculate the effective illumination of each MT for a selected target. A detailed project management plan and documentation framework were also created, including a prototype development path, a project cost estimate, and a schedule to completion. Following a review of the project near the end of this concept development phase, the decision was taken to put the project on hold. Although the MTs were deemed to be technically feasible, a more detailed science case was required in order to proceed with the project. In addition, several personnel-intensive projects to improve the performance and reliability of the telescope, either now underway or soon to be started, would need to be completed prior to beginning a project of this magnitude and complexity. However, a number of valuable tools and results that will benefit SALT emerged from this concept development phase, and are outlined here.
The addition of multiple “mini-trackers” (MTs) to the Southern African Large Telescope (SALT) would create in effect several four-to-six-meter class telescopes that take advantage of the SALT 10-meter diameter primary mirror’s 35 degree diameter uncorrected field-of-view. These devices, with a 100 square degree patrol area, would provide valuable follow up capability for the large astronomical surveys either in operation (e.g. MeerKAT, eROSITA, Gaia), or expected to begin operations soon (e.g. LSST, SKA, Euclid). A feasibility study was conducted to evaluate the technical practicality associated with the design, fabrication, integration, and testing of a prototype MT for SALT. The study determined that the development of a mini-tracker was indeed feasible, and work has begun on the concept design phase of the project.
Astronomical instrumentation development used to be much simpler than it is today. The quest for new discoveries and more light has driven the design and construction of new generations of ever-larger telescopes, which in turn created the need for correspondingly larger and more complex instruments. Large instrument teams composed of scientists and engineers from many technical disciplines have been brought together to design and build these instruments. Engineers trained in these disciplines have become key members of instrument development teams, taking responsibility for these areas of instrument design. With the engineers came an engineering culture and way of thinking that is often at odds with the scientific culture of astronomers. Project management techniques can help organize such an effort, but they have important limitations in a research environment and cannot ensure success. Training in the so-called “soft skills” can improve how a diverse team functions, but this, too, is not the complete answer. Only by immersing the engineer in an observing environment can one hope to overcome the cultural differences and inherent conflicts between scientists and engineers that can cause instrument projects to fail.
The MeerKAT radio telescope array, the Large Synoptic Survey Telescope (LSST), and eventually the Square Kilometer Array (SKA) will usher in a remarkable new era in astronomy, with thousands of transients being discovered and transmitted to the astronomical community in near-real-time each night. Immediate spectroscopic follow-up will be critical to understanding their early-time physics – a task to which the Southern African Large Telescope (SALT) is uniquely suited, given its southerly latitude and the 14-degree-diameter uncorrected field (patrol area) of its 10-m spherical primary mirror. A new telescope configuration is envisioned, incorporating multiple “mini-trackers” that range around a much larger patrol area of 35 degrees in diameter. Each mini-tracker is equipped with a small spherical aberration corrector feeding an efficient, low resolution spectrograph to perform contemporaneous follow-up observations.
Considerable effort has gone into improving the performance and reliability of the SAAO’s 74-inch telescope. This included replacing the telescope encoders, refining the pointing model and increasing the telescope throughput. The latter involved re-aluminising the primary and formulating a procedure to ensure optimal alignment of the telescope mirrors. To this end, we developed the necessary hardware and techniques to ensure that such alignment is achieved and maintained, particularly following re-aluminising of the mirrors. In essence, the procedure involves: placing a Taylor Hobson Alignment Telescope on the mechanical rotation axis of the 74-inch (which we define to be the optical axis, since the Cassegrain instruments attach to the associated turntable), then adjusting the tip/tilt of the secondary mirror to get it onto that axis and, lastly, adjusting the tip/tilt of the primary mirror to eliminate coma. An eyepiece (or wavefront camera) is installed at the Cassegrain port for this final step since comatic star images indicate the need to tip/tilt the primary mirror to align it to the secondary. Tuning out any brightness gradients seen in an out-of-focus image of a bright star may also be used for feedback when adjusting the tip/tilt of the primary mirror to null coma.
The Hobby-Eberly Telescope (HET) is an innovative large telescope located in West Texas at the McDonald Observatory. The HET operates with a fixed segmented primary and has a tracker, which moves the four-mirror optical 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 substantially increase the pupil size to 10 meters (from 9.2 m) and the field of view to 22 arcminutes (from 4 arcminutes) by replacing the corrector, tracker, and prime focus instrument package. In addition to supporting existing instruments, and a new low resolution spectrograph, 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§). The upgrade is being installed and this paper discusses the current status.
A major upgrade of the HET is in process that increases the pupil size to 10 meters and the field of view to 22’ by replacing the four-mirror corrector and prime focus instrument package to track the sidereal and non-sidereal motions of astronomical targets. To support the new payload a new Tracker, comprising 13 axes, and weighing 20 tons, was designed, built and tested at the University of Texas Center for Electromechanics, in Austin, Texas. It was then disassembled and installed on the HET. Structural modifications were performed on the upper hexagon of the telescope structure to support the net increase of 15% to the total mass of the system and maintain fundamental mode performance of 5Hz. Testing in the laboratory, as well as subsequent commissioning tests on the HET, confirm that the Tracker will position the payload to acquire and track within the specified +/-9.5um de-center, +/-15um de-focus, and +/-4.4 arc-sec tip/tilt requirement*.
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.
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.
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.
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.
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.
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.
Hexapod systems (6 legged Stewart Platforms), offer advantages in accuracy over other positioning systems and are
finding applications in numerous telescopes. However, instruments with increased sophistication for modern telescopes
continue to grow in size and required positioning accuracy. This paper details an alternative hexapod configuration and
design approach, particularly focused on relatively large, high precision hexapod systems supporting high mass
payloads. The new configuration improves accuracy, reduces actuator mass, simplifies design, and reduces system cost
but requires modest additional control algorithm sophistication.
A 4-mirror prime focus corrector is under development to provide seeing-limited images for the 10-m aperture Hobby-
Eberly Telescope (HET) over a 22 arcminute wide field of view. The HET uses an 11-m fixed elevation segmented
spherical primary mirror, with pointing and tracking performed by moving the prime focus instrument package (PFIP)
such that it rotates about the virtual center of curvature of the spherical primary mirror. The images created by the
spherical primary mirror are aberrated with 13 arcmin diameter point spread function. The University of Arizona is
developing the 4-mirror wide field corrector to compensate the aberrations from the primary mirror and present seeing
limited imaged to the pickoffs for the fiber-fed spectrographs. The requirements for this system pose several challenges,
including optical fabrication of the aspheric mirrors, system alignment, and operational mechanical stability.
Hexapods are finding increased use in telescope applications for positioning large payloads. Engineers from The
University of Texas at Austin have been working with engineers from ADS International to develop large, high force,
highly precise and controllable hexapod actuators for use on the Wide Field Upgrade (WFU) as part of the Hobby Eberly
Telescope Dark Energy Experiment (HETDEX). These actuators are installed in a hexapod arrangement, supporting
the 3000+ kg instrument payload which includes the Wide Field Corrector (WFC), support structure, and other
optical/electronic components. In addition to force capability, the actuators need to meet the tracking speed (pointing)
requirements for accuracy and the slewing speed (rewind) requirements, allowing as many observations in one night as
possible. The hexapod actuator stroke (retraction and extension) was very closely monitored during the design phase to
make sure all of the science requirements could be met, while minimizing the risk of damaging the WFC optical
hardware in the unlikely event of a hexapod actuator or controller failure. This paper discusses the design trade-offs
between stiffness, safety, back-drivability, accuracy, and leading to selection of the motor, high ratio worm gear, roller
screw, coupling, end mounts, and other key components.
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.
To enable the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), the McDonald Observatory (MDO) and
the Center for Electro-mechanics (CEM) at the University of Texas at Austin are developing a new HET tracker in
support of the Wide-Field Upgrade (WFU) and the Visible Integral-Field Replicable Unit Spectrograph (VIRUS). The
precision tracker is required to maintain the position of a 3,100 kg payload within ten microns of its desired position
relative to the telescope's primary mirror. The hardware system to accomplish this has ten precision controlled
actuators. Prior to installation on the telescope, full performance verification is required of the completed tracker in
CEM's lab, without a primary mirror or the telescope's final instrument package. This requires the development of a
laboratory test stand capable of supporting the completed tracker over its full range of motion, as well as means of
measurement and methodology that can verify the accuracy of the tracker motion over full travel (4m diameter circle,
400 mm deep, with 9 degrees of tip and tilt) at a cost and schedule in keeping with the HET WFU requirements. Several
techniques have been evaluated to complete this series of tests including: photogrammetry, laser tracker, autocollimator,
and a distance measuring interferometer, with the laser tracker ultimately being identified as the most viable method.
The design of the proposed system and its implementation in the lab is presented along with the test processes, predicted
accuracy, and the basis for using the chosen method*.
The upcoming Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) has provided motivation for upgrading
the Hobby-Eberly Telescope (HET) at the McDonald Observatory. This upgrade includes an increase in
the field-of-view to accommodate the new and revolutionary Visible Integral-field Replicable Unit Spectrograph
(VIRUS). VIRUS is the instrument designed to conduct the HETDEX survey and consists of 150 individual
integral-field spectrographs fed by 33,600 total optical fibers covering the 22 arc-minute field-of-view of the
upgraded HET. The spectrographs are mounted in four enclosures, each 6.0×3.0×1.4 meters in size. Each
spectrograph contains a CCD detector that must be cryogenically cooled, presenting an interesting cryogenic
and vacuum challenge within the distribution system. In this paper, we review the proposed vacuum jacketed,
thermal siphon, liquid nitrogen distribution system used to cool the array of detectors and discuss recent developments.
We focus on the design, prototyping, and testing of a novel "make-break" thermal connector, built
from a modified cryogenic bayonet, that is used to quickly detach a single spectrograph pair from the system.
The Visible Integral-Field Replicable Unit Spectrograph (VIRUS) is an integral field spectrograph to support
observations for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). The VIRUS instrument is fed by
more than 33,000 optical fibers and consists of 150 spectrographs in 75 individual, identical units. This paper discusses
the evolution in mechanical design of the VIRUS unit spectrographs to maximize the cost benefit from volume
production. Design features which enable volume manufacture and assembly are discussed. Strategies for reducing
part count while enabling precision alignment are detailed. Design considerations for deployment, operation, and
maintenance en mass at the Hobby-Eberly Telescope are also made. In addition, several enabling technologies are
described including the use of cast aluminum in vacuum housings, use of cast Invar, and processing cast parts for
precision tolerances.
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).
KEYWORDS: Mirrors, Telescopes, Monochromatic aberrations, Image quality, Cameras, Simulation of CCA and DLA aggregates, Image segmentation, Wavefront sensors, Wavefronts, Interfaces
Construction of the Southern African Large Telescope (SALT) was largely completed by the end of 2005 and since then
it has been in intensive commissioning. This has now almost been completed except for the telescope's image quality
which shows optical aberrations, chiefly a focus gradient across the focal plane, along with astigmatism and other less
significant aberrations. This paper describes the optical systems engineering investigation that has been conducted since
early 2006 to diagnose the problem. A rigorous approach has been followed which has entailed breaking down the
system into the major sub-systems and subjecting them to testing on an individual basis. Significant progress has been
achieved with many components of the optical system shown to be operating correctly. The fault has been isolated to a
major optical sub-system. We present the results obtained so far, and discuss what remains to be done.
The Hobby-Eberly Telescope (HET) is an existing innovative large telescope of 9.2 meter aperture, located at the
McDonald Observatory in West Texas. The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) requires a
major upgrade to the HET, including a substantial increase in the telescope field of view, as well as the development and
integration of a revolutionary new integral field spectrograph called VIRUS. The Visible Integral-Field Replicable Unit
Spectrograph (VIRUS) is an instrument comprising approximately 150 individual IFU-fed spectrographs which will be
mounted on the telescope structure. Each spectrograph has a CDD camera detector package which must be cryogenically
cooled during scientific operation. In order to cool each of these camera systems a liquid nitrogen system has been
proposed and design study completed. The proposed system includes: a liquid nitrogen source, vacuum jacket
distribution system, local storage on the telescope, and distribution under a thermal siphon to the individual
spectrographs and local thermal connectors.
The Hobby-Eberly Telescope (HET) is an innovative large telescope of 9.2 meter aperture, located in West Texas at the
McDonald Observatory. The HET operates with a fixed segmented primary and has a tracker which moves the fourmirror
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 substantially increase the field of view 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.
We describe the design, construction, and performance of VIRUS-P (Visible Integral-field Replicable Unit
Spectrograph - Prototype), the prototype for 150+ identical fiber-fed integral field spectrographs for the Hobby-Eberly
Telescope Dark Energy Experiment (HETDEX). VIRUS-P was commissioned in 2007, is in regular service on the
McDonald Observatory 2.7 m Smith telescope, and offers the largest field of any integral field spectrograph. The 246-fiber IFU uses a densepak-type fiber bundle with a 1/3 fill factor. It is fed at f/3.65 through a telecentric, two-group
dioptric focal reducer. The spectrograph's double-Schmidt optical design uses a volume phase holographic grating at
the pupil between the articulating f/3.32 folded collimator and the f/1.33 cryogenic prime focus camera. High on-sky
throughput is achieved with this catadioptric system by the use of high reflectivity dielectric coatings, which set the
340-670 nm bandwidth. VIRUS-P is gimbal-mounted on the telescope to allow short fibers for high UV throughput,
while maintaining high mechanical stability. The instrument software and the 18 square arcmin field, fixed-offset guider
provide rapid acquisition, guiding, and precision dithering to fill in the IFU field. Custom software yields Poisson noise
limited, sky subtracted spectra. The design characteristics are described that achieved uniformly high image quality with
low scattered light and fiber-to-fiber cross talk. System throughput exceeds requirements and peaks at 40%. The
observing procedures are described, and example observations are given.
The Hobby-Eberly Telescope (HET) is an innovative large telescope of 9.2 meter aperture, located in West Texas at McDonald Observatory. 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. The HET has been taking science data for six years. Work over the past two years has improved performance significantly, replacing the mirror coatings and installing metrology equipment to provide feedback that aids tracking and alignment of the primary mirror segments. The first phase of HET instrumentation includes three facility instruments: the Low Resolution Spectrograph (LRS), the Medium Resolution Spectrograph (MRS), and High Resolution Spectrograph (HRS). The current status of these instruments is described. A major upgrade of HET is planned that will increase the field of view to 22 arcminutes diameter, replacing the corrector, tracker and prime focus instrument package. 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).
The HET is a modified Arecibo-style telescope with a segmented spherical primary and a four-mirror spherical
aberration corrector (SAC). Objects are tracked by driving the SAC along the focal sphere of the primary. In the original
design of the telescope the alignment of the SAC was to be maintained passively. In practice, this could not be done to
specifications, leading to degraded imaging quality. We have developed a metrology system to actively control the
alignment of the SAC. An autocollimator maintains the optical axis of the SAC normal to the primary mirror beneath it.
An absolute distance measuring interferometer (DMI) monitors the SAC/primary mirror distance, maintaining focus.
Both systems work at a wavelength of 1.5 microns, well above the operating wavelength of current or planned science
instruments and therefore do not interfere with observations. The performance of the system is measured via Hartmann
testing.
Several upgrades are implemented in the primary mirror control system, including calibration of individual edge
sensors, new control system software, and a new method of setting and controlling the overall radius of curvature of the
primary array. New techniques were developed to efficiently piston the segments onto the proper sphere radius.
A major performance upgrade for the Hobby-Eberly Telescope (HET) is in the conceptual design phase. The extensive upgrade will include a wide field optical corrector, a new HET tracker with increased payload capacity, and improved telescope pointing and tracking accuracy. The improvements will support the HET Dark Energy Experiment (HETDEX), which seeks to characterize the evolution of dark energy by mapping the imprint of baryonic oscillations on the large scale structure of the Universe. HETDEX will use the increased field-of-view and payload to feed an array of approximately 145 fiber-fed spectrometers, called VIRUS for "Visible Integral field Replicable Unit Spectrograph". The new corrector will have a science field-of-view diameter of 18 arcminutes, in contrast to the original corrector's 4 arcminute field, a twenty-fold increase in area. A new HET tracker with increased payload capacity will be designed to support the wide field corrector. Improved pointing and tracking will be accomplished using new autocollimation and distance measuring metrology combined with real-time wavefront sensing and correction. The upgrade will maintain operation of the current suite of facility instruments, consisting of low, medium, and high resolution spectrometers.
The Hobby-Eberly Telescope (HET) is a fixed-elevation, 9.2-m telescope with a spherical primary mirror and a tracker at prime focus to follow astronomical objects. The telescope was constructed for $13.9M over the period 1994-1997. A series of extensive engineering upgrades and corrective actions have been completed recently, resulting in significantly improved delivered image quality and increased operational efficiency. The telescope's Spherical Aberration Corrector (SAC) optics were recoated with a highly reflective and durable broadband coating at Lawrence Livermore National Laboratory. The software mount model that maintains optical alignment of the SAC with the 11-m primary mirror array was recalibrated and improved. The acquisition and guiding optics for both the High Resolution Spectrograph (HRS) and the Low Resolution Spectrograph (LRS) were reworked and improved, allowing for better focus and SAC alignment monitoring and control. Recoating of the primary mirror segment array was begun. Telescope images of 0.82 arcseconds have been recorded for sustained periods in preliminary testing following the engineering upgrade, an improvement of 50% over previous best performance. Additional engineering upgrades are scheduled to consolidate these performance gains and to continue improving delivered image quality, throughput, and telescope operational efficiency. The HET is now capable of the science performance for which it was designed.
KEYWORDS: Sensors, Mirrors, Space telescopes, Telescopes, Control systems, Calibration, Sensor calibration, Temperature metrology, Image quality, Process control
The Segment Alignment Maintenance System (SAMS) is a control system to maintain the alignment of the 91 segment Hobby-Eberly Telescope (HET) primary mirror array. The system was developed by Blue-Line Engineering (Colorado Springs, CO) and NASA-Marshall Space Flight Center (Huntsville-Al). The core of the system is a set of 480 inductive edge sensors which measure relative shear between adjacent segments. The relative shear is used to calculate segment tip/tilt and piston corrections. Although the system has dramatically improved the performance of the HET it does not meet its error budget due to thermal drifts in the sensors. The system is now sufficiently stable that it routinely requires only one primary mirror alignment at the beginning of the night. We describe methods to calibrate this sensor drift.
KEYWORDS: Telescopes, Mirrors, Image segmentation, Domes, Mars, Image quality, Simulation of CCA and DLA aggregates, Sensors, Electronics, Optical instrument design
The Hobby-Eberly Telescope (HET) is a fixed-elevation, 9.2-m telescope with a spherical primary mirror and a tracker at prime focus to follow astronomical objects. The telescope was constructed for $13.9M over the period 1994-1997. A number of telescope performance deficiencies were identified and corrected following construction. Remaining problems included: 1) Dome seeing, 2) inadequate initial mirror segment alignment accuracy, and 3) mirror segment misalignment with time. The HET Completion Project was created in May 2001 to attack these problems and to identify and solve the next tier of problems. To address dome seeing, large louvers were installed and in operation by May 2002. Efforts are also underway to eliminate or suppress heat sources within the dome environment. To address segment alignment accuracy, a prototype Shack-Hartmann device, the Mirror Alignment Recovery System (MARS), was built and is in routine use at HET. The Segment Alignment Maintenance System (SAMS) is in early operation and has markedly improved telescope performance. Two Differential Image Motion Monitor (DIMM) telescopes were brought into regular operation in July 2001 to quantify atmospheric seeing at HET. As these improvements have been implemented, telescope image quality has improved significantly. Plans are in place to address additional performance issues.
The Hobby-Eberly Telescope (HET) enclosure is receiving a series of modifications to improve dome seeing, including removal of residual heat loads from the optical path, increased insulation of the enclosure, and ventilation of the enclosure ring wall and dome. Analysis indicates that the contribution of dome seeing with the new system will be less than 0.05 arc seconds. The design of the HET enclosure lends itself to very large ventilation openings in the stationary portion of the enclosure also referred to as the "ring wall", with supplementary openings in the rotating dome. The ventilation design implemented has opened approximately 58% of the ring wall, and will open 8% of the dome, in the coming year, in order to achieve natural ventilation of 22 air changes per hour at the minimum design wind velocity of 3.5 mph. A system of ventilation louvers similar in design to the Kitt Peak Observatories 4-meter telescope was determined to be the most practical and cost effective design for use in the ring wall. Conventional off-the-shelf louvers are proposed for the dome, due to constraints in its design. Special considerations for retrofitting an operating facility included a custom hoist mounted on the dome for installation of the louvers (5000lb./ea.), and an inflatable curtain to protect the telescope during foul weather. The ring wall ventilation system has been in full operation since early April 2002 and is part of a program in progress to substantially improve the HET dome seeing.
The Segment Alignment Maintenance System (SAMS) was installed on McDonald Observatory's Hobby-Eberly Telescope (HET) in August 2001. The SAMS became fully operational in October 2001. The SAMS uses a system of 480 inductive edge sensors to correct misalignments of the HET's 91 primary mirror segments when the segments are perturbed from their aligned reference positions. A special observer estimates and corrects for the global radius of curvature (GRoC) mode, a mode unobservable by the edge sensors. The SAMS edge sensor system and GRoC estimator are able to maintain HET's primary figure for longer durations than previously had been observed. This paper gives a functional description of the SAMS control system and presents performance verification data.
KEYWORDS: Mirrors, Image segmentation, Simulation of CCA and DLA aggregates, Polarization, Cameras, Telescopes, Semiconductor lasers, Phase shifts, Interferometers, Space telescopes
The Center of Curvature Alignment Sensor (CCAS) was the original instrument installed in the center of curvature (CoC) tower on the Hobby-Eberly Telescope (HET) for aligning the 91 primary mirror segments. The CCAS is a polarization shearing interferometer with HeNe and diode laser sources that illuminate the HET primary mirror with polarized coherent light. Returns from each mirror segment focus back at the CoC and can be viewed on a faceplate at the front of the instrument for coarse alignment of the primary mirror, or sent into the interferometer for fine alignment. Inside the interferometer, Wollaston prisms separate the HET primary mirror image into two polarization components which are spatially shifted by the distance of one mirror segment. This overlaps images of segments with their neighbors to generate interference fringes. The beam is then split into 4 legs, each of which introduces phase shifts to the polarization. Fringe patterns shifted by 0, 90, 180, and 270 degrees are recorded on each leg by a CCD camera. The intensity in each pixel is measured and used in the standard 4-bucket algorithm to calculate the relative phase shift between the two mirror segments, and thus their tip/tilt misalignment. Segment piston is determined from the location of the peak in the fringe contrast function, using all four camera images and light at four laser diode wavelengths.
Although the CCAS has recently been replaced with a Shack-Hartmann sensor for mirror alignment on the HET, its operation and performance are described. Under less environmentally challenging conditions, such as laboratory or space-based applications, this instrument could be used for aligning segmented mirrors to high precision.
KEYWORDS: Mirrors, Sensors, Telescopes, Space telescopes, Control systems, Image segmentation, Simulation of CCA and DLA aggregates, Image quality, Calibration, Observatories
A sensing and control system for maintaining the optical alignment of the ninety-one 1-meter diameter hexagonal segments forming the Hobby-Eberly Telescope (HET) primary mirror array has been developed by NASA - Marshall Space Flight Center (Huntsville, AL) and Blue Line Engineering (Colorado Springs, CO) and implemented. This Segment Alignment Maintenance System (SAMS) employs 480 edge sensors to measure the relative shear motion between each segment edge pair and compute individual segment tip, tilt and piston position errors. Error information is sent to the HET primary mirror control system, which then corrects the physical position of each segment every 90 seconds. On-site installation of the SAMS sensors, ancillary electronics and software was completed in September 2001. Since that time, SAMS has undergone engineering testing. The system has operated almost nightly, improving HET's overall operational capability and image quality performance. SAMS has not yet, however, demonstrated performance at the specified levels for tip, tilt, piston and Global Radius of Curvature (GRoC) maintenance. Additional systems development and in situ calibration are expected to bring SAMS to completion and improved operation performance by the end of this year.
A long term program to quantify the intrinsic site seeing at McDonald Observatory, using two differential image motion monitors (DIMMs) has been initiated on Mt. Fowlkes where the Hobby-Eberly Telescope (HET) is located. Raw DIMM data are corrected to the zenith and to a uniform 10msec integration time. Nightly median seeing measurements (FWHM) along with the max/min range are presented for 186 nights over the 13 month period between July 2001 and July 2002. A definite seasonal effect is present in the dataset with the median seeing in the spring-summer-fall months (0.93±0.18 arcsec) being significantly better than the winter months (1.24±0.33 arcsec). The measured seeing was better than 0.70 arcsec about 9% of the time. Since DIMM units were operated at ground level these data are not quite lower limits to the site seeing performance. Even so, the seeing of this West Texas continental site at 6,650ft (2,027m) elevation in the Davis Mountains is superior to what has been assumed in the past, based on less direct seeing measurements.
Future plans are described for moving a DIMM telescope to a tower mounted, semi-automated observatory to sample the site seeing at an elevation above the ground similar to the HET mirror.
KEYWORDS: Mirrors, Image segmentation, Mars, Telescopes, Simulation of CCA and DLA aggregates, Beam splitters, Wavefront sensors, Collimators, Sensors, Image quality
The Mirror Alignment Recovery System (MARS) is a Shack-Hartmann based sensor at the center of curvature (CoC) of the Hobby-Eberly Telescope (HET) spherical primary mirror used to align the 91 mirror segments. The instrument resides in a CoC tower next to the HET dome, a location which provides a challenging set of problems including wind shake and seeing from two different domes. The system utilizes an internal light source to illuminate the HET and a reference mirror to provide focused spot locations from a spherical surface. A custom lenslet array is sized to the HET pupil image, matching a single hexagonal lenslet to each mirror segment. Centroids of the HET mirror segment spots are compared to the reference spot locations to measure tip/tilt misalignments of each segment. A MARS proof-of-concept (POC) instrument, tested on the telescope in 2001, utilized a commercial wavefront sensor from Adaptive Optics Associates. The final system uses the same concept, but is customized for optimal performance on the HET.
MARS replaces previous burst-antiburst alignment techniques and provides a more intuitive method of aligning the primary mirror for telescope operators. The POC instrument has improved median HET stack sizes by 0.3" EE50, measured at the CoC tower. The current alignment accuracy is 0.14" rms (0.28" rms on the sky), resolution is 0.014", measurement precision is 0.027" rms, and segment capture range is ± 5". With continuing improvements in HET dome ventilation and the addition of software customized for removal of tower motion during measurement, the alignment accuracy is expected to reach approximately 0.04" rms in the final MARS, to be installed in late 2002.
KEYWORDS: Mirrors, Solids, Image segmentation, Telescopes, Simulation of CCA and DLA aggregates, Image quality, Space telescopes, Mirror mounts, Observatories, Optical instrument design
To improve the image quality performance of the Hobby-Eberly Telescope's (HET) segmented primary mirror and to assist in the requirements definition for an optical alignment sensing and control system, multiple engineering tests have been designed and executed. The most significant of these tests have been the alignment maintenance baseline and solid mount tests. Together, these engineering tests defined the complex thermal and non-thermal response modes of the steel HET primary mirror truss and quantified the performance of the segment support system. We discuss the configuration and performance of the HET primary mirror, and discuss our engineering test motivation, goals, design, implementation and results. We also discuss the implications of our primary mirror performance test results for conceptually similar next generation telescope designs, such as the Extremely Large Telescope.
KEYWORDS: Telescopes, Image segmentation, Mirrors, Control systems, Domes, Simulation of CCA and DLA aggregates, Image quality, Cameras, Computing systems, Camera shutters
The HET is unique among 9-meter class telescopes in featuring an Arecibo-like design with a focal surface tracker. The focal surface tracker causes image quality and pointing/tracking performance to interact in a complex way that has no precedent in astronomical telescope system design and that has presented unusual demands upon commissioning. The fixed-elevation, segmented primary-mirror array offers some simplifications over traditional telescope design in principle, but has presented challenges in practice. The sky access characteristics of the HET also place unique demands on observational planning and discipline. The HET is distinguished by uniquely low construction and operating costs which affected commissioning. In this contribution, we describe those aspects of our commissioning experience that may impact how similar telescopes are designed, especially those with larger aperture, and review the challenges and lessons learned from commissioning a 9-meter class telescope with a small technical team.
KEYWORDS: Mirrors, Sensors, Telescopes, Image segmentation, Control systems, Space telescopes, Telecommunications, Actuators, Digital signal processing, Process control
A sensing and control system for maintaining optical alignment of ninety-one 1-meter mirror segments forming the Hobby-Eberly Telescope (HET) primary mirror array is now under development. The Segment Alignment Maintenance System (SAMS) is designed to sense relative shear motion between each segment edge pair and calculated individual segment tip, tilt, and piston position errors. Error information is sent to the HET primary mirror control system, which corrects the physical position of each segment as often as once per minute. Development of SAMS is required to meet optical images quality specifications for the telescope. Segment misalignment over time is though to be due to thermal inhomogeneity within the steel mirror support truss. Challenging problems of sensor resolution, dynamic range, mechanical mounting, calibration, stability, robust algorithm development, and system integration must be overcome to achieve a successful operational solution.
The Hobby-Eberly telescope (HET) is a recently completed 9- meter telescope designed to specialize in spectroscopy. It saw first light in December 1996 and during July 1997, it underwent its first end-to-end testing acquiring its first spectra of target objects. We review the basic design of the HET. In addition we summarize the performance of the telescope used with a commissioning spherical aberration correlator and spectrograph, the status of science operations and plans for the implementation of the final spherical aberration corrector and facility class instruments.
KEYWORDS: Mirrors, Telescopes, Image segmentation, Image quality, Stars, Spectrographs, Control systems, Observatories, Simulation of CCA and DLA aggregates, Spherical lenses
Experience in bringing into operation the 91-segment primary mirror alignment and control system, the focal plane tracker system, and other critical subsystems of the HET will be described. Particular attention is given to the tracker, which utilizes three linear and three rotational degrees of freedom to follow sidereal targets. Coarse time-dependent functions for each axis are downloaded to autonomous PMAC controllers that provide the precise motion drives to the two linear stages and the hexapod system. Experience gained in aligning the sperate mirrors and then maintaining image quality in a variable thermal environments will also be described. Because of the fixed elevation of the primary optical axis, only a limited amount of time is available for observing objects in the 12 degrees wide observing band. With a small core HET team working with McDonald Observatory staff, efficient, reliable, uncomplicated methodologies are required in all aspects of the observing operations.
KEYWORDS: Telescopes, Mirrors, Stars, Control systems, Computer programming, Optical instrument design, Control systems design, Manufacturing, Space telescopes, Kinematics
A large prime-focus robotic star tracking device has been designed and constructed and is now undergoing commissioning atop the 9.2-meter Hobby-Eberly Telescope at McDonald Observatory in West Texas. The novel, cost-effective tracker represents a major departure in the way very large astronomical telescopes are controlled in pointing, tracking, and guiding. The tracker development and design implementation included detailed structural analysis, the application of minimum constraint kinematic design to a large gantry-type motion control system, and the unique application of a large precision hexapod to solve the dynamic tilting and focus motion problems. Challenging fabrication, test, and on-telescope assembly problems were overcome. Performance data of the completed device demonstrate that the tracker design and implementation efforts were successful.
The Spectroscopic Survey Telescope is being constructed by a consortium of universities at McDonald Observatory in the Davis Mountains of Texas. Principal partners are the University of Texas at Austin and the Pennsylvania State University. Also participating are Stanford University and the University of Munich and University of Gottingen in Germany. We describe the specific design attributes which enable the SST to be constructed for a fraction of the cost of astronomical telescopes of comparable size. Such unique features as identical spherical mirror segments, selective figuring for constant mirror mount deformation, air bearing azimuth rotation system, and pre-fabricated architectural type domes are employed. Emphasis is on simplification of design, reduction of part count and mass, and utilization of lessons learned from other recent large telescope projects.
A focal reducer which converts the f/13.5 primary Cassegrain beam of the McDonald 2.1 m telescope to f/3.0 is described. The resulting plate scale is 30 microns/arcsec, which is a good match to the resolution of typical CCDs for seeing conditions of about 1 arcsec. The present system uses a SID501 RCA CCD with 30-micron pixels; however, a TI 800 x 800 CCD with 15-micron pixels and other low-noise, small pixel devices will be used eventually. The focal reducer uses primarily reflective optics to provide good sensitivity down to the atmospheric cutoff at 3100A.
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