Proc. SPIE. 9911, Modeling, Systems Engineering, and Project Management for Astronomy VI
KEYWORDS: Actuators, Telescopes, Mirrors, 3D modeling, Space telescopes, Finite element methods, Computer aided design, Large Synoptic Survey Telescope, Large Synoptic Survey Telescope, Systems modeling, Solid modeling
During this early stage of construction of the Large Synoptic Survey Telescope (LSST), modeling has become a crucial system engineering process to ensure that the final detailed design of all the sub-systems that compose the telescope meet requirements and interfaces. Modeling includes multiple tools and types of analyses that are performed to address specific technical issues. Three-dimensional (3D) Computeraided Design (CAD) modeling has become central for controlling interfaces between subsystems and identifying potential interferences. The LSST Telescope dynamic requirements are challenging because of the nature of the LSST survey which requires a high cadence of rapid slews and short settling times. The combination of finite element methods (FEM), coupled with control system dynamic analysis, provides a method to validate these specifications. An overview of these modeling activities is reported in this paper including specific cases that illustrate its impact.
This paper describes the status and details of the large synoptic survey telescope1,2,3 mount assembly (TMA). On June 9th, 2014 the contract for the design and build of the large synoptic survey telescope mount assembly (TMA) was awarded to GHESA Ingeniería y Tecnología, S.A. and Asturfeito, S.A. The design successfully passed the preliminary design review on October 2, 2015 and the final design review January 29, 2016. This paper describes the detailed design by subsystem, analytical model results, preparations being taken to complete the fabrication, and the transportation and installation plans to install the mount on Cerro Pachón in Chile. This large project is the culmination of work by many people and the authors would like to thank everyone that has contributed to the success of this project.
Motivated by a desire to improve the KPNO Mayall 4m telescope’s pointing and tracking performance prior to the start of the DESI installation and by a need to improve the maintainability of its telescope control system (TCS), we recently completed a major modernization of that system based heavily on recent changes made at the CTIO Blanco 4m, as described by Warner et al (2012). We describe here the things we did differently from the Blanco upgrade. We also present results from the as-built performance of the new servo and pointing systems.
We describe design modifications to the SOAR telescope intended to reduce the impact of future major earthquakes, based on the facility’s experience during recent events, most notably the September 2015 Illapel earthquake. Specific modifications include a redesign of the encoder systems for both azimuth and elevation, seismic trigger for the emergency stop system, and additional protections for the telescope secondary mirror system. The secondary mirror protection may combine measures to reduce amplification of seismic vibration and “fail-safe” components within the assembly. The status of these upgrades is presented.
In recent years the V. M. Blanco 4-m telescope at Cerro Tololo Inter-American Observatory (CTIO) has been renovated for use as a platform for a completely new suite of instruments: DECam, a 520-megapixel optical imager, COSMOS, a multi-object optical imaging spectrograph, and ARCoIRIS, a near-infrared imaging spectrograph. This has had considerable impact, both internally to CTIO and for its wider community of observers. In this paper, we report on the performance of the renovated facility, ongoing improvements, lessons learned during the deployment of the new instruments, how practical operations have adapted to them, unexpected phenomena and subsequent responses. We conclude by discussing the role for the Blanco telescope in the era of LSST and the new generation of extremely large telescopes.
The Large Synoptic Survey Telescope (LSST) is a large (8.4 meter) wide-field (3.5 degree) survey telescope, which will be located on the Cerro Pachón summit in Chile. Both the Secondary Mirror (M2) Cell Assembly and Camera utilize hexapods to facilitate optical positioning relative to the Primary/Tertiary (M1M3) Mirror. A rotator resides between the Camera and its hexapod to facilitate tracking. The final design of the hexapods and rotator has been completed by Moog CSA, who are also providing the fabrication and integration and testing. Geometric considerations preclude the use of a conventional hexapod arrangement for the M2 Hexapod. To produce a more structurally efficient configuration the camera hexapod and camera rotator will be produced as a single unit. The requirements of the M2 Hexapod and Camera Hexapod are very similar; consequently to facilitate maintainability both hexapods will utilize identical actuators. The open loop operation of the optical system imposes strict requirements on allowable hysteresis. This requires that the hexapod actuators use flexures rather than more traditional end joints. Operation of the LSST requires high natural frequencies, consequently, to reduce the mass relative to the stiffness, a unique THK rail and carriage system is utilized rather than the more traditional slew bearing. This system utilizes two concentric tracks and 18 carriages.
The response of the SOAR telescope to the September 2015 Illapel earthquake is documented and placed in the context of other recent, nearby seismic events. Accelerometer data collected on the telescope during these events suggest that observed intensities due to events occurring to the south of the SOAR telescope site are higher than predicted by simple models. Amplification of accelerations occurs at several places within the telescope system, most notably the telescope top end and secondary mirror assembly, and the azimuth encoder system. Damage in these areas is described, and an overview of the earthquake recovery effort is presented.
The Large Synoptic Survey Telescope (LSST) has a 10 degrees square field of view which is achieved through a 3 mirror optical system comprised of an 8.4 meter primary, 3.5 meter secondary (M2) and a 5 meter tertiary mirror. The M2 is a 100mm thick meniscus convex asphere. The mirror surface is actively controlled by 72 axial electromechanical actuators (axial actuators). Transverse support is provided by 6 active tangential electromechanical actuators (tangent links). The final design has been completed by Harris Corporation. They are also providing the fabrication, integration and testing of the mirror cell assembly, as well as the figuring of the mirror. The final optical surface will be produced by ion figuring. All the actuators will experience 1 year of simulated life testing to ensure that they can withstand the rigorous demands produced by the LSST survey mission. Harris Corporation is providing optical surface metrology to demonstrate both the quality of the optical surface and the correctablility produced by the axial actuators.
The Large Synoptic Survey Telescope instrument include four guiding and wavefront sensing subsystems called corner
raft subsystems, in addition to the main science array of 189 4K x 4K CCDs. These four subsystems are placed at the
four corners of the instrumented field of view. Each wavefront/guiding subsystem comprises a pair of 4K x 4K guide
sensors, capable of producing 9 frames/second, and a pair of offset 2K x 4K wavefront curvature sensors from which the
images are read out at the cadence of the main camera system, providing 15 sec integrations. These four
guider/wavefront corner rafts are mechanically and electrically isolated from the science sensor rafts and can be installed
or removed independently from any other focal plane subsystem. We present the implementation of this LSST
subsystem detailing both hardware and software development and status.
TripleSpec 4 (TS4) is a near-infrared (0.8um to 2.45um) moderate resolution (R ~ 3200) cross-dispersed spectrograph
for the 4m Blanco Telescope that simultaneously measures the Y, J, H and K bands for objects reimaged
within its slit. TS4 is being built by Cornell University and NOAO with scheduled commissioning in 2015.
TS4 is a near replica of the previous TripleSpec designs for Apache Point Observatory's ARC 3.5m, Palomar
5m and Keck 10m telescopes, but includes adjustments and improvements to the slit, fore-optics, coatings and
the detector. We discuss the changes to the TripleSpec design as well as the fabrication status and expected
sensitivity of TS4.
The Large Synoptic Survey Telescope (LSST) is an 8.4 meter, 3.5 degree, wide-field survey telescope. The survey mission requires a short slew, settling time of 5 seconds for a 3.5 degree slew. Since it does not include a fast steering mirror, the telescope has stringent vibration limitations during observation. Meeting these requirements will be facilitated by a stiff compact Telescope Mount Assembly (TMA) riding on a robust pier and by added damping. The TMA must also be designed to facilitate maintenance. The design is an altitude over azimuth welded and bolted assembly fabricated from mild steel.
In preparation for the arrival of the Dark Energy Camera (DECam) at the CTIO Blanco 4-m telescope, both the hardware
and the software of the Telescope Control System (TCS) have been upgraded in order to meet the more stringent
requirements on cadence and tracking required for efficient execution of the Dark Energy Survey1. This upgrade was
also driven by the need to replace obsolete hardware, some of it now over half a century old.
In this paper we describe the architecture of the new mount control system, and in particular the method used to develop
and implement the servo-driver portion of the new TCS. This portion of the system had to be completely rethought,
when an initial approach, based on commercial off the shelf components, lacked the flexibility needed to cope with the
complex behavior of the telescope. Central to our design approach was the early implementation of extensive telemetry,
which allowed us to fully characterize the real dynamics of the telescope. These results then served as input to extensive
simulations of the proposed new servo system allowing us to iteratively refine the control model. This flexibility will be
important later when DECam is installed, since this will significantly increase the moving mass and inertia of the
Based on these results, a fully digital solution was chosen and implemented. The core of this new servo hardware is
modern cRIO hardware, which combines an embedded processor with a high-performance FPGA, allowing the
execution of LabVIEW applications in real time.
The Large Synoptic Survey Telescope will be located on a seismically active Chilean mountain. Seismic ground
accelerations produce the telescope's most demanding load cases. Consequently, accurate prediction of these
accelerations is required. These seismic accelerations, in the form of Peak Spectral Acceleration (PSA), were compared
for site specific surveys, the Chilean building codes and measured seismic accelerations. Methods were also investigated
for adjusting for variations in damping level and return period. The return period is the average interval of time between
occurrences of a specific intensity.
Results from determining the optical turbulence profile (OTP) on the LSST site, El
Peñon, located on Cerro Pachón (Chile) are presented. El Peñón appears to be an
excellent observatory site with a surface layer seeing contribution on the order of 0.15”
with most of this seeing being produced below 20m. These measurements also helped to
confirm that the telescope is elevated high enough above ground. As part of the LSST site
characterization campaign, microthermal measurements were taken in order to determine
the contribution of the surface layer turbulence to the atmospheric seeing. Such
measurements are commonly used for this purpose where pairs of microthermal sensors
mounted on a tower measure atmospheric temperature differences. In addition, the lunar
scintillometer LuSci was installed on El Peñon for short campaigns near full moon for the
same purpose. LuSci is a turbulence profiler based on measuring spatial correlation of
moonlight scintillations. The comparison of the results from both instruments during
simultaneous data acquisition showed a remarkable temporal correlation and very similar
The 3.5-meter diameter Large Synoptic Survey Telescope (LSST) secondary (M2) mirror utilizes a 100mm thick
meniscus ULE™ blank completed by Corning Incorporated in 2009. Sub-aperture interferometry will guide the
polishing process to meet mirror structure function requirements. The convex asphere is actively supported by 72
axial actuators and 6 tangential links. These tangent links utilize an embedded lever system to meet the
requirements. The axial actuators have force limiting devices. The control system utilizes a higher level "outer loop
controller" for monitoring and commanding the tangent links and axial actuators. Numerous sensors determine the
assembly status. To prevent thermally induced image degradation, the interior air of the M2 cell is conditioned.
The Dark Energy Camera (DECam) is a new prime focus, wide-field imager for the V. M. Blanco 4-m telescope at CTIO. Instrumentation includes large, five-lens optical corrector mounted on hexapod mechanism for fine adjustment, filters, and a 519 Megapixel camera vessel; all integrated in a cage similar to the existing telescope prime focus structure. Currently Blanco allows a flip of this structure such that the f/8 secondary mirror, mounted on the back of the cage, points towards the primary mirror for Ritchey-Chretien observations. DECam will maintain this capability by attaching the existing F/8 mirror cell to the front of the new cage. Installation of this 8,600 kg instrument required the removal from the telescope of the primary mirror, the removal of the old prime focus assembly, and fine adjustment of large, over-constrained mechanisms followed by reassembly. A large facility shutdown was scheduled for this upgrade and several tools, fixtures, monitoring systems and procedures were developed in order to identify and then recover the optical alignment of the system, to control the distribution of stresses during tuning of the installation and to maintain the balance of the telescope with significant added mass. The final goal has been to maintain high performance of the telescope for both the existing f/8 Ritchey-Chretien focus mounted instruments and the new DECam instrument now in commissioning. The challenges presented in handling large elements, real-time monitoring, alignment, verification and feedback are described.
The LSST Telescope has critical requirements on tracking error to meet image quality specifications, and will require
closing a guiding loop, with the telescope servo control, to meet its mission. The guider subsystem consists of eight
guiding sensors located inside the science focal plane at the edge of the 3.5deg field of view. All eight sensors will be
read simultaneously at a high rate, and a centroid average will be fed to the telescope and rotator servo controls, for
tracking error correction. A detailed model was developed to estimate the sensors centroid noise and the resulting
telescope tracking error for a given frame rate and telescope servo control system.
The centroid noise depends on the photo-electron flux, seeing conditions, and guide sensor specifications. The model for
the photo-electron flux takes into consideration the guide star availability at different galactic latitudes, the atmospheric
extinction, the optical losses at different filter bands, the detector quantum efficiency, the integration time and the
number of stars sampled. A 7-layer atmospheric model was also developed to estimate the atmospheric decorrelation
between the different guide sensors due to the 3.5deg field of view, to predict both correlated and decorrelated
atmospheric tip/tilt components, and to determine the trade-offs of the guider servo loop.
The SOAR telescope fast tip-tilt tertiary mirror, was delivered by the Goodrich Optical and Space Systems Division,
Danbury, CT, and integrated into the SOAR optical system in 2004. It consist of a plane, light weighted 655×470 mm
elliptical mirror, controllable over a range of ±1 mrad, in two axes, with a required position loop bandwidth of 50 Hz. It
operates using the signal from a fast read-out guide camera to generate position commands, in an outer loop fashion.
The original tertiary mirror controller consisted of several analog circuit boards, incorporating the position control loop
compensation, and power amplifiers. This system was limited by the difficulty of making any modifications, to optimize
the control loop, and meet the required bandwidth. The analog controller was replaced with a digital controller based on
a National Instruments Compact RIO/FPGA device. This allows the full optimization of the control system, and also
allows closing the torque (acceleration) loop using the optical feedback of the guide signal alone, which should result in
even higher performance. This paper will describe the models, design, and performance tests, of the new digital control
The very short slew times and resulting high inertial loads imposed upon the Large Synoptic Survey Telescope (LSST) create new challenges to the primary mirror support actuators. Traditionally large borosilicate mirrors are supported by pneumatic systems, which is also the case for the LSST. These force based actuators bear the weight of the mirror and provide active figure correction, but do not define the mirror position. A set of six locating actuators (hardpoints) arranged in a hexapod fashion serve to locate the mirror. The stringent dynamic requirements demand that the force actuators must be able to counteract in real time for dynamic forces on the hardpoints during slewing to prevent excessive hardpoint loads. The support actuators must also maintain the prescribed forces accurately during tracking to maintain acceptable mirror figure. To meet these requirements, candidate pneumatic cylinders incorporating force feedback control and high speed servo valves are being tested using custom instrumentation with automatic data recording. Comparative charts are produced showing details of friction, hysteresis cycles, operating bandwidth, and temperature dependency. Extremely low power actuator controllers are being developed to avoid heat dissipation in critical portions of the mirror and also to allow for increased control capabilities at the actuator level, thus improving safety, performance, and the flexibility of the support system.
The Blanco 4-meter telescope has been in operation for over 30 years and is now subject to an extensive upgrade of its
control system, both of the hardware and software aspects. The motivation for the upgrade, besides the normal
replacement of obsolete components, is the preparation of the telescope for the installation of the DECAM instrument,
which makes greater operational demands than can't be met by the current system. The architecture of the new system is
in line with the designs proposed for modern telescopes like the Large Synoptic Survey Telescope (LSST), and its
implementation utilizes similar technologies as proposed for that project. In this paper we present a detailed description
of the upgraded system, including tape encoders, control algorithms, the use of trajectories to optimize motions,
communications middleware, and its performance as a whole.
The CTIO V. M. Blanco 4-m telescope is to be the host facility for the Dark Energy Survey (DES), a large area optical
survey intended to measure the dark energy equation of state parameter, w. The survey is expected to use ~30% of the
telescope time over 5 years and use a new 520 megapixel CCD prime focus imaging system: the Dark Energy Camera
(DECam). The Blanco telescope will also be the southern hemisphere platform for NEWFIRM, a large area infrared
imager currently being commissioned at the Mayall Telescope at KPNO. As part of its normal cycle of continuing
upgrades and in preparation for the arrival of these new instruments, the Blanco telescope control system (TCS) will be
upgraded to provide a modern platform for observations and maximize the efficiency of survey operations. The
upgraded TCS will be based on that used at the SOAR telescope and will be a prototype of the TCS to be used by LSST.
It will be optimized for programmed and queued survey observations, will provide extended real-time telemetry of site
and facility characteristics, and will incorporate a distributed observer interface allowing for on- and off-site
observations and real time quality control. Hardware modifications will include the use of absolute tape encoders and a
modern servo control and power driver systems.
A wind pressures PSD measured on the Gemini South Telescope was applied to the FEA model of
the LSST telescope to determine the RMS motions of the principal optical systems. These motions
were then converted to the time domain. The time domain motions were analyzed in the ZEMAX®
software to determine the wind induced image degradation. This degradation was shown to be
Heidenhain position tape encoders are in use on almost all modern telescopes with excellent results. Performance of
these systems can be limited by minor mechanical misalignments between the tape and read head causing errors at the
grating period. The first and second harmonics of the measured signal are the dominant errors, and have a varying
frequency dependant on axis rate. When the error spectrum is within the mount servo bandwidth it results in periodic
telescope pointing jitter. This paper will describe an adaptive error correction using elliptic interpolation of the raw
signals, based on the well known compensation technique developed by Heydemann . The approach allows the
compensation to track in real time with no need of a large static look-up table, or frequent calibrations. This paper also
presents the results obtained after applying this approach on data measured on the SOAR telescope.
An active tangent link system was developed to provide transverse support for large thin meniscus mirrors. The support
system uses six tangent links to control position and distribute compensating force to the mirror. Each of the six tangent
links utilizes an electromechanical actuator and an imbedded lever system working through a load cell and flexure. The
lever system reduces the stiffness, strength and force resolution requirements of the electromechanical actuator and
allows more compact packaging. Although all six actuators are essentially identical, three of them are operated quasi
statically, and are only used to position the optic. The other three are actively operated to produce an optimal and
repeatable distribution of the transverse load. This repeatable load distribution allows for a more effective application of
a look up table and reduces the demands on the active optics system.
A control system was developed to manage the quasi static force equilibrium servo loop using a control matrix that
computes the displacements needed to correct any force imbalance with good convergence and stability.
This system was successfully retrofitted to the 4.3 meter diameter, 100 mm thick SOAR primary mirror to allow for
more expeditious convergence of the mirror figure control system. This system is also intended for use as the transverse
support system for the LSST 3.4 meter diameter thin meniscus secondary mirror.
The Large Synoptic Survey Telescope (LSST) will be a large, wide-field ground-based telescope designed to obtain sequential images of the entire visible sky every few nights. The LSST, in spite of its large field of view and short 15 second exposures, requires a very accurate pointing and tracking performance. The high efficiency specified for the whole system implies that observations will be acquired in blind pointing mode and tracking demands calculated from blind pointing as well.
This paper will provide a high level overview of the LSST Control System (LCS) and details of the Telescope Control System (TCS), explaining the characteristics of the system components and the interactions among them. The LCS and TCS will be designed around a distributed architecture to maximize the control efficiency and to support the highly robotic nature of the LSST System. In addition to its control functions, the LCS will capture, organize and store system wide state information, to make it available for monitoring, evaluation and calibration processes. An evaluation of the potential communications middleware software to be utilized for data transport, is also included.
The Infrared Side Port Imager ISPI is a facility infrared imager for
the CTIO Blanco 4-meter telescope. ISPI has the following capabilities: 1-2.4 micron imaging with an 2K x 2K HgCdTe array, 0.3
arcsec/pixel sampling matched to typical f/8 IR image quality of ~0.6
arcsec and a 10.5 x 10.5 arcmin field of view. First light with ISPI
was obtained on September 24 2002, and since January 2003 ISPI has
been in operation as a common user instrument. In this paper we discuss operational aspects of ISPI, the behavior of the array and we report on the performance of ISPI during the first one and half year of operation.
The 4.1-meter SOuthern Astrophysical Research (SOAR) Telescope mount and drive systems have been commissioned and are in routine operation. The telescope mount, the structure and its full drive systems, was fully erected and tested at the factory prior to reassembly and commissioning at the observatory. This successful approach enabled complete integration, from a concrete pier to a pointing and tracking telescope, on the mountain, in a rapid 3-month period. The telescope mount with its high instrument payload and demanding efficiency requirements is an important component for the success of the SOAR scientific mission. The SOAR mount utilizes rolling element bearings for both azimuth and elevation support, counter torqued sets of gear motors on azimuth and two frameless torque motors built into the elevation axles. Tracking jitter and its associated spectra, pointing errors and their sources, bearing friction and servo performances are critical criteria for this mount concept and are important factors in achieving the mission. This paper addresses the performance results obtained during the integration, commissioning, and first light periods of the telescope mount system.
Development of the 4.1 meter SOuthern Astrophysical Research (SOAR) Telescope is now complete. All baseline systems are in place and extensive commissioning activities have been performed with and without the primary optics installed in the telescope. The facility and dome have been under observatory operations and TCS control for a year of testing and tuning. The altitude over azimuth telescope mount was integrated on the mountain in a rapid 3-month period due to the complete assembly and testing performed at the factory prior to delivery. Early mount testing and successful integration into the Telescope Control System (TCS) without the optical system was accomplished on the sky through use of two separate small aperture telescopes fixed to the structure. One of these, the "feed telescope" was also pivotal in early testing of the calibration wavefront sensor and SOAR optical imager by directing focused light to these separate instruments. The SOAR optical system, with its 4.1 meter clear aperture, 100 cm thick, ULEtm primary mirror, its lightweight ULEtm secondary, and its fast tip tilt ULEtm tertiary has been delivered and installed in the telescope. This system was also assembled as an electrically connected system and individually optically tested under a visible interferometer at the factory enabling rapid integration and a short commissioning period on telescope. In this paper we present the project status, a summary of the commissioning period, and the performance data for the completed telescope and its major components.
The MONSOON Image Acquisition System has been designed to meet the need for scalable, multichannel, high-speed image acquisition required for the next-generation optical and infared detectors and mosaic projects currently under development at NOAO as described in other papers at this proceeding such as ORION, NEWFIRM, QUOTA, ODI and LSST. These new systems with their large scale (64 to 2000 channels) and high performance (up to 1Gbyte/s) raise new challenges in terms of communication bandwidth, data storage and data processing requirements which are not adequately met by existing astronomical controllers. In order to meet this demand, new techniques for not only a new detector controller, but rather a new image acquisition architecture, have been defined. These extremely large scale imaging systems also raise less obvious concerns in previously neglected areas of controller design such as physical size and form factor issues, power dissipation and cooling near the telescope, system assembly/test/ integration time, reliability, and total cost of ownership. At NOAO we have taken efforts to look outside of the astronomical community for solutions found in other disciplines to similar classes of problems. A large number of the challenges raised by these system needs are already successfully being faced in other areas such as telecommunications, instrumentation and aerospace. Efforts have also been made to use true commercial off the shelf (COTS) system elements, and find truly technology independent solutions for a number of system design issues whenever possible. The Monsoon effort is a full-disclosure development effort by NOAO in collaboration with the CARA ASTEROID project for the benefit of the astronomical community.
The new operations model for the CTIO Blanco 4-m telescope will use a small suite of fixed facility instruments for imaging and spectroscopy. The Infrared Side Port Imager, ISPI, provides the infrared imaging capability. We describe the optical, mechanical, electronic, and software components of the instrument. The optical design is a refractive camera-collimator system. The cryo-mechanical packaging integrates two LN2-cooled dewars into a compact, straightline unit to fit within space constraints at the bent Cassegrain telescope focus. A HAWAII 2 2048 x 2048 HgCdTe array is operated by an SDSU II array controller. Instrument control is implemented with ArcVIEW, a proprietary LabVIEW-based software package. First light on the telescope is planned for September 2002.