This paper presents a comprehensive framework for integrated modeling, simulation and analysis of optical telescopes. This framework is called DOCS (Dynamics-Optics-Controls-Structures) and supports model development, model integration, analysis and multidisciplinary design optimization of this class of precision opto-mechanical systems. First the research background and literature in this young filed is discussed. Next the structure and nominal process of an integrated modeling, simulation and analysis study for a generic optical telescope using the DOCS framework is discussed in detail. The major steps include subsystems modeling, model assembly, model reduction and conditioning, initial performance assessment, sensitivity analysis, uncertainty analysis, redesign, design optimization and isoperformance analysis. Such a comprehensive analysis is demonstrated for the NEXUS Space Telescope precursor mission. This mission was designed as a technology testbed for the Next Generation Space Telescope. The challenge is to achieve a very tight pointing accuracy with a sub-pixel line-of-sight (LOS) jitter budget and a root-mean-square (RMS) wavefront error smaller than λ/50 despite the presence of electronic and mechanical disturbance sources. The framework suggested in this paper has the potential for becoming a general prescription for analyzing future, innovative telescope projects. Significant challenges remain in enabling fast simulations for large models, analytical sensitivity analysis for all sub-models, incorporation of slow-varying thermal or impulsive transient effects and the effective use of experimental results.
As the complexity of telescope systems have increased, system engineering trades related to cost and performance issues have become correspondingly complex. The traditional methodology for end-to-end system modeling depends upon focused analysis and data handoff between disciplines - aptly termed the “bucket brigade” approach.
For the last 7 years, Ball Aerospace has supported development of an integrated modeling environment for telescope performance modeling and analysis. The Integrated Telescope Model (ITM), a realization of this effort, has been used on several current large telescope programs such as the VLT, NGST, TPF and MAXIM. It permits the user to do both time simulations and analytical work in the spatial/temporal frequency domains. The individual discipline models in structural dynamics, optics, controls, signal processing, detector physics and disturbance modeling are seamlessly integrated into one cohesive model to efficiently support system level trades and analysis. The core of the model is formed by the optical toolbox implemented in MATLAB and realized in object-oriented Simulink environment. Both geometric and physical optical models can be constructed and interfaced to disturbances and detection models. The geometric approach includes ray tracing for exact modeling or sensitivity matrices for rapid execution. Spectral, transmission and polarization information is carried with each ray. The physical optics modules do wavefront propagation for analyzing diffraction effects under either with coherent or incoherent conditions. Coupling of the static offset models, quasi-static thermal deformations and structural dynamics with an optical model allows one to view the full range of disturbance effects on the resulting PSF.
This paper addresses the overall model architecture, considerations and issues related to model execution speed, complexity and model resolution/validity. Example of a recent use of the model is reviewed.
This article presents a software package for “integrated modeling” of single- and multi-aperture optical telescopes. Integrated modeling is aiming at time-dependent system analysis combining different technical disciplines such as optics, mechanical structure, control system with sensors and actuators. Various, environmental and internal disturbances can be taken into account. Software design and development is done in a joint effort by the European Southern Observatory (ESO), Astrium GmbH and the Institute of Lightweight Structures (LLB), Technical University of Munich. The architectures of the two most advanced modules generating dynamic models of the mechanical structure and the optical system are described. A “real-life” example related to the Very Large Telescope Interferometer (VLTI) illustrates the application in practice.
In cooperation with the European Southern Observatory (ESO), the Institute of Lightweight Structures (LLB), Technische Universtitaet Muenchen, has developed the Structural Modeling Interface Toolbox (SMI), a Matlab based software package for creation of a dynamical model of a telescope structure. It is called Interface, since it uses the modal data of a finite element (FE) analysis and creates a dynamic model to be used within a time-dependent control loop simulation in the Matlab/Simulink environment. SMI is part of the Integrated Modeling Toolbox (IMT) developed in a joint effort by ESO, Astrium GmbH and LLB.
Since SMI can read modal data in a general format, it is not depending on the FE-software. In addition to that, an interface to the FE-package ANSYS has been developed. It allows the variation of parameters and some settings for the FE-analysis directly within SMI.
Both, force excitation like windloads and base excitation like micro seismic perturbations can be included. Several tools for model reduction are provided. Some of them are modal based, like effective modal masses, others are general model reduction procedures from control engineering like balanced truncation.
For the evaluation of the reduced models, transfer functions of different models can be displayed in a Bode-plot. Time characteristics like step response or impulse response are also available. Moreover, for a typical excitation PSD the response PSD can be computed. This response can either be compared to the response of an exact model or to measured data and the rms-error can be calculated.
The final result is a linear statespace model of the structure and a Simulink block, which can be included into a Simulink model.
LOFAR, a new radio telescope, will be designed to observe with up to 8 independent beams, thus allowing several simultaneous observations. Scheduling of multiple observations parallel in time, each having their own constraints, requires a more intelligent and flexible scheduling function then operated before.
In support of the LOFAR radio telescope project, and in co-operation with Leiden University, Fokker Space has started a study to investigate the suitability of the use of evolutionary algorithms applied to complex scheduling problems. After a positive familiarization phase, we now examine the potential use of evolutionary algorithms via a demonstration project. Results of the familiarization phase, and the first results of the demonstration project are presented in this paper.
It is shown how an object-oriented approach to modelling in optics can help to create reusable and flexible code. We present a class system which has found its applicaiton as in the task of extended object imaging through the turbulent atmosphere, and the simulation of diffraction properties of the segmented telescope.
Ground-based telescopes operate in a turbulent atmosphere that affects the optical path across the aperture by changing both the mirror positions (wind induced vibrations) and the air refraction index. Although the characteristics of the atmosphere are well understood in the inertial range, the validity of the homogeneous, isotropic field assumption is questionable inside the enclosure and in the close vicinity of the structure. To understand the effect of wind on an actual telescope, we conducted extensive wind measurements at the Gemini South Telescope. Simultaneous measurements were made of pressures at multiple points on the mirror surface, as well as wind velocity and direction at several locations inside and outside the dome. During the test we varied the dome position relative to the wind, the telescope elevation angle, the position of windscreens in the observing slit, and the size of the openings in the ventilation gates. The data sets have been processed to provide the temporal and spatial characteristics of the pressure variations on the primary mirror in comparison to the theory of atmospheric turbulence. Our investigation is part of an effort leading to the development of a scalable wind model for large telescope simulations, which describes the forces due to air turbulence on the primary mirror and telescope structure reasonably well even inside an enclosure.
The Euro50 is a proposed optical telescope with an equivalent aperture of 50 m. It will have a segmented primary mirror and full adaptive optics. To study the interaction of the telescope structure, the control system and the optics, an integrated simulation model has been formulated. The mechanical model is a modal version of an Ansys finite element model. The optics model is based on ray tracing and physical optics. The segments model takes the alignment servos and the segment dynamics into account. Wind variation over the primary mirror is included. Segment control system modeling is in progress. First results clearly demonstrate that a good enclosure is needed to protect the telescope well against wind. The results also suggest that the segment alignment system must have a bandwidth well above the lowest eigenfrequencies of the telescope.
During the conceptual design of the GTC (Gran Telescopio Canarias) it was suggested to develop a Global Model of the behaviour of the GTC system to be used as a tool for the system engineering. This Global Model should be a dynamical simulation capable to predict the pointing, tracking, guiding and image quality of the GTC system in several simulation scenarios depending on the behavior of each subsystem. It was decided to develop the simulation in the Matlab/Simulink® environment. The kernel of the Global Model was a Simulink® model of the telescope mechanics. The model included the structural dynamics, control loops of the main axis (azimuth, elevation and rotators), and load models (wind, gravity, seism). Each component included error sources inherent to it (cogging and ripple on motors, encoding errors, bearing run-out, etc). The model permitted large rotations in elevation axis, which was necessary to test pointing performances. A specific simulation was developed within the project office for the analysis of the image quality of the optical system. It includes polishing defects of the optical surfaces (M1 segments, M2 and M3), low spatial frequency distortions of the optical surfaces (due to fabrication, gravity of instability) and misalignment between the primary mirror segments.
In this paper we describe the development of a C++ class library for the simulation of adaptive optics systems. This library includes functionality to simulate the propagation of electromagnetic waves through a randomly generated turbulent atmosphere and through an adaptive optical system. It includes support for extended emitters and laser guide stars, and for different types of wavefront sensors and reconstructors. The library also aims to support parallelization of simulations across symmetric multiprocessor and cluster supercomputers.
As part of an “Integrated Modeling Toolbox” described in Wilhelm, Koehler et al. 2002 (these proceedings) the optical modeling tool BeamWarrior has been developed. Its main purpose is the creation of optical models for integration into a dynamic control system simulation. Offering a versatile set of geometrical and wave optical propagation algorithms it can also be used for sophisticated static optical analysis. The article summarizes the functional features of the tool and describes its algorithms - both, from a theoretical and practical point of view.
The design of VISTA (Visible and Infrared Survey Telescope for Astronomy) requires close interaction between the science requirements, the optical and active mechanical design of the telescope and its instrumentation with the wavefront sensing. The optical design is based on an integrated approach of the telescope with tow separate cameras, one working in the IR waveband and the other working in the Visible waveband. The large field of view (2 degrees in the visible and 1.65 degrees in the IR), the seeing-limited resolution required (FWHM of 0.4 arcsec for the visible and 0.5 arcsec for the IR), the technological advance in active telescopes and large IR arrays and the f/1 quasi Ritchey-Chretien telescope design, makes this telescope a very powerful tool in performing high resolution and large astronomical surveys. A system analysis, modeling the various sources of errors such as optical aberrations, surface errors, control errors, environmental effects and detector effects is presented in this paper.
The Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST) is a national large scientific project in China at the beginning of this century. It is an unconventional designed modern optical telescope and has the both large field of view and large aperture. The spherical primary mirror MB in LAMOST is a segmented mirror with 37 sub-mirrors. The MB will be supported by a very stable truss structure and the mirror surface will be kept in a high optical accuracy. This paper presents the work on the finite element model of the truss structure of MB and gives the results of static and dynamic analysis with this model especially for the optimization of the higher stiffness and the lighter weight.
The Chinese ever-ambitious project of Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) has brought about a tremendous challenge for the software engineers. This paper describes the strategy of software simulation for the telescope control system as a whole, which is vital before the actual integration at the telescope with electronics, and mechanics. The development process of the simulator itself is envisioned from level-0 upgrade to level-2, and a demonstration of such a simulator at its level-0 phase is illustrated in detail in this paper.
We have developed an imaging simulation package for ALMA within the GILDAS software environment. The simulation takes into account most of the possible sources of errors affecting image quality: array configuration, pointing errors, tracking errors, atmospheric phase, gain calibration errors, primary beam errors, and thermal noise as well as the effects of observing strategies used to calibrate the zero order effects. Particular care has been taken to optimize for speed, since the evaluation of the imaging quality requires a large number of simulations. To do this, approximate, but sufficiently accurate solutions have been implemented rather than using the exact, but slower method.
The simulation package is driven by a simple GUI interface, on top of a command line interface. This system permits easy use by occasional users, as well as simple setup for large series of simulations. Example of simulation results are given, and the importance of chosen modeling methods are discussed.
The functional end-to-end modeling of large mm- and radio-telescopes extends beyond the simulation of the physical telescope structures and geometries. The complexity of the RF and IF chains, the digital signal processing systems and the increasing importance of data-processing pipelines forces us to model the full dataflow through the system. This is true is particular for telescopes based on phased array technology, where the physical antenna structure is static and relatively simple.
This paper distinguishes between various categories of modeling and simulation. In functional modeling the full functional breakdown of the system is simulated. This allows for the identification of bottlenecks in the flow, gives options for failure mode analysis, for the study of control topologies and real-time responsiveness and for the verification of interfaces and functional behavior. In fact a proper functional model is a virtual mockup of the entire system. This immediately points at a limitation of functional models. They usually do not operate in real-time on representative datavolumes. Analytical simulations take a number of defined shortcuts to generate representative datasets. Such simulations do not describe the system proper, but make motivated assumptions on e.g. the main disturbing factors. It is important to combine the results of the various categories, since they usually serve a mutual verifications.
The topics involved are illustrated with the ALMASim and LOFARSim simulators, both built on a generic C++ class library designed at ASTRON for large-scale functional simulations on cluster computers. ALMASim has been used in the feasibility study for the second generation ALMA correlators. LOFARSim is in use as an analysis and verification tool for the LOFAR telescope.
Integrated models are inherently complex and often obscure to any but those who write them. Their usefulness can be greatly enhanced through well-structured, object-oriented design. A robust and computationally efficient Simulink/C++ library of optics, control, finite-element, and visualization routines for modeling radio telescope performance under various operating conditions is being developed and is described.
The library is being developed in conjunction with an end-to-end model of the Atacama Large Millimeter Array (ALMA) antennas. The model includes the mechanical structure, optics, servos, and potential laser gyros, and can be used to investigate such issues as tracking performance, compliance with error budgets, wind sensitivity, and effectiveness of an internal metrology system. It will also be a good tool for comparison of different antenna designs.
In order to predict, optimize, control, and correct the performance of a radio telescope from a computer model it is necessary to consider the static influence of gravity and the time-variable influences of temperature and wind. An end-to-end model of a radio telescope, which includes these effects, can be used for design studies, but also for actual observational operation using as input the gravity load and measured temperature and wind distributions across the telescope structure. We discuss measurements of gravity, temperature, and wind effects which illustrate present and proposed possibilities to probe the actual state of a telescope, to be used in the integral model for instantaneous predictions. For the end-to-end modeling of a radio telescope several sub-sets of (FE) models are available; however, an integral model has yet to be constructed and proven to work.
At the IRAM 30m telescope on Pico Veleta, Spain, an extended net of temperature sensors has been installed in 1996 and their data recorded since. A finite element (FE) model of the antenna has been used to analyse these measurements and to refine the sensor network. Details on the optimum choice of sensor locations will be presented, and how their readings are interpolated onto the model grid. From the model, structural deformations are obtained and converted into observable telescope parameters. These parameters, like focus, pointing and large-scale surface deformations, will be used to upgrade the real-time instrumental performance and to provide the astronomer with data for an eventual correction of observations.
For the new generation of radio telescopes based on phased array principles, the complexity of the embedded systems is growing. The main changes are a very high bandwidth, multi-beaming, RFI suppression and scaling to thousands of elements. To handle the complexity and to facilitate the optimum design of a very large system, a high level architecture model is needed. The architecture exploration at system level is of high value, as it orients the intermediate and detailed design steps of a telescope. Moreover, the impact of detailed application selections should be visible at an early stage of the system design. Therefore we propose to improve the quality of the system models using possible combinations of processing and routing up to board and component levels to model generic hardware blocks. The blocks are connected at various levels of abstraction to the application process network. The model of the system architecture presented here is consequently derived from the high level system description and simulation, refined and scaled up using modular hardware blocks as mapping targets. The exercise allowed comparisons of telescope schemes for the station digital processing and the correlation based on existing and tested generic blocks.
Airborne telescopes have, compared with earthbound or space telescopes, by far the most worse environment during operations. They must not only deal with aircraft vibrations, but also with large temperature differences and aero-acoustic loads including standing waves. System simulations are particularly useful for verifying the design performance in this environment. They should include the behavior of the overall opto-mechanical system, the environmental loads and the pointing control system itself. SOFIA - the “Stratospheric Observatory for Infrared Astronomy” - is a 2,7 m infrared telescope in an open cavity of a Boeing 747 aircraft. At present the telescope is under construction in the MAN premises in Augsburg. Simulations during the design phase of the telescope showed, that there are resonance effects in the telescope excited by the cavity acoustics. The excitations disturb the pointing behavior above the allowances. This paper describes, how the overall system was simulated, how the resonance modes will be compensated by “flexible body control”, and how the pointing control system will be finally optimized during test flights in an pointing improvement phase.
Space-based optical systems that perform tasks such as laser communications, Earth imaging, and astronomical observations require precise line-of-sight (LOS) pointing. A general approach is described for integrated modeling and analysis of these types of systems within the MATLAB/Simulink environment. The approach can be applied during all stages of program development, from early conceptual design studies to hardware implementation phases. The main objective is to predict the dynamic pointing performance subject to anticipated disturbances and noise sources. Secondary objectives include assessing the control stability, levying subsystem requirements, supporting pointing error budgets, and performing trade studies. The integrated model resides in Simulink, and several MATLAB graphical user interfaces (GUI’s) allow the user to configure the model, select analysis options, run analyses, and process the results. A convenient parameter naming and storage scheme, as well as model conditioning and reduction tools and run-time enhancements, are incorporated into the framework. This enables the proposed architecture to accommodate models of realistic complexity.