OWL: Further steps in designing the telescope mechanical structure and in assessing its performance Enzo Brunetto, Franz Koch, Marco Quattri European Southern Observatory ABSTRACT The baseline concept for the OWL mechanical structure is further developed and studied on the basis of the six mirrors optical concept. The primary mirror supporting structure is elaborated in deeper detail, the impact on the design of using lightweight mirrors is analyzed and also a trade-off among the so-called iso-static and hyper-static configurations is discussed. The performance of the telescope under seismic and wind survival load cases is analyzed. Keywords: OWL, extremely large telescope, opto-mechanic design, performance analyses. 1. INTRODUCTION The evolution of OWL's optical design, since June 1999 [1] [2], has lead to a redefinition of OWL's mechanical design, which is based on the new optical design [3]. The corresponding mechanical design benefits from the reduction of the distance between the primary and secondary mirrors (from 120m to 95m). This new design resulted in a more compact and sturdy mechanical structure. The current design phase of the opto-mechanical structure concentrates on the following domains: Further design development and optimization. Trade-off between Iso-static and hyper-static configuration. Investigation of alternative mirror material (SiC for M1 and M2). Investigation of survival load cases. Preliminary control simulation of the altitude tracking error under wind disturbance. 2. TELESCOPE ARCHITECTURE AND FUNCTIONS The synergy between the mechanical structure and the infrastructure of the observatory has been taken into account already in the early design phases. The following figures show the general architecture of the telescope's opto-mechanical structure integrated into the infrastructure of the observatory along with its main functions. The infrastructure and its buildings are shown in Figure 1. The main functions and description of the buildings are summarized below. Mirror Maintenance Building: Assembly and disassembly of M1- and M2 modules. Assembly and disassembly of M3, M4, M5 and M6. Re-coating and Maintenance of all the mirrors and cells. Park location of one sector of the primary mirror cover. Corrector Unit Maintenance Building: Assembly, disassembly and handling of Corrector Unit. Integration of telescope instrumentation. Park location for one sector of the primary mirror cover. Control Building: Control room. Engineering office. First aid facilities. Park location for one sector of the primary mirror cover. SPIE Vol. 4004, Munich, March 2000
Workshop and Storage Building: Optical, mechanical and electronics workshops. Assembly and disassembly of OWL instrumentation. Storage area and warehouse of spare parts. Park location for one sector of the primary mirror cover. Sectors 1, 2 and 3 of the primary mirror cover: Protection of ¾ of the Primary mirror surface. Each sector fits in any of the 4 docking locations of M1. Sectors center of gravity aligned with altitude axis. Tube rotation with mounted mirror covers possible. Sector 4 of the primary mirror covers: Same functions as Sectors 1,2 and 3 Handling and integration of the M1 mirror modulus. Built in washing facilities of the Primary mirror. Inspection and maintenance of M1mirror modulus. Sectors 1 and 2 of the secondary mirror covers: Protection of secondary mirror. Lightweight structure (minimization of altitude unbalance). Rotation of 90 o to change from parking position (downwards) to docking position (upwards). Gantry crane: Handling of Corrector Unit and Correctors mirrors. 200 tons load capacity. Figure 1: General perspective view of OWL telescope at Zenith position.
Figure 2 shows the overall opto-mechanical dimensions, the distance between the main optical elements and the size of the optical elements. The location of the Azimuth cable wrap, the cooling, the compressor, the oil pumping facilities and the Auxiliary drive are indicated as well. The Telescope at parking and maintenance position is shown in Figure 3. The tube can be parked in front of any of the 4 building facilities. One sector of the secondary mirror cover is shown attached to the telescope tube. One sector of the secondary mirror cover is shown in docking position (upwards). The following operations can be carried out in the parking position: Maintenance of the lower half of the secondary mirror (dedicated maintenance tower). Handling and integration of mirror modulus. Washing of mirrors. Inspection and maintenance. Figure 2: Front and side view of OWL telescope at zenith. Figure 3: General perspective view of OWL telescope at 90 o from Zenith. Parking position.
Figure 4 shows the covering operation of the Primary mirror. Sector 1 is shown in completed docking location. Sector 2 is shown approaching its docking location. Sector 3 is shown approaching the telescope structure. Sector 4 is shown still in its parking location. Figure 4: Primary mirror covers, docking operation The handling of the corrector unit is displayed in Figure 5. The Corrector Unit is shown in 3 positions. Position 1: Disassembled in the Corrector Unit assembly hall. Position 2: On the transporter, approaching access "tunnel" of the telescope tube. Part of the altitude horse shoe shall be temporarily removed. Position 3: Sliding inside the telescope tube tunnel towards its operational location, between M1 and M2. Figure 5: Corrector Unit handling (partial section of the Tube).
Figures 6 to 8 represent the operational range of the telescope. The altitude drive range covers the maximum observational range of the telescope (sky coverage down to 60 o from zenith). Auxiliary drive covers a range of 180 o, thus allowing park position of the tube on both sides and access of the secondary mirror from both sides. Altitude brakes cover a range of 180 o. The altitude brake is always operative, independently from the operational mode. For safety reasons, the brakes are located on the azimuth ring close to the altitude drive stator. The minimum distance from the infrastructure buildings to the telescope is indicated in Figure 9, which shows the extreme Tube position during operation at 60 o from Zenith. Figure 6: Tube at 15 o from Zenith position Figure 7: Tube at 60 o from Zenith position Figure 8: Side view of OWL telescope at its maximum observational range at 60 o from zenith.
3. ISO-STATIC VERSUS HYPERSTATIC CONFIGURATION Two different support configurations for the Tube structure have been investigated. In the Iso-static configuration (see Figure 9) the Tube is supported only at the two Altitude bearing journals on both sides of the Altitude axis, whereas the Hyper-static configuration (see Figure 10) contains additional radial bearings in the central Tube plane perpendicular to the Altitude axis. The advantages of both configurations are summarized below. Iso-Static configuration: Easier definition of the geometry during site assembly Less sensitivity to high thermal gradient. No mechanical over-constraint. Relief of mechanical tolerance of tracks. Simpler concept, less parts to be realized. Less maintenance demanding concept. Higher reliability. Figure 9: The Azimuth Ring, Iso-static configuration Hyper-static configuration: Better static and dynamic performance. Lower stress on the structural parts. Even distribution of loads over azimuth tracks. Increased design volume available for Altitude drive stator. Figure 10: The Azimuth Ring, Hyper-static configuration The Azimuth ring, shown in Figure 9 and 10, is the telescope sub-assembly which is most affected by the trade-off among Iso- and Hyper-static configurations. It provides mechanical support for the altitude bearing, altitude drive stator, azimuth drive, Altitude brakes, azimuth and altitude cable wrap facilities and azimuth auxiliary drive. The design presented offers a mechanical de-coupling of 5 degrees of freedom between the 2 altitude bearing journals. Only the rotation degree of freedom on azimuth axis is constrained, thus relieving the azimuth tracks mechanical tolerances. The Azimuth ring is embedded in the foundation.
4. ANALYSIS AND STRUCTURAL PERFORMANCE Analytical Models The structural behavior of three different design concepts has been analytically investigated by appropriate static and dynamic Finite Element Analyses (FEA) and has been compared with the original concept (120 m optical design). The new configurations (2 nd baseline) are based on the six mirrors 95 m optical concept. The topology of the structural framework is almost identical for all configurations, whereas the dimensions of the tubular beams have been pre-optimized independently for each design variant. The optimization goal was to maximize the altitude locked rotor frequency and to minimize the overall weight, while keeping the Tube s center of gravity close to the altitude axis. The internal designation and the main differences of the three concepts are indicated in the table below. Name Concept Tube support M1 and M2 material BL1-HZ 1 st baseline (June 99) Hyper-static Zerodur BL2-IZ 2 nd baseline Iso-static Zerodur BL2-HZ 2 nd baseline hyper-static Hyper-static Zerodur BL2-IS 2 nd baseline SiC Iso-static SiC Each FE Model represents the complete Tube structure and the Azimuth Ring. The latter was not taken into account in the 1 st baseline investigations presented in the June 99 paper [2]. The mirrors M1, M2 and the Corrector structure (M3 to M6) are simulated by distributed mass elements that are directly connected to the Tube structure. The structure is fixed to ground at the Azimuth supports and motors along the corresponding support directions. In case of the Iso-static mount the Tube is connected to the Fork- Azimuth structure by two hinges at the location of the mechanical bearings, i.e. only the three translational degrees of freedom are coupled. The altitude motor interface is simulated by a constraint equation, which links the appropriate parts of the Azimuth and the Tube structures in the tangential direction. In case of the hyper-static configuration, nodal links in radial direction represent the additional pads of the altitude bearings. The x-axis of the global co-ordinate system is identical to the Altitude axis, the z-axis points to zenith (=optical axis) and the y-axis is perpendicular to the xz-plane. Tube orientations different from Zenith are taken into account by rotating the structure about the altitude axis and adapting the boundary conditions of the Altitude motors and bearings accordingly. Each of the FE Models comprises about 8000 elements (beam, rod, shell and mass elements). The FE Model of the 2 nd baseline concept (BL2-IZ) is shown in Figure 11. Figure 11: FE Model of 2 nd baseline concept (BL2-IZ). Dynamic Performance Based on the FE models described before Modal, Harmonic Response Spectrum and Earthquake Analyses have been performed to evaluate the dynamic performance of the structure in terms of eigenfrequency, effective mass, mode shape, transfer function and earthquake response. The mass and inertia breakdown and a summary of the Altitude and Azimuth locked rotor frequencies of all configurations are presented in the table below. Concept Mass Inertia [Tons] [kgm 2. 10 6 ] Tube Optics Azimuth Total Altitude Azimuth BL1-HZ 6500 2300 8000 16800 20000 50000 BL2-IZ 6850 2350 4500 13700 13200 32500 BL2-HZ 7500 2350 4600 14450 14200 33800 BL2-IS 4450 900 4500 9850 8500 27000 Locked Rotor Frequency [Hz] Altitude Azimuth 1.37 1.13 1.50 1.98 2.40 1.98 1.55 1.83
It is evident that the present SiC concept BL2-IS provides an enormous reduction of mass (4000 tons) and inertia compared to the Zerodur concepts, but the desired improvement of dynamic performance is not yet obtained. Further investigations and design optimizations will be done. A significant increase of the Altitude locked rotor frequency of about 1 Hz can be obtained by using the Hyperstatic configuration (2.4 Hz). A typical locked rotor mode shape (BL2-IZ) is shown in Figure 12. This mode at 1.5 Hz represents almost a rigid body rotation of the Tube about the Altitude axis combined with a lateral motion of the inner tower structure of the Tube in y- direction. The Transfer Functions in Figure 13 provide an important information for the control behavior of the various design concepts. They represent the M2 motion in y-direction under Altitude motor torque when the Tube is free to rotate about the Altitude axis. The 2 nd baseline and the hyper-static concept are compared with the 1 st baseline concept of June 99 (dashed line). The lowest locked rotor modes are characterized in the plots as the first dominant amplitude peaks. It is evident, that in spite of its iso-static mount the 2 nd baseline concept shows better performance than the original design because of higher frequency and lower amplitude. Compared to both baseline concepts an enormous improvement can be achieved with the hyper-static configuration: The 1 st amplitude peak occurs at about 2.9 Hz, and is about three times smaller than the one of the 2 nd baseline concept. The curves in Figure 13 clearly demonstrate the remarkable performance improvement of the new design concepts bearing in mind, that the original concept was based on hyper-static mount and did not consider the Azimuth ring. Figure 12: Figure 13: Locked Rotor mode shape of 2 nd baseline concept at 1.5 Hz. Transfer functions M2 unit (Motor torque, free rotor).
Static Performance The results of the gravity load case for the 2 nd baseline concepts BL2-IZ and BL2-HZ are summarized in the table below. It shows the differential displacements and rotations between Tube pointing to zenith and 60 o from Zenith. As expected the maximum decenter appears for all configurations at M2 level. However, due to the flatness of M2 in the present optical design, this motion does not need to be corrected. The piston values of each configuration are nearly the same for all the Mirror Decenter Piston Tilt [mm] [mm] [arcsec] BL2-IZ BL2-HZ BL2-IZ BL2-HZ BL2-IZ BL2-HZ M1 67 67 37 36 72 74 M2 93 88 39 38 4 2 M3 82 80 39 38 20 8 M4 84 80 39 38 17 5 mirrors, because the whole central part of the Tube sags almost rigidly. The maximum stresses in the structures under dead weight load have been calculated for various Tube positions. It turns out that the maximum stress levels under pure gravity load are in general quite high, i.e. maximum calculated value is 330 MPa. Although the mechanical design and its FE model representation is not yet detailed enough to calculate exact values, it is possible that the maximum stresses might exceed 300 MPa. Nevertheless, measures like structural reinforcement or selection of stronger material can help to cure this problem. Survival load analysis To assess the sensitivity of the OWL structure to survival loads, preliminary earthquake analyses on the basis of the 2 nd baseline concept (BL2-IZ) were carried out. Since the site of the telescope and the corresponding level of seismic activities are not yet known, two different values of maximum horizontal ground acceleration have been investigated (0.05g and 0.10g). These values represent quite low seismic activities compared to the Maximum Likely Earthquake (MLE) value valid on Paranal (0.34 g). According to the rules of Eurocode 8 the global seismic response was obtained and combined with the gravity load case. The worst results are obtained for the Tube pointing to 60 o from Zenith. In this case maximum stresses in the steel structure of about 400 MPa (0.05 g) and 470 MPa (0.10 g) respectively have been calculated. In case of the 0.10 g earthquake the support system has to withstand horizontal loads in the order of 7000 tons. These results demonstrate that due to the permanent presence of high stresses from gravity, additional sources causing high stresses should be minimized, e.g. selection of site with low seismic activity. 5. CONCLUSION The results emerging from this current design phase of the opto-mechanical structure can be summarized as follow: Due to the reduction of distance between M1 and M2 (from 120 m to 95 m), the design of the 2 nd baseline concept has been improved into a more compact and sturdy mechanical structure. The telescope s architecture, its functions and the observatory s infrastructure have been elaborated in more detail. The lowest locked rotor eigenfrequency has been increased to 1.5 Hz for the iso-static configuration. The main advantage of the Hyper-static design concept is a remarkable improvement of its dynamic performance, i.e. Altitude locked rotor frequency of 2.4 Hz and reduction of highest Transfer Function peak by a factor of 3. Using SiC for M1 and M2 mirrors leads to significant reduction of mass and inertia, but, based on the present design status, the improvement in dynamic performance is minor. Further investigations will be carried out. Since the stress level under gravity load is already quite high, additional sources causing stress increase (e.g. earthquake, wind, etc.) should be minimized by appropriate site selection. 6. REFERENCES [1] P. Dierickx, J. Beletic, B. Delabre, M. Ferrari, R. Gilmozzi, N. Hubin, The optics of the owl 100-m adaptive telescope, Baeckaskog Workshop on Extremely Large telescope. Sweden, June1-2,1999. [2] E. Brunetto, F. Koch, M. Quattri, OWL: First steps towards designing the mechanical structure, Baeckaskog Workshop on Extremely Large telescope. Sweden, June1-2,1999. [3] P. Dierickx, B. Delabre, L. Noethe, OWL Optical design, active optics and error budget, SPIE 2000 Munich, 4003.