MICADO is the Multi-AO Imaging Camera for Deep Observations, a first light instrument for the Extremely Large Telescope (ELT). It will provide the ELT with diffraction limited imaging capacity over a ~53-arcsec field of view, while operating with the Multi-Conjugate Adaptive Optics (MCAO) module MAORY (0.8-2.5 μm). Here, we present the design status of the MICADO derotator, which at the same time serves (i) as crucial mechanical interface between the cryo-opto-mechanical camera assembly and the instrument support structure and (ii) as high-precision image and wavefront sensor derotator to allow for 50 µas astrometry over the entire MCAO corrected field. Additionally, first test results are presented which were obtained with a derotator prototype based on a scaled 1:2 test bearing. The derotator test stand is essential to explore the limitations of the preferred bearing type in the context of the given requirements. The technical difficulties addressed by the design include: (i) design of adequate mechanical interfaces to minimize mass, deformation and the effect of the warping moment on the bearing and (ii) analysis of the friction-related stick-slip effects at low tracking velocities for the implementation of a suitable position-velocity closed-loop control system. Furthermore, our prototype setup is used to develop and test the required control concept of this high-precision application.
The telescope structure of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is subject to vibration excitation due to aircraft motions and turbulence from the airflow coming into the telescope cavity. A proper understanding of the dynamical behavior of the telescope structure under operational loads is crucial for pointing control and measures against higher order optical aberrations. During design and construction a Finite Element model of the telescope assembly has been created in order to assess the structural integrity and the early performance. This legacy model used conservative assumptions and had a coarse approach on the approximation of some structural features. We present an updated Finite Element model of the SOFIA Primary Mirror Assembly, which represents support members as well as the primary mirror itself in greater detail, in order to support ongoing development for performance optimization. An iterative approach employing structural optimization was used to tune the model in order to fit modal parameters of the Primary Mirror Assembly which were measured in a test campaign prior to integration into the full telescope structure. The updated and tuned model is used to calculate deformations due to gravity, thermal loads and dynamic excitation. These deformations serve as input for ray-tracing analyses to investigate alterations in the light path in order to evaluate pointing errors and higher order optical aberrations.
The Multi-AO Imaging Camera for Deep Observations (MICADO), a first light instrument for the 39 m European Extremely Large Telescope (E-ELT), is being designed and optimized to work with the Multi-Conjugate Adaptive Optics (MCAO) module MAORY (0.8-2.5 μm). The current concept of the MICADO instrument consists of a structural cryostat (2.1 m diameter and 2 m height) with the wavefront sensor (WFS) on top. The cryostat is mounted via its central flange with a direct interface to a large 2.5-m-diameter high-precision bearing, which rotates the entire camera (plus wavefront sensor) assembly to allow for image derotation without individually moving optical elements. The whole assembly is suspended at 3.6 m above the E-ELT Nasmyth platform by a Hexapod-type support structure. We describe the design of the MICADO derotator, a key mechanism that must precisely rotate the cryostat/SCAO-WFS assembly around its optical axis with an angular positioning accuracy better than 10 arcsec, in order to compensate the field rotation due to the alt-azimuth mount of the E-ELT. Special attention is being given to simulate the performance of the derotator during the design phase, in which both static and dynamics behaviors are being considered in parallel. The statics flexure analysis is done using a detailed Finite Element Model (FEM), while the dynamics simulation is being developed with the mathematical model of the derotator implemented in Matlab/Simulink. Finally, both aspects must be combined through a realistic end-to-end model. The experiment designed to prove the current concept of the MICADO derotator is also presented in this work.
The Stratospheric Observatory for Infrared Astronomy (SOFIA) uses its compact and highly integrated Secondary Mirror Mechanism (SMM) to switch between target positions on the sky in a square wave pattern. This chopping motion excites eigenmodes of the mechanism structure, which limit controller and observatory performance. We present the setup and results of experimental modal tests performed on different building stages of a test-bench model as well as on the original flight hardware. Test results were correlated to simulations employing a finite element model in order to identify excited mode shapes and contributing flexible components of the Secondary Mirror Mechanism. It was possible to isolate the motion of the compensation ring and its elastic mounts as the vibration mode inducing the main disturbance at about 300 Hz, which is currently the main mode shape limiting the performance of the chopping controller.
SOFIA has reached in the last two years its full operational capabilities and is producing now great science on typically three observing flights per week. The telescope is the backbone of the observatory and is working nearly perfectly. This may be the right time to have a look on the design history of the telescope and some of the major subsystems, which ensure the functionality under the harsh aero-acoustic environment inside the aircraft cavity. A comparison with SOFIA’s predecessor KAO gives insight in to the challenges of airborne telescope design. The paper describes the development of the optical subsystem, the telescopes structure, the telescope mount and the interface to the aircraft from the conceptual design up to the finally as-built telescope, and comments on their influence on the overall observatory performance.
The Stratospheric Observatory for Infrared Astronomy SOFIA consists of a B747-SP aircraft, which carries aloft a 2.7-meter reflecting telescope. The image stability goal for SOFIA is 0:2 arc-seconds rms. The performance of the telescope structure is affected by elastic vibrations induced by aeroacoustic and suspension disturbances. Active compensation of such disturbances requires a fast way of estimating the structural motion. Integrated navigation systems are examples of such estimation systems. However they employ a rigid body assumption. A possible extension of these systems to an elastic structure is shown by different authors for one dimensional beam structures taking into account the eigenmodes of the structural system. The rigid body motion as well as the flexible modes of the telescope assembly, however, are coupled among the three axes. Extending a special mathematical approach to three dimensional structures, the aspect of a modal observer based on integrated motion measurement is simulated for SOFIA. It is in general a fusion of different measurement methods by using their benefits and blinding out their disadvantages. There are no mass and stillness properties needed directly in this approach. However, the knowledge of modal properties of the structure is necessary for the implementation of this method. A finite-element model is chosen as a basis to extract the modal properties of the structure.
The original pointing accuracy requirement of the Stratospheric Observatory for Infrared Astronomy SOFIA was defined
at the beginning of the program in the late 1980s as very challenging 0.2 arcsec rms. The early science flights of the
observatory started in December 2010 and the observatory has reached in the mean time nearly 0.7 arcsec rms, which is
sufficient for most of the SOFIA science instruments. NASA and DLR, the owners of SOFIA, are planning now a future
4 year program to bring the pointing down to the ultimate 0.2 arcsec rms.
This may be the right time to recall the history of the pointing requirement and its verification and the possibility of its
achievement via early computer models and wind tunnel tests, later computer aided end-to-end simulations up to the first
commissioning flights some years ago. The paper recollects the tools used in the different project phases for the
verification of the pointing performance, explains the achievements and may give hints for the planning of the upcoming
final pointing improvement phase.
The Stratospheric Observatory for Infrared Astronomy SOFIA started in December 2010 with the first series of science
flights, and has successfully completed about 38 science missions until fall 2011. The science instruments flown included
HIPO, FORCAST, GREAT and FLITECAM. Beside their scientific results (see related papers in these proceedings)
the flights delivered an extensive data base which is now used for the telescope performance characterization and the
operational optimization of the telescope in its unique environment. In this progress report we summarize recent
achievements of the observatory as well as the status of the telescope and give an update of the SOFIA pointing system
completed by intended future pointing optimization activities.
After 8 years of development, the telescope of the Stratospheric Observatory for Infrared Astronomy, SOFIA has been
integrated into the aircraft and has just started with the first observation test flights. Due to its rather unique environment
in the open port of a Boeing 747SP, the telescope optics of SOFIA is exposed to extreme aero-acoustic excitations. The
telescope pointing system is equipped with several design features, such as a vibration isolation system, a flexible body
control system and - potentially - active mass dampers, to handle excitations in different frequency ranges. Final performance
features of these systems will only be available after the first test flights, which will happen in the first half of
2010. A progress report is presented and describes the recent achievements as well as the status of the telescope, and
gives an update of the SOFIA pointing system, and the planned commissioning tests.
The NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA) employs a 2.5-meter reflector telescope in
a Boeing 747SP. The telescope is housed in an open cavity and will be subjected to aeroacoustic and inertial
disturbances. The image stability goal for SOFIA is 0.2 arc-seconds (RMS). Throughout the development phase of the
project, analytical models were employed to predict the image stability performance of the telescope, and to evaluate
pointing performance improvement measures. These analyses clearly demonstrated that key aspects which determined
1) Disturbance environment and relevant load-paths
2) Telescope modal behavior
3) Sensor and actuator placement
4) Control algorithm design
The SOFIA program is now entering an exciting phase in which the characteristics of the telescope and the cavity
environment are being verified through ground and airborne testing. A modal survey test (MST) was conducted in early
2008 to quantify the telescope modal behavior. We will give a brief overview of analytical methods which have been
employed to assess/improve the pointing stability performance of the SOFIA telescope. In this context, we will describe
the motivation for the MST, and the pre-test analysis which determined the modes of interest and the required MST
sensor/shaker placement. A summary will then be given of the FEM-test correlation effort, updated end-to-end
simulation results, and actual data coming from telescope activation test flights.