Andreas Reinacher

SOFIA observatory performance and characterization

The Stratospheric Observatory for Infrared Astronomy (SOFIA) has recently concluded a set of engineering flights for Observatory performance evaluation. These in-flight opportunities have been viewed as a first comprehensive assessment of the Observatorys performance and will be used to address the development activity that is planned for 2012, as well as to identify additional Observatory upgrades. A series of 8 SOFIA Characterization And Integration flights have been conducted from June to December 2011. The HIPO science instrument in conjunction with the DSI Super Fast Diagnostic Camera (SFDC) have been used to evaluate pointing stability, including the image motion due to rigid-body and flexible-body telescope modes as well as possible aero-optical image motion. We report on recent improvements in pointing stability by using an Active Mass Damper system installed on Telescope Assembly. Measurements and characterization of the shear layer and cavity seeing, as well as image quality evaluation as a function of wavelength have been performed using the HIPO+FLITECAM Science Instrument conguration (FLIPO). A number of additional tests and measurements have targeted basic Observatory capabilities and requirements including, but not limited to, pointing accuracy, chopper evaluation and imager sensitivity. This paper reports on the data collected during these flights and presents current SOFIA Observatory performance and characterization.

Characterization of the mechanical properties of the SOFIA secondary mirror mechanism in a multi-stage approach

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.

Modeling of the SOFIA secondary mirror controller

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5m infrared telescope built into a Boeing 747SP. During observations the telescope will not only be subject to aircraft vibrations and maneuver loads - by opening a large door to give the observatory an unhindered view of the sky, there will also be aerodynamic and aeroacoustic disturbances. A critical factor in the overall telescope performance is the SOFIA Secondary Mirror Assembly (SMA). The 35cm silicon carbide mirror is mounted on the Secondary Mirror Mechanism (SMM), which has five degrees-offreedom and consists of two parts: The slow moving base for focusing and centering, and on top of that the Tilt Chop Mechanism (TCM) for chopping with a frequency of up to 20Hz and a chop throw of up to 10arcmin. The development of the controller that is used to meet the stringent performance requirements relys heavily on a state space model of the system. A pole-placement controller is compared to an optimal LQG control approach which makes use of the model to calculate all required system states in real-time. The paper explains the modeling of the TCM with linear differential equations and the optimization via a grey-box model approach with system identification data. Simulated data is then compared to measurements taken on ground and in flight.

SOFIA's secondary mirror assembly: in-flight performance and control approach

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5m infrared telescope built into a Boeing 747 SP. In 2014 SOFIA reached its Full Operational Capability milestone and nowadays takes off about three times a week to observe the infrared sky from altitudes above most of the atmosphere’s water vapor content. An actively controlled 352mm SiC secondary mirror is used for infrared chopping with peak-to-peak amplitudes of up to 10 arcmin and chop frequencies of up to 20Hz and also as actuator for fast pointing corrections. The Swiss-made Secondary Mirror Mechanism (SMM) is a complex, highly integrated and compact flexure based mechanism that has been performing with remarkable reliability during recent years. Above mentioned capabilities are provided by the Tilt Chopper Mechanism (TCM) which is one of the two stages of the SMM. In addition the SMM is also used to establish a collimated telescope and to adjust the telescope focus depending on the structure’s temperature which ranges from about 40°C at takeoff in Palmdale, CA to about −40◦C in the stratosphere. This is achieved with the Focus Center Mechanism (FCM) which is the base stage of the SMM on which the TCM is situated. Initially the TCM was affected by strong vibrations at about 300 Hz which led to unacceptable image smearing. After some adjustments to the PID-type controller it was finally decided to develop a completely new control algorithm in state space. This pole placement controller matches the closed loop system poles to those of a Bessel filter with a corner frequency of 120 Hz for optimal square wave behavior. To reduce noise present on the position and current sensors and to estimate the velocity a static gain Kalman Filter was designed and implemented. A system inherent delay is incorporated in the Kalman filter design and measures were applied to counteract the actuators’ hysteresis. For better performance over the full operational temperature range and to represent an amplitude dependent non-linearity the underlying model of the Kalman filter adapts in real-time to those two parameters. This highly specialized controller was developed over the course of years and only the final design is introduced here. The main intention of this contribution is to present the currently achieved performance of the SOFIA chopper over the full amplitude, frequency, and temperature range. Therefore a range of data gathered during in-flight tests aboard SOFIA is displayed and explained. The SMM’s three main performance parameters are the transition time between two chop positions, the stability of the Secondary Mirror when exposed to the low pressures, low temperatures, aerodynamic, and aeroacoustic excitations present when the SOFIA observatory operates in the stratosphere at speeds of up to 850 km/h, and finally the closed-loop bandwidth available for fast pointing corrections.

A new test environment for the SOFIA secondary mirror assembly to reduce the required time for in-flight testing

The Stratospheric Observatory For Infrared Astronomy (SOFIA) reached its full operational capability in 2014 and takes off from the NASA Armstrong Flight Research Center to explore the universe about three times a week. Maximizing the programs scientific output naturally leaves very little flight time for implementation and test of improved soft- and hardware. Consequently, it is very important to have a comparable test environment and infrastructure to perform troubleshooting, verifications and improvements on ground without interfering with science missions. SOFIAs Secondary Mirror Mechanism is one of the most complex systems of the observatory. In 2012 a first simple laboratory mockup of the mechanism was built to perform basic controller tests in the lower frequency band of up to 50Hz. This was a first step to relocate required engineering tests from the active observatory into the laboratory. However, to test and include accurate filters and damping methods as well as to evaluate hardware modifications a more precise mockup is required that represents the system characteristics over a much larger frequency range. Therefore the mockup has been improved in several steps to a full test environment representing the system dynamics with high accuracy. This new ground equipment allows moving almost the entire secondary mirror test activities away from the observatory. As fast actuator in the optical path, the SMM also plays a major role in SOFIAs pointing stabilization concept. To increase the steering bandwidth, hardware changes are required that ultimately need to be evaluated using the telescope optics. One interesting concept presented in this contribution is the in- stallation of piezo stack actuators between the mirror and the chopping mechanism. First successful baseline tests are presented. An outlook is given about upcoming performance tests of the actively controlled piezo stage with local metrology and optical feedback. To minimize the impact on science time, the laboratory test setup will be expanded with an optical measurement system so that it can be used for the vast majority of testing.

SOFIA image motion compensation

We describe a laboratory simulation of an image motion compensation system for SOFIA that uses high-speed image acquisition from the science instrument HIPO as the sensing element of the system and a Newport voice-coil actuated fast steering mirror as the correcting actuator. Performance of the system when coupled to the SOFIA secondary mirror is estimated based on the known current performance of the secondary mirror controller. The system is described and the observed performance is presented together with expectations for applicability in flight with SOFIA.

Design of an innovative observer based feedback enabling faster telescope control in SOFIA

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a Cassegrain telescope with a 2.7m primary mirror flying at altitudes up to 45 kft. One particular challenging aspect of an airborne observatory is the pointing control and image stability. The main control system consists of three cascaded SISO attitude and rate loops. The Fine Drive (FD) as the main actuator can move the telescope in all three axes by + / - 3°. Its bandwidth is currently limited to 3-5 Hz (depending on the axis), which prevents it from compensating higher frequency eigenmodes and especially modes in between 5-10 Hz. The compensation of these modes and of higher frequency excitation is currently achieved with a feed-forward loop to the active secondary mirror. A faster main actuator (the Fine Drive) could better counteract low frequency disturbances and would reduce the load on the secondary mirror. Flexible modes above 10 Hz are the main task of the Flexible Body Compensation system and not part of the FD scope. As the eigenfrequencies nonetheless occur on the gyro sensor measurements in the FD loop, the controller gains are conservatively chosen to not amplify these modes. This paper discusses first the derivation of a very accurate telescope plant model for simulation and then a specifically designed observer which minimizes the impact of the telescope resonance frequencies on the FD feedback. The flexible modes are part of the observer noise model. It is shown that this observer can stabilize the closed loop system and minimize the necessity of compensation filters, thereby enabling a faster FD controller.

An end-to-end simulation to predict the in-flight performance improvement of a modified SOFIA secondary mirror mechanism

One of the most complex systems of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is the Secondary Mirror Assembly (SMA) providing fast mirror steering capability for image stabilization and infrared square wave chopping. Since its integration in 2002 the performance of the SMM is limited by a strong structural resonance caused by the deformation of a ring-shaped reaction mass. Constraining this resonance would not only lead to a wider actuation bandwidth and therefore a faster transition between the chop positions but also reduce the image jitter introduced by external disturbances acting on the active mechanism itself. Concepts have been developed to attenuate this resonance by structural modifications on the hardware level. To predict the later in-flight performance of these concepts an end-to-end simulation has been setup. The design changes are implemented into a finite element model of the SMA to compute the open loop system response of the mechanism. Subsequently the new dynamic system behavior is implemented into a controller model to simulate the closed loop controlled SMA. Next to the new steering bandwidth, the disturbance rejection capability is analyzed by applying a white noise excitation simulating wind loads and process noise. Moreover, the transition time between the chop positions is determined by applying a square wave input signal to the simulation.

Simulating SOFIA's image jitter performance and how the results compare to in-flight measurements

SOFIA, the Stratospheric Observatory for Infrared Astronomy is an airborne telescope and in full operation since 2014. It has already successfully conducted over 400 flights and can be equipped with eight different science instruments which range from the visible to the far infrared wavelength regime. In order to reach SOFIA’s scientific goals, the telescope has to provide a stable platform with the ambitous image jitter requirements of less than 0.4 ”rms. Such a steady operating environment is especially important for slit spectrometers like EXES (Echelon - Cross - Echelle Spectrograph), that aim to keep the star in the area of a very thin slit for integration. Currently, image motion is mainly caused by deformation and excitation of the telescope structure in a wide range of frequencies. These disturbances are counteracted by the so-called Flexible Body Compensation system which uses a set of accelerometers to estimate the resulting image motion. To better study optimization possibilities of SOFIA’s control system, a simulation tool has been developed which not only implements system identification data and analytically derived models, but also allows the implementation and verification with sensor data from in flight measurements. Results of the simulation as well as in flight measurements will be presented and improvement strategies will be discussed.

SOFIA pointing and chopping: performance and prospect

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5m infrared telescope built into a Boeing 747 SP. In 2014 SOFIA reached its Full Operational Capability milestone and nowadays takes off about three times a week to observe the infrared sky from altitudes above most of the atmosphere’s water vapor content. Despite reaching this major milestone the work to improve the observatory’s performance is continuing in many areas. This paper focuses on the telescope’s current pointing and chopping performance and gives an overview over the ongoing and foreseen work to further improve in those two areas. Pointing performance as measured with the fast focal plane camera in flight is presented and based on that data it is elaborated how and in which frequency bands a further reduction of image jitter might be achieved. One contributor to the remaining jitter as well as the major actuator to reduce jitter with frequencies greater than 5 Hz is SOFIA’s Secondary Mirror Assembly (SMA) or Chopper. As-is SMA jitter and chopping performance data as measured in flight is presented as well as recent improvements to the position sensor cabling and calibration and their effect on the SMA’s pointing accuracy. Furthermore a brief description of a laboratory mockup of the SMA is given and the intended use of this mockup to test major hardware changes for further performance improvement is explained.