Ms. Friederike M. Graf Profile

Friederike M. Graf

Upgrades and Controls Engineer at NASA Ames Research Ctr.

SPIE Involvement:

Author

Publications (6)

Proceedings Article | 6 July 2018 Paper

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

Friederike Graf, Johannes Reinhart, Andreas Reinacher, Stefanos Fasoulas

Proceedings Volume 10700, 107005O (2018) https://doi.org/10.1117/12.2306089

KEYWORDS: Telescopes, Data modeling, Gyroscopes, System identification, Space telescopes, Filtering (signal processing), Performance modeling, Control systems, Optical instrument design, Mirrors

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Proceedings Article | 5 September 2017 Presentation + Paper

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

Friederike Graf, Andreas Reinacher, Daniel Spohr, Holger Jakob, Stefanos Fasoulas

Proceedings Volume 10401, 1040112 (2017) https://doi.org/10.1117/12.2273453

KEYWORDS: Data modeling, Control systems design, Telescopes, Astronomy, Observatories, Infrared astronomy, Motion estimation, Image analysis, Control systems, Device simulation, Modeling and simulation

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Proceedings Article | 14 September 2016 Presentation + Paper

SOFIA pointing and chopping: performance and prospect

Andreas Reinacher, Yannick Lammen, Friederike Graf, Holger Jakob

Proceedings Volume 9973, 99730L (2016) https://doi.org/10.1117/12.2237789

KEYWORDS: Infrared telescopes, Shape memory alloys, Telescopes, Cameras, Actuators, Mirrors, Motion estimation, Position sensors, Control systems, Sensors

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Proceedings Article | 27 July 2016 Paper

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

Yannick Lammen, Andreas Reinacher, Rick Brewster, Benjamin Greiner, Friederike Graf, Alfred Krabbe

Proceedings Volume 9906, 99064T (2016) https://doi.org/10.1117/12.2232152

KEYWORDS: Actuators, Mirrors, Observatories, Cameras, Telescopes, Shape memory alloys, Sensors, Position sensors, Aluminum, Manufacturing

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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 program's 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. SOFIA's 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 SOFIA's 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.

Proceedings Article | 27 July 2016 Paper

Pointing and control system performance and improvement strategies for the SOFIA Airborne Telescope

Friederike Graf, Andreas Reinacher, Holger Jakob, Ulrich Lampater, Enrico Pfueller, Manuel Wiedemann, Jürgen Wolf, Stefanos Fasoulas

Proceedings Volume 9906, 99061T (2016) https://doi.org/10.1117/12.2231803

KEYWORDS: Telescopes, Control systems, Cameras, Electroluminescence, Sensors, Mirrors, Motion estimation, Ray tracing, Finite element methods, Gyroscopes

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Proceedings Article | 27 July 2016 Paper

SOFIA secondary mirror mechanism heavy maintenance and improvements

Yannick Lammen, Andreas Reinacher, Ivar Kjelberg, Serge Droz, Holger Jakob, Friederike Graf, Michael Lachenmann, Alfred Krabbe

Proceedings Volume 9906, 99061R (2016) https://doi.org/10.1117/12.2231352

KEYWORDS: Mirrors, Calibration, Temperature metrology, Observatories, Autocollimators, Telescopes, Sensors, Shape memory alloys, Sensor calibration, Position sensors

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The Stratospheric Observatory For Infrared Astronomy (SOFIA) reached its full operational capability in 2014 and completed hundreds of observation flights. Since its installation in 2002, the Secondary Mirror Mechanism was subject to thousands of operating hours equivalent to millions of load cycles. During the aircraft heavy maintenance in fall 2014, a four month time window enabled the removal of the mechanism from the telescope structure for service and improvements. Next to visual corrosion- and crack-inspection of the flexures, critical electronic components (in particular the set of three eddy current position sensors that determine the mirror tilt) were replaced. Moreover, a detailed temperature dependent position calibration of the system was performed in a cold chamber to improve the pointing accuracy. Until then, a simple temperature independent linear gain was used to translate the sensor output voltage into a position. For accurate positioning across the whole temperature range, a temperature dependent correction function had to be developed. This calibration would have cost hours of observing time when performed in flight which made it an essential goal for completion during the maintenance period. An autocollimator was used as optical reference camera to measure the tip-tilt position of the secondary mirror in the cold chamber. Using this calibration setup, a pattern of many mirror positions in the tip-tilt domain was approached at several temperature points to provide a high resolution data set for the new multidimensional calibration function. Follow-up in-flight verification measurements confirmed a large improvement in pointing accuracy as soon as the temperature measurements were included into the position correction. Improvements of up to a factor of 10 were especially noticed in the lower temperature range. This contribution provides an insight into the work performed during the SOFIA - Secondary Mirror Mechanism maintenance with the focus on the temperature dependent position calibration.

Showing 5 of 6 publications

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