Browsing by Author "Harms, Franziska"

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    A contribution to characterizing and calibrating the pointing control system of the SOFIA telescope
    (2009) Harms, Franziska; Röser, Hans-Peter (Prof. Dr. rer. nat.)
    SOFIA, the Stratospheric Observatory for Infrared Astronomy, is an airborne observatory that will study the universe in the infrared spectrum. A Boeing 747-SP aircraft will carry a 2.5 m telescope designed to make sensitive infrared measurements of a wide range of astronomical objects. It will fly at and above 12 km, where the telescope collects radiation in the wavelength range from 0.3 micrometers to 1.6 millimeters of the electromagnetic spectrum. During flight, a door will be opened to allow clear optical observations from the cavity environment where the telescope is mounted. The telescope pointing control is achieved during science observations by an array of sensors including three imagers, gyroscopes and accelerometers. In addition, throughout alignment and calibration of the telescope assembly, the High-speed Imaging Photometer for Occultation (HIPO) is used as a reference instrument. A theoretical concept has been developed to compensate the perturbations in the airborne environment and to correct them within the attitude control loop. A set of Cartesian reference frames is established to describe and manipulate the orientations of the various subsystems, sensor and pointing orientations. The dissertation proposes the alignment strategy for these reference frames. By means of sky observations, reduced to make fundamental measurements of control performance, the calibration parameters for the transformation matrices between the reference frames are determined. The alignment maneuvers are described in order to measure the misalignment between the gyroscopes, the imagers and the focal plane. The alignment strategy incorporates the compensation concept for pointing errors due to static structural deformation and combines it with the alignment measurements. To avoid a false calibration due to misinterpretation of the measured data, sensor errors and external perturbations that are present in the sensor signals are studied in detail. This refers mainly to the three gyroscopes which are the primary feedback sensors for the telescope pointing control loop. A set of data reduction techniques is presented that is used for the analysis of the sensor performance and alignment measurements. This includes the data reduction for the gyroscopes and for astrometric measurements with the imagers and HIPO. An extensive test series of gyroscope measurements is presented. They characterize the sensor performance after the three gyroscopes are integrated into the telescope system. Various system effects caused by other TA subsystems, aircraft systems or ground support equipment are analyzed and typified. During the first on sky tests of the SOFIA telescope in 2004, a variety of alignment measurements, structural flexure measurements and pointing measurements were performed. In particular, the gyroscope misalignment was measured and compensated. Measurements over the entire operational elevation range provide additional alignment and sensor performance information. The results are presented and discussed within this work. Due to the fact that all of the optical components will be demounted for coating and cleaning from time to time, the alignment between the imagers and other sensors and the reference instrument is changing. Thus, a semi-autonomous method for the calibration is pursued and recommendations are established based on the proposed calibration concept and the measurement results.