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978-3-8439-2772-7, Reihe Raumfahrt
Improving the Sensitivity of the SOFIA Target Acquisition and Tracking Cameras
217 Seiten, Dissertation Universität Stuttgart (2016), Softcover, A5
The Stratospheric Observatory for Infrared Astronomy (SOFIA) was developed for the exploration of our solar system, the milky way, and extragalactic objects in the infrared and sub-millimeter wavelength range. A 2.5m telescope was integrated into the aft section of a Boeing 747SP’s fuselage. In flight, the telescope cavity door opens and allows an unobstructed view of the universe at 11.5 to 13.7km altitude. The observation altitude at the border to the stratosphere is high enough to be above > 99% of the atmospheric water vapor, enabling observations in the infrared and sub-millimeter spectrum, radiation, which is otherwise almost completely absorbed in the troposphere before it can reach the ground.
The telescope structure is exposed to mechanical, thermal, and aerodynamic loads during observations, which makes a precise pointing of the telescope a challenge. For the stabilization of the telescope various active and passive systems are used. These include three on-axis target acquisition and tracking cameras to determine the orientation of the telescope in inertial space with the help of stars. The three cameras, as originally delivered with telescope, were identical and used different optics with different fields of view. The “Focal Plane Imager” (FPI) is mounted inside the temperature controlled aircraft cabin and is the primary tracking camera. It uses the visible portion of the light from the SOFIA telescope with a field of view of 9x9 arcmin. The “Fine Field Imager” (FFI) is the secondary tracking camera with a field of view of 70x70 arcmin. The “Wide Field Imager” (WFI) has a field of 6x6 degrees and is primarily used for target acquisition. The FFI and WFI are mounted with their own optics to the front ring of the SOFIA telescope and are therefore exposed to stratospheric conditions in flight.
Especially the original FPI was not sensitive enough to satisfy the SOFIA project requirement of being able to track on a 16th magnitude star. This was mainly due to the high dark current by operating the CCD camera at T≈20°C ambient temperature without active cooling. Furthermore, the original cameras’ quantum efficiency was about a factor of five lower than modern back-illuminated CCDs, which impacted all three cameras equally.
This thesis describes the upgrade of the SOFIA imagers from theoretical analysis and development to integration and verification testing of the new FPI and the ongoing development of the new WFI and FFI cameras and optics.