SOFIA3 - The Third Power of a SOFIA PhD Exam

The three freshly baked Doctors of Engineering Science: Enrico Pfüller,
Prashant Kaswekar,and Manuel Wiedemann, right after their successful
exams (Copyright: DSI).

On June 9th 2016 three PhD exams were administered at the University of Stuttgart DSI (German SOFIA Institute) to Prashant Kaswekar, Enrico Pfüller, and Manuel Wiedemann, who defended their PhD theses with excellent presentations and exam performances. The special situation here was that, in unique ways, their studies all dealt with the question of how to optimize the image stability of the SOFIA telescope. During observations the SOFIA telescope is exposed to various disturbing factors such as vibrations of the aircraft, wind loads, and temperature fluctuations. In order to satisfy the needs of the scientists using the telescope, its pointing accuracy must be 0.2 arcsec or better. That tiny angle is equal to the apparent size of a penny viewed at a distance of 16 kilometers.

 

The Fast Diagnostic Camera (alias Focal Plane Imager) attached to the
SOFIA telescope (Copyright: DSI).

Enrico Pfüller developed SOFIA’s Fast Diagnostic Camera (FDC) – a CCD-camera that could discretely be integrated into the observatory in place of of the Focal Plane Imager (FPI) to characterize the pointing stability and image quality of the flying observatory. Taking as many as 400 images per second, it detects even minor shifts of a calibrator star at the camera’s sensor, directly indicating disturbing motions of the telescope. This enables the camera to optimize the various interacting telescope control systems.

The FDC also acts as the upgraded Focal Plane Imager+ (FPI+) and has been integrated into the telescope system of the observatory by Pfüller’s colleague Manuel Wiedemann (see below). In early 2016 the former FDC / FPI+ was officially added to SOFIA’s suites of scientific instruments. The FPI+ allows astronomers to conduct observations in visible light parallel to the mounted infrared instrument, and can be applied to investigations of exoplanet transits, comets or occultation of so-called Trans-Neptunian-Objects. “I would never have dreamed that the FDC would become a real scientific instrument that allows us to observe events like the Pluto occultation in June 2015”, said Enrico Pfüller as he beamed with pride.

 

Simulations of the first bending eigen-frequency of the SOFIA telescope
system (Copyright: P. Kaswekar).

Prashant Kaswekar worked on the origins of the disturbances of the SOFIA telescope. At a speed of 800 km/h (500 mph) and an altitude of 13 km (43,000 feet), the telescope experiences significant wind loads once the cavity door is opened. Because the telescope is mounted like a dumb-bell it can easily be stimulated into oscillations with particular eigen-frequencies. To measure these states and the corresponding frequencies reliably, different sensors needed to be placed at particular locations on the telescope. In combination with kinematic models, these sensor measurements can tell which states do have an especially high impact on image movement in the focal plane. But even before the measurements can be taken the interaction between sensor data and the model – in particular the systematic errors and measurement inaccuracies – needed to be investigated and understood in detail by intense simulations. “I am just the simulation guy”, Prashant Kaswekar admits. “But seeing SOFIA in operation in Palmdale where the observatory is based significantly inspired my efforts.”

Manuel Wiedemann and Enrico Pfüller while testing the guiding cameras

during a SOFIA observing flight (copyright: DSI).

In contrast to that, Manuel Wiedemann dealt in practice with the inhospitable conditions of the stratosphere. To determine the actual orientation of the telescope, three different auxiliary cameras are used. The “Focal Plane Imager” (FPI) is the primary guiding camera of the observatory. With a field of view of 9 x 9 arc minutes, it has the smallest field of view, but highest accuracy. and sees exactly the same section of the sky as the scientific infrared instruments. The “Fine Field Imager” (FFI) has a larger field of view of 70 x 70 arc minutes and can serve as the secondary guiding camera if there is no proper guide star in the FPI field of view. The ”Wide Field Imager“ (WFI) has an even larger field of view of 6 x 6 degrees. It is used as the finding camera and, with the help of well-known stellar patterns, allows the initial alignment of the SOFIA telescope to the right position on the sky. Manuel Wiedemann’s task was to upgrade the existing set of auxiliary cameras. In order to increase the sensitivity of the originally available instruments, his first approach was to develop a very new camera system just for SOFIA in cooperation with industry – but finally had to reject that idea. The primary guiding camera – the FPI – the Fast Diagnostic Camera (FDC) that was developed by Enrico Pfüller as described above proved to be a perfect solution: The FDC is 100 times more sensitive than the original FPI and the geometry of the sensors fulfills the requirements for the SOFIA observatory’s the primary guiding camera. Hence, Wiedemann “only” needed to integrate this upgraded FPI+ into the whole system so that it could act as the primary guiding camera for SOFIA. For the FFI and WFI, however, the solution was a little more complicated because both are mounted at the front ring of the SOFIA-telescope and are exposed to the stratosphere and a flight velocity of 800 km/s. Manuel Wiedemann managed to convert an available commercial camera that finally satisfied the needed requirements with respect to sensitivity, reliability and safety. ”The path I had to take was not always straight and easy“, Manuel Wiedemann admits, “but finally I am very happy with the results“.

On one point, all the three examinees do agree: It is great the stress is over now!

Links:

Neuer Focal Plane Imager meistert Generalprobe mit Bravour - DSI News vom 13. März 2013