Manuel Wiedemann

Active damping of the SOFIA Telescope assembly

The NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA) employs a 2.5-meter reflector telescope in a Boeing 747SP. The telescope is housed in an open cavity and is subjected to aeroacoustic and inertial disturbances in flight. To meet pointing requirements, SOFIA must achieve a pointing stability of approximately 0.5 arcseconds RMS. An active damping control system is being developed for SOFIA to reduce image jitter and image degradation due to resonance of the telescope assembly. Our paper discusses the history of the active damping design for SOFIA, from early concepts to the current implementation which has recently completed a ground and flight testing for proof-of-concept. We describe some milestones in the analysis and testing of the telescope assembly which guided the development of the vibration control system. The control synthesis approach and current implementation of the active damping control system is presented. Finally, we summarize the performance observed in early flight tests and the steps that are currently foreseen to completing the development of this system.

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.

Pointing stability and image quality of the SOFIA Airborne Telescope during initial science missions

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is an airborne observatory for astronomical observations at wavelengths ranging from 0.3-1600 µm. It consists of a telescope with an effective aperture of 2.5 m, which is mounted in a heavily modified Boeing 747SP. The aircraft features an open port cavity that gives the telescope an unobstructed view of the sky. Hence the optical system is subject to both aerodynamic loads from airflow entering the cavity, and to inertial loads introduced by motion of the airborne platform. A complex suspension assembly was designed to stabilize the telescope. Detailed end-to-end simulations were performed to estimate image stability based on the mechatronic design, the expected loads, and optical influence parameters. In December 2010 SOFIA entered its operational phase with a series of Early Science flights, which have relaxed image quality requirements compared to the full operations capability. At the same time, those flights are used to characterize image quality and image stability in order to validate models and to optimize systems. Optimization of systems is not based on analytical models, but on models derived from system identification measurements that are performed on the actual hardware both under controlled conditions and operational conditions. This paper discusses recent results from system identification measurements, improvements to image stability, and plans for the further enhancement of the system.

Evaluation of the aero-optical properties of the SOFIA cavity by means of computional fluid dynamics and a super fast diagnostic camera

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5 m reflecting telescope housed in an open cavity on board of a Boeing 747SP. During observations, the cavity is exposed to transonic flow conditions. The oncoming boundary layer evolves into a free shear layer being responsible for optical aberrations and for aerodynamic and aeroacoustic disturbances within the cavity. While the aero-acoustical excitation of an airborne telescope can be minimized by using passive flow control devices, the aero-optical properties of the flow are difficult to improve. Hence it is important to know how much the image seen through the SOFIA telescope is perturbed by so called seeing effects. Prior to the SOFIA science fights Computational Fluid Dynamics (CFD) simulations using URANS and DES methods were carried out to determine the flow field within and above the cavity and hence in the optical path in order to provide an assessment of the aero-optical properties under baseline conditions. In addition and for validation purposes, out of focus images have been taken during flight with a Super Fast Diagnostic Camera (SFDC). Depending on the binning factor and the sub-array size, the SFDC is able to take and to read out images at very high frame rates. The paper explains the numerical approach based on CFD to evaluate the aero-optical properties of SOFIA. The CFD data is then compared to the high speed images taken by the SFDC during flight.

Development of the FPI+ as facility science instrument for SOFIA cycle four observations

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a heavily modified Boeing 747SP aircraft, accommodating a 2.5m infrared telescope. This airborne observation platform takes astronomers to flight altitudes of up to 13.7 km (45,000ft) and therefore allows an unobstructed view of the infrared universe at wavelengths between 0.3 m and 1600 m. SOFIA is currently completing its fourth cycle of observations and utilizes eight different imaging and spectroscopic science instruments. New instruments for SOFIAs cycle 4 observations are the High-resolution Airborne Wideband Camera-plus (HAWC+) and the Focal Plane Imager (FPI+). The latter is an integral part of the telescope assembly and is used on every SOFIA flight to ensure precise tracking on the desired targets. The FPI+ is used as a visual-light photometer in its role as facility science instrument. Since the upgrade of the FPI camera and electronics in 2013, it uses a thermo-electrically cooled science grade EM-CCD sensor inside a commercial-off-the-shelf Andor camera. The back-illuminated sensor has a peak quantum efficiency of 95% and the dark current is as low as 0.01 e-/pix/sec. With this new hardware the telescope has successfully tracked on 16th magnitude stars and thus the sky coverage, e.g. the area of sky that has suitable tracking stars, has increased to 99%. Before its use as an integrated tracking imager, the same type of camera has been used as a standalone diagnostic tool to analyze the telescope pointing stability at frequencies up to 200 Hz (imaging with 400 fps). These measurements help to improve the telescope pointing control algorithms and therefore reduce the image jitter in the focal plane. Science instruments benefit from this improvement with smaller image sizes for longer exposure times. The FPI has also been used to support astronomical observations like stellar occultations by the dwarf planet Pluto and a number of exoplanet transits. Especially the observation of the occultation events benefits from the high camera sensitivity, fast readout capability and the low read noise and it was possible to achieve high time resolution on the photometric light curves. This paper will give an overview of the development from the standalone diagnostic camera to the upgraded guiding/tracking camera, fully integrated into the telescope, while still offering the diagnostic capabilities and finally to the use as a facility science instrument on SOFIA.

Upgrade of the SOFIA target acquisition and tracking cameras

The Stratospheric Observatory for Infrared Astronomy (SOFIA) uses three visible range CCD cameras with different optics for target acquisition and tracking. The Wide Field Imager (WFI with 68mm f/2.0 optics) and the Fine Field Imager (FFI with 254mm f/2.8 optics) are mounted on the telescope front ring and are therefore exposed to stratospheric conditions in flight. The Focal Plane Imager (FPI) receives visible light from the 2.5m Cassegrain/Nasmyth telescope via a dichroic tertiary mirror and is mounted inside the pressurized aircraft cabin at typically +20°C. An upgrade of these three imagers is currently in progress. The new FPI was integrated in February 2013 and is operating as SOFIA’s main tracking camera since then. The new FFI and WFI are planned to be integrated in summer of 2015. Andor iXonEM+ DU- 888 cameras will be used in all three imagers to significantly increase the sensitivity compared to the previous CCD sensors. This will allow for tracking on fainter stars, e.g. the new FPI can track on a 16mag star with an integration time of 2 sec. While the FPI uses a commercial off the shelf camera, the cameras for FFI and WFI are extensively modified to withstand the harsh stratospheric environment. The two front ring imagers will also receive new optics to improve the image quality and to provide a stable focus position throughout the temperature range that SOFIA operates in. In this paper we will report on the results of the new FPI and the status of the FFI/WFI upgrade work. This includes the selection and design of the new optics and the design and testing of a prototype camera for the stratosphere. We will also report on preparations to make the new FPI available for scientific measurements.

Testing the e2v CCD47-20 as the new sensor for the SOFIA target acquisition and tracking cameras

The telescope of the Stratospheric Observatory for Infrared Astronomy (SOFIA) has three target acquisition and tracking cameras, the Wide Field Imager (WFI), Fine Field Imager (FFI) and Focal Plane Imager (FPI). All three cameras use Thompson TH7888A CCD sensors (now offered by e2v) which are quite suitable in terms of their geometry and readout speed. However, their quantum efficiency and dark current rate are not comparable to newer CCD sensors now widely used in astronomy. The Deutsches SOFIA Institut (DSI) under contract of the German Aerospace Center (DLR) has therefore initiated an upgrade project of the cameras with high-sensitivity and low dark current CCD sensors, the e2v CCD47-20 BI AIMO. The back-illuminated architecture allows for high quantum efficiency, while the inverted mode operation lowers the dark current significantly. Both features enable the cameras to use fainter stars for tracking. The expected improvements in sensitivity range between 1.2 and 2.5 stellar magnitudes for the three cameras. In this paper we present results of laboratory and on-sky tests with the new sensor, obtained with a commercial camera platform.

Deutsches SOFIA Institut (DSI) at the SOFIA Science Center: engineering and scientific contributions to the airborne observatory

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5-meter infrared telescope built into a Boeing 747SP. 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 atmospheres water vapor content. Despite reaching this major milestone, efforts to improve the observatorys performance are continuing in many areas. The team of the Deutsches SOFIA Institut, DSI (German SOFIA Institute) at the SOFIA Science Center in Moffett Field, CA works in several engineering areas to improve the observatorys performance and its efficiency. DSI supports the allocation process of SOFIAs observation time for guest observers, provides and supports two facility science instruments and conducts an observing program of stellar occultations by small objects of the solar system. This paper summarizes results and ongoing work on a spare secondary mirror made of aluminum, the new and improved Focal Plane Imager (FPI+) that has become a facility science instrument, the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS), new cameras and optics for the Fine Field and Wide Field Imagers (FFI+ and WFI+), real-time astrometric solution of star field images, ground support equipment and astronomical observations.

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

The Stratospheric Observatory for Infrared Astronomy (SOFIA) has already successfully conducted over 300 flights. In its early science phase, SOFIAs pointing requirements and especially the image jitter requirements of less than 1 arcsec rms have driven the design of the control system. Since the first observation flights, the image jitter has been gradually reduced by various control mechanisms. During smooth flight conditions, the current pointing and control system allows us to achieve the standards set for early science on SOFIA. However, the increasing demands on the image size require an image jitter of less than 0.4 arcsec rms during light turbulence to reach SOFIAs scientific goals. The major portion of the remaining image motion is caused by deformation and excitation of the telescope structure in a wide range of frequencies due to aircraft motion and aerodynamic and aeroacoustic effects. Therefore the so-called Flexible Body Compensation system (FBC) is used, a set of fixed-gain filters to counteract the structural bending and deformation. Thorough testing of the current system under various flight conditions has revealed a variety of opportunities for further improvements. The currently applied filters have solely been developed based on a FEM analysis. By implementing the inflight measurements in a simulation and optimization, an improved fixed-gain compensation method was identified. This paper will discuss promising results from various jitter measurements recorded with sampling frequencies of up to 400 Hz using the fast imaging tracking camera.

Improved sensitivity of the SOFIA target acquisition and tracking cameras and a high speed diagnostics camera for telescope movements in flight

The telescope of the Stratospheric Observatory for Infrared Astronomy (SOFIA) uses three CCD based visible light cameras for target acquisition and tracking. All three cameras use the TH7888A CCD chips which are quite suitable in terms of their geometry and readout speed. However, their quantum efficiency and dark current are not comparable to newer high-sensitivity CCD chips now widely used in astronomy. The Deutsche SOFIA Institute (DSI) under contract of the German Aerospace Center (DLR) has therefore initiated an upgrade project of the cameras with back-illuminated, high-sensitivity and low dark current CCD chips, e2v 47-20. The expected improvements in sensitivity range between 1.2 and 2.5 stellar magnitudes for the three cameras. In addition, DSI and DLR plan to provide a high-speed camera which can monitor stellar images of the SOFIA main telescope in the visible spectral range at frame rates of up to ~ 300 frames per second. Analysis of image movements at such speeds will help to identify sources of instabilities in flight, such as vibrations and wind loads. Knowledge of such disturbances and their influence on the telescope system will be essential to achieve the requirement of 0.6 arc-seconds (rms) pointing stability.