Enrico Pfüller

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.

Optical characterization of the SOFIA telescope using fast EM-CCD cameras

The Stratospheric Observatory for Infrared Astronomy (SOFIA) has recently demonstrated its scientific capabilities in a first series of astronomical observing flights. In parallel, special measurements and engineering flights were conducted aiming at the characterization and the commissioning of the telescope and the complete airborne observatory. To support the characterization measurements, two commercial Andor iXon EM-CCD cameras have been used, a DU-888 dubbed Fast Diagnostic Camera (FDC) running at frame rates up to about 400 fps, and a DU-860 as a Super Fast Diagnostic Camera (SFDC) providing 2000 fps. Both cameras have been mounted to the telescope’s Focal Plane Imager (FPI) flange in lieu of the standard FPI tracking camera. Their fast image sequences have been used to analyze and to improve the telescope’s pointing stability, especially to help tuning active mass dampers that suppress eigenfrequencies in the telescope system, to characterize and to optimize the chopping secondary mirror and to investigate the structure and behavior of the shear layer that forms over the open telescope cavity in flight. In June 2011, a collaboration between the HIPO science instrument team, the MIT’s stellar occultation group and the FDC team, led to the first SOFIA observation of a stellar occultation by the dwarf planet Pluto over the Pacific.

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.

First exoplanet transit observation with the Stratospheric Observatory for Infrared Astronomy: confirmation of Rayleigh scattering in HD 189733 b with the High-Speed Imaging Photometer for Occultations

Abstract. Here, we report on the first successful exoplanet transit observation with the Stratospheric Observatory for Infrared Astronomy (SOFIA). We observed a single transit of the hot Jupiter HD 189733 b, obtaining two simultaneous primary transit lightcurves in the B and z′ bands as a demonstration of SOFIA’s capability to perform absolute transit photometry. We present a detailed description of our data reduction, in particular, the correlation of photometric systematics with various in-flight parameters unique to the airborne observing environment. The derived transit depths at B and z′ wavelengths confirm a previously reported slope in the optical transmission spectrum of HD 189733 b. Our results give new insights to the current discussion about the source of this Rayleigh scattering in the upper atmosphere and the question of fixed limb darkening coefficients in fitting routines.

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.

Optical measurement of the pointing stability of the SOFIA Telescope using a fast EM-CCD camera

The goal of the Stratospheric Observatory for Infrared Astronomy (SOFIA) is to point its airborne telescope at astronomical targets stable within 0.2 arcseconds (rms). However, the pointing stability will be affected in flight by aircraft vibrations and movements and constantly changing aerodynamic conditions within the open telescope compartment. Model calculations indicate that initially the deviations from targets may be at the order of several arcseconds. The plan is to carefully analyse and characterize all disturbances and then gradually fine tune the telescopes attitude control system to improve the pointing stability. To optically measure how star images change their position in the focal plane, an Andor DU-888 electronmultiplying (EM) CCD camera will be mounted to the telescope instead of its standard tracking camera. The new camera, dubbed Fast Diagnostic Camera (FDC) has been extensively tested and characterized in the laboratory and on ground based telescopes. In ground tests on the SOFIA telescope system it proofed its capabilities by sampling star images with frame rates up to 400 frames per second. From this data the stars location (centroid) in the focal plane can be calculated every 1/400th second and by means of a Fourier transformation, the stars movement power spectrum can be derived for frequencies up to 200 Hz. Eigenfrequencies and the overall shape of the measured spectrum confirm the previous model calculations. With known disturbances introduced to the telescopes fine drive system, the FDC data can be used to determine the systems transfer function. These data, when measured in flight will be critical for the refinement of the attitude control system. Another subsystem of the telescope that was characterized using FDC data was the chopping secondary mirror. By monitoring a star centroid at high speed while chopping, the chopping mechanism and its properties could be analyzed. This paper will describe the EM-CCD camera and its characteristics and will report on the tests that lead up to its first use in a SOFIA flight.

Adding a second spectral channel to the SOFIA FPI+ science instrument

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a heavily modified Boeing 747SP aircraft, accommodating a 2.7 meter infrared telescope. This airborne observation platform operates at flight altitudes of up to 13.7 km (45,000 ft) and therefore allows a nearly unobstructed view of the visible and infrared universe at wavelengths between 0.3 µm and 1600 µm. The Focal Plane Imager (FPI+) is SOFIA’s main tracking camera. It uses a commercial, off-the-shelf camera with a thermoelectrically cooled EM-CCD. The back-illuminated sensor has a peak quantum efficiency greater than 95% at 550 nm and the dark current is as low as 0.01 e-/pix/sec. Since 2015, the FPI+ has been available to the community as a Facility Science Instrument, and can be used to observe stellar occultations by solar system objects such as dwarf planets, moons, asteroids, and comets, and transits of extra-solar planets. To date, SOFIA has conducted multi-channel observations of occultations, e.g. the occultation by Pluto in June of 2015 or the occultation by Triton in October 2017, using three instruments, HIPO and FLITECAM at the main instrument flange of the telescope, and the FPI+. This multi-wavelength sampling is important for enabling discrimination of particle sizes and constituents of hazes in the atmosphere of bodies such as Pluto and Triton, and the coma material of comets. Multi-wavelength observations also serve to allow us to place constraints on the chemical compositions of these formations. After the retirement of the two other instruments, the FPI+ is now SOFIA’s only remaining observing tool for occultations. In order to preserve some of the multi-color observing capability of the platform, we here discuss the addition of a second spectral channel to the FPI+. In a first upgrade step, a beamsplitter will split the incoming light and send it to two EMCCD cameras, one working in the ”blue”, e.g. SLOAN g’ band, and the other working in the ”red”, e.g. SLOAN i’ or z’ band. In a second upgrade step, the ”red” channel could be equipped with a NIR camera in order to provide a wider wavelength separation of the two bands. This will however require a modified dichroic coating on the tertiary (Nasmyth) mirror of the SOFIA telescope. This paper presents a preliminary design study of the opto-mechanical configuration of the dual channel FPI+.

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.

A Fast EM-CCD Camera as Performance Monitor for the SOFIA Telescope with Science Capabilities

One of the most challenging requirements for the Stratospheric Observatory for Infrared Astronomy (SOFIA) is the pointing stability of 0.2 arcseconds (rms) of its 2.7 m telescope onboard a Boeing 747SP. To support the analysis of pointing disturbances in flight, an EM-CCD camera has been prepared to measure star positions in the focal plane at speeds up to about 400 frames per second. Currently, the camera is planned to be mounted for special engineering flights, a procedure that requires overhead for mechanical work and optical alignment, including star tests on the night sky from the ground. This paper summarizes the status of the project and explores possibilities to mount the camera permanently to the SOFIA telescope. Permanent mounting would make it available for continuous performance monitoring and as a trouble shooting tool if needed. In addition, the camera could serve as a versatile high speed photometer in the visible and very near IR wavelength range for astronomical observations, e.g. for the photometry of stellar occultations and of transits of extra-solar planets.