Peter T. Zell

THE SOFIA OBSERVATORY AT THE START OF ROUTINE SCIENCE OPERATIONS: MISSION CAPABILITIES AND PERFORMANCE

The Stratospheric Observatory for Infrared Astronomy (SOFIA) has recently concluded a set of engineering flights for observatory performance evaluation. These in-flight opportunities are viewed as the first comprehensive assessment of the observatorys performance and are used to guide future development activities, as well as to identify additional observatory upgrades. Pointing stability was evaluated, 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 the telescope. Measurements and characterization of the shear layer and cavity seeing, as well as image quality evaluation as a function of wavelength have also been performed. Additional tests targeted basic observatory capabilities and requirements, including 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.

Laboratory demonstration of high-contrast imaging at inner working angles 2 λ/D and better

Coronagraph technology is advancing and promises to enable direct imaging and spectral characterization of extrasolar Earth-like planets in the 2020 decade with a telescope as small as 1.5m. A small Explorer-sized telescope can also be launched in the 2010 decade capable of seeing debris disks as dim as tens of zodis and potentially a few large planets. The Phase Induced Amplitude Apodization (PIAA) coronagraph makes such aggressive performance possible, providing high throughput and high contrast close to the diffraction limit. We report on the latest results from a testbed at NASA Ames that is focused on developing and testing the PIAA coronagraph. This laboratory facility was built in 2008 and is designed to be flexible, operated in an actively thermally stabilized air environment, and to complement collaborative efforts at NASA JPLs High Contrast Imaging Testbed. For our wavefront control we are using small Micro-Electro- Mechanical-System deformable mirrors (MEMS DMs), which promise to reduce the size of the beam and overall instrument, a consideration that becomes very important for small telescopes. We describe our lab progress and results, which include (as of August 2011): the demonstration of 1.9x10-8 average raw contrast in a dark zone from 2.0 - 3.4 λ/D and of 1.2x10-6 contrast from 1.5-2.0 λ/D (in monochromatic light); the testing of the next-generation reflective PIAA mirror set built by Tinsley and designed for broadband; and finally, discuss our most important past limiting factors as well as expected future ones.

Laboratory demonstration of high-contrast imaging at 2 λ/D on a temperature-stabilized testbed in air

Direct imaging of extrasolar planets in visible light, and Earth-like planets in particular, is an exciting but difficult problem requiring a telescope imaging system with 10-10 contrast at separations of 100mas and less. Furthermore, only a small 1-2m space telescope may be realistic for a mission in the foreseeable future, which puts strong demands on the performance of the imaging instrument. Fortunately, an efficient coronagraph called the Phase Induced Amplitude Apodization (PIAA) coronagraph may enable Earth-like planet imaging for such small telescopes if any exist around the nearest stars. In this paper, we report on the latest results from a testbed at the NASA Ames Research Center focused on testing the PIAA coronagraph. This laboratory facility was built in 2008 and is designed to be flexible, operated in a highly stabilized air environment, and to complement efforts at NASA JPLs High Contrast Imaging Testbed. For our wavefront control we are focusing on using small Micro-Electro-Mechanical-System deformable mirrors (MEMS DMs), which promises to reduce the size of the beam and overall instrument, a consideration that becomes very important for small telescopes. In this paper, we briefly describe our lab and methods, including the new active thermal control system, and report the demonstration of 5.4×10-8 average raw contrast in a dark zone from 2.0 - 5.2 λ/D. In addition, we present an analysis of our current limits and solutions to overcome them.

Demonstration of broadband contrast at 1.2λ/D and greater for the EXCEDE starlight suppression system

Abstract. The EXoplanetary Circumstellar Environments and Disk Explorer (EXCEDE) science mission concept uses a visible-wavelength phase-induced amplitude apodization (PIAA) coronagraph to enable high-contrast imaging of circumstellar debris systems and some giant planets at angular separations reaching into the habitable zones of some of the nearest stars. We report on the experimental results obtained in the vacuum chamber at the Lockheed Martin Advanced Technology Center in 10% broadband light centered about 650 nm, with a median contrast of 1×10−5 between 1.2 and 2.0λ/D simultaneously with 3×10−7 contrast between 2 and 11λ/D for a single-sided dark hole using a deformable mirror (DM) upstream of the PIAA coronagraph. These results are stable and repeatable as demonstrated by three measurement runs with DM settings set from scratch and maintained on the best 90% out of the 1000 collected frames. We compare the reduced experimental data with simulation results from modeling observed experimental limits. The observed performance is consistent with uncorrected low-order modes not estimated by the low-order wavefront sensor. Modeled sensitivity to bandwidth and residual tip/tilt modes is well matched to the experiment.

EXCEDE technology development I: First demonstrations of high contrast at 1.2 λ/D for an explorer space telescope mission

Coronagraph technology is advancing and promises to enable space telescopes capable of seeing debris disks as well as seeing and spectrally characterizing exo-Earths. Recently, NASAs explorer program has selected the EXCEDE (EXoplanetary Circumstellar Environments and Disk Explorer) mission concept for technology development. EXCEDE is a 0.7m space telescope concept designed to achieve raw contrasts of 1e-6 at an inner working angle of 1.2 λ/D and 1e- 7 at 2 λ/D. In addition to doing fundamental science on debris disks, EXCEDE will also serve as a technological and scientific precursor for an exo-Earth imaging mission. EXCEDE uses a Starlight Suppression System (SSS) based on the Phase Induced Amplitude Apodization (PIAA) coronagraph to provide high throughput and high contrast close to the diffraction limit, enabling aggressive performance on small telescopes. We report on the latest progress in developing the SSS and present coronagraphic performance results from our air testbed at NASA Ames. Our results include a lab demonstration of 1e-5 contrast at 1.2 λ/D, 1.3e-6 contrast at 1.4 λ/D and 2e-8 at 2 λ/D in monochromatic light. In addition, we discuss tip-tilt instabilities, which are believed to be our main limiting factor at present, and ways of characterizing them.

Aerodynamic flow quality and acoustic characteristics of the 40- by 80-foot test section circuit of the National Full-Scale Aerodynamic Complex

The 40- by 80-foot wind tunnel circuit of the National Full-Scale Aerodynamic Complex (NFAC) has recently undergone major modifications and subsequently completed final acceptance testing. The initial testing and calibration of the wind tunnel are described and in many cases these results are compared with predictions derived from model tests and theoretical analyses. The wind tunnel meets or exceeds essentially all performance objectives. The facility runs smoothly and routinely at its maximum test-section velocity of 300 knots (Mach number = 0.45). An effective cooling air exchange system enables the wind tunnel to operate indefinitely at this maximum power condition. Throughout the operating envelope of the wind tunnel the test-section dynamic pressure is uniform to within + or - 0.5 deg, and the axial component of turbulence is generally less than 0.5 percent. Acoustic measurements indicate that, due to the low noise fans and acoustic treatment in the wind-tunnel circuit and test section, the background noise level in the test section is comparable to other large-scale acoustic wind tunnels in the United States and abroad.

Capabilities, performance, and status of the SOFIA science instrument suite

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is an airborne observatory, carrying a 2.5 m telescope onboard a heavily modified Boeing 747SP aircraft. SOFIA is optimized for operation at infrared wavelengths, much of which is obscured for ground-based observatories by atmospheric water vapor. The SOFIA science instrument complement consists of seven instruments: FORCAST (Faint Object InfraRed CAmera for the SOFIA Telescope), GREAT (German Receiver for Astronomy at Terahertz Frequencies), HIPO (High-speed Imaging Photometer for Occultations), FLITECAM (First Light Infrared Test Experiment CAMera), FIFI-LS (Far-Infrared Field-Imaging Line Spectrometer), EXES (Echelon-Cross-Echelle Spectrograph), and HAWC (High-resolution Airborne Wideband Camera). FORCAST is a 5–40 μm imager with grism spectroscopy, developed at Cornell University. GREAT is a heterodyne spectrometer providing high-resolution spectroscopy in several bands from 60–240 μm, developed at the Max Planck Institute for Radio Astronomy. HIPO is a 0.3–1.1 μm imager, developed at Lowell Observatory. FLITECAM is a 1–5 μm wide-field imager with grism spectroscopy, developed at UCLA. FIFI-LS is a 42–210 μm integral field imaging grating spectrometer, developed at the University of Stuttgart. EXES is a 5–28 μm high-resolution spectrograph, developed at UC Davis and NASA ARC. HAWC is a 50–240 μm imager, developed at the University of Chicago, and undergoing an upgrade at JPL to add polarimetry capability and substantially larger GSFC detectors. We describe the capabilities, performance, and status of each instrument, highlighting science results obtained using FORCAST, GREAT, and HIPO during SOFIA Early Science observations conducted in 2011.

Preflight performance of the Echelon-Cross-Echelle Spectrograph for SOFIA

The Echelon-Cross-Echelle Spectrograph (EXES) is one of the first generation instruments for the Stratospheric Observatory for Infrared Astronomy (SOFIA). The primary goal of EXES is to provide high-resolution, cross-dispersed spectroscopy, with resolutions of 50,000-100,000 and wavelength coverage of 0.5-1.5% between 4.5 μm and 28.3 μm. EXES will also have medium (R=5000-25000) and low (R=1500-4000) modes available, as well as a target acquisition imaging mode and a pupil-imaging mode for alignment testing. EXES is scheduled for commissioning flights in February 2014. It will be available to the public for shared-risk observations in SOFIA’s Cycle 2. Here we give an overview of the design and capabilities of EXES as well as its laboratory performance to date.

Retro rocket plume actuated heat shield exhaust ports

A preliminary scheme was developed for base-mounted solid-propellant retro rocket motors to self-penetrate the Orion Crew Module heat shield for configurations with the heat shield retained during landings on Earth. In this system the motors propel impactors into structural push plates, which in turn push through the heat shield ablator material. The push plates are sized such that the port created in the ablator material is large enough to provide adequate flow area for the motor exhaust plume. The push plate thickness is sized to assure structural integrity behind the ablative thermal protection material. The concept feasibility was demonstrated and the performance was characterized using a gas gun to launch representative impactors into heat shield targets with push plates. The tests were conducted using targets equipped with Fiberform® and PICA as the heat shield ablator material layer. The PICA penetration event times were estimated to be less than 30 ms from the start of motor ignition. The mass of the system (not including motors) was estimated to be less than 2.3 kg (5 lbm) per motor. The configuration and demonstrations are discussed in this paper.

Testing of a germanium immersion grating

Germanium Immersion Gratings (GIGs) may be an important component for a compact, high-resolution spectrograph for the infrared. Germanium’s large index of refraction reduces the length of the grating by a factor of four compared to conventional reflection gratings. Germanium transmits light from roughly 2 to 11.5 μm, which includes spectral regions largely unavailable from the ground because of molecules in Earth’s atmosphere. This combination makes GIGs a compelling technology for space missions focused on molecules in astrophysical environments. We are beginning testing of a GIG supplied by Canon, Inc., and anticipate eventual detailed testing of the Canon grating and a similar GIG supplied by LLNL. We also discuss potential science observations that demonstrate the significance of high-resolution, infrared spectroscopy from space.