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 Observatory's 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
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.
The Stratospheric Observatory for Infrared Astronomy (SOFIA) is the next generation of airborne astronomical observatories. Funded by the U.S. and German space agencies, SOFIA is scheduled for science flights beginning in late-2008. The observatory consists of a 747-SP modified to accommodate a 2.7-meter telescope with an open port design. Academic and government laboratories spanning both the U.S. and Germany are developing science instruments for SOFIA. Using state-of-the-art technologies, SOFIA will explore the emission of astronomical sources with an unprecedented level of angular resolution (θ[arc-seconds] = 0.1 x wavelength [μm]) and spectral line sensitivity at infrared and sub-millimeter wavelengths. The current status of SOFIA is available from the observatory web site at http://sofia.arc.nasa.gov/ and is updated frequently.
HAWC (High-resolution Airborne Wideband Camera) is a facility science instrument for SOFIA (Stratospheric Observatory for Infrared Astronomy). It is a far-infrared camera designed for diffraction-limited imaging in four spectral passbands centered at wavelengths of 53, 89, 155, and 216 μm. Its detector is a 12x32 array of bolometers cooled to 0.2 K by an adiabatic demagnetization refrigerator. In this paper, we report on the development and testing of the instrument and its subsystems.
We present an overview of the science instrument program for the Stratospheric Observatory for Infrared Astronomy (SOFIA). Funded for an initial suite of facility and PI instruments, the SOFIA instrument development program includes imagers and spectrometers from both U.S. and German development teams. With an emphasis on lessons learned, we review the development of the facility instrument suite. We conclude with the anticipated role for SOFIA as a new technology test bed for the latest far-infrared detectors.
The Stratospheric Observatory for Infrared Astronomy (SOFIA) will carry a 2.5 meter effective aperture telescope onboard a Boeing 747SP aircraft to altitudes of 41,000 to 45,000 ft, above most of the atmosphere's IR-absorbing water vapor. SOFIA will start its astrophysical observations in early 2005, flying from Moffett Field, California with a suite of specialized cameras and spectrometers covering wavelengths between 0.3 and 600 m. A high-speed visible range CCD camera will use the airborne observatory to chase the shadows of celestial bodies during occultations. The SOFIA telescope was designed and built in Germany and has been delivered to the U.S. in September 2002. Its integration into the B747SP is well advanced so flight-testing will start in mid of 2004. After an initial test phase dedicated to the re-certification of the modified aircraft, functional and performance tests of the telescope and other scientific systems will commence. At NASA's Ames Research Center the ground support facilities for the observatory are being prepared.
The Stratospheric Observatory For Infrared Astronomy SOFIA will become operational with the next two years. It will be the biggest astronomical airborne observatory ever build, comprising a 3m-class telescope onboard a Boeing 747SP. A suite of first-light instruments is under development striving for cutting edge technology to make SOFIA a milestone in infrared astronomy. Here we present an overview over the instrumentation and an update on the current status.
When SOFIA enters operation, it will be the largest far- infrared telescope available, so it will have the best intrinsic angular resolution. HAWC (High-resolution Airborne Wideband Camera) is a far-infrared camera designed to cover the 40 - 300 micron spectral range at the highest possible angular resolution. Its purpose is to provide a sensitive, versatile, and reliable facility-imaging capability for SOFIA's user community during its first operational use.
We report on first observation run with the Achromatic Interfero Coronagraph (AIC) developed at Observatoire de la Cote d'Azure, France. Observations took place last Fall at Observatoire de Haute Provence with the 1.52 m telescope equipped at that time with adaptive optics. The AIC is an imaging device providing the nulling of a star without nulling the close environment of this star. Nulling results from a destructive interference process. Morphological features located as close to the star as the first angular Airy ring can be detected, thus breaking a limitation of the classical Lyot coronagraphs. The objectives of the observation run is to demonstrate that the AIC can image faint companions very close to the diffraction limit with ground-based telescope. After a short reminding of the principle of the AIC, conditions of observations are reported and first coronagraphed-images are shown. Finally limitations are discussed and improvements to carry on are described.
Many IR sources are dusty; embedded stars are obscured, often completely, and their light is absorbed. The starlight heats the dust, typically to temperatures of tens or hundreds of Kelvin, and the heated dust radiates in the far IR, at wavelengths for which the Stratospheric Observatory for IR Astronomy (SOFIA) is optimized. These dusty targets radiate most or all of their energy in the far IR: broadband imaging with the highest possible spatial resolution is the natural starting point form which to develop an understanding of their morphology and energetics. Because SOFIA is the largest far IR telescope, it delivers the best spatial resolution. The wealth of detail revealed when resolution improves often result in startling insights, as new pictures of old favorites from the Hubble Space Telescope so regularly remind us. We therefore believe that most SOFIA studies will begin with high spatial resolution broadband imaging, and that a facility science instrument is required to serve this heavy and continuing workload.
The joint US and German SOFIA project to develop and operate a 2.5 meter IR airborne telescope in a Boeing 747-SP is now in its second year. The Universities Space Research Association , teamed with Raytheon E-Systems and United Airlines, is developing and will operate SOFIA. The 2.5 meter telescope will be designed and built by a consortium of German companies led by MAN. Work on the aircraft and the preliminary mirror has started. First science flights will begin in 2001 with 20 percent of the observing time assigned to German investigators. The observatory is expected to operate for over 20 years. The sensitivity, characteristics, US science instrument complement, and operations concept for the SOFIA observatory, with an emphasis on the science community's participation are discussed.
Single Photon Ionization Time-of-Flight Mass Spectroscopy (SPI-TOFMS) is used as an in situ optical characterization technique to monitor chemical reactions occurring at semiconductor surfaces during molecular beam epitaxial (MBE) growth be detecting gaseous species. In this approach, 118 nm (10.5 eV) laser photons are generated and passed on front of a semiconductor substrate in the ultra-high vacuum (UHV) chamber. Here, the photons ionize the gaseous scattered and desorbed growth species which are detected by time-of-flight mass spectroscopy. The photons are produced by frequency tripling the fundamental Nd:YAG output to 355 nm and tripling again in a static cell of Xe/Ar to 118 nm. The 10.5 eV photons have sufficient energy to ionize III-V species of interest, but not fragment them, allowing simple interpretation of mass spectra. Gated boxcars allow for rapid data acquisition of growth species in real time. SPI-TOFMS has been used to study Asn/Si(100) desorption kinetics and, more recently, MBE growth of GaAs. Results are presented on the real-time monitoring of Gan and Asn growth species. Simultaneous monitoring of growth with Reflection High-Energy Electron Diffraction (RHEED) is also discussed. Future work includes SPI_TOFMS studies of Si delta-doping in GaAs and surfactant-enhanced epitaxy of Ge on Si. SPI-TOFMS is an in situ UHV optical probe used to study the growth chemistry of semiconductor surfaces. This noninstrusive, species-specific real-time monitor of growth can be applied to increase the quality of device manufacturing.
Epitaxial growth of III-V semiconductor materials is probed in a molecular beam epitaxy reactor by single photon ionization of the gaseous fluxes using vacuum ultraviolet (VUV) laser radiation. The ninth harmonic of the Nd:YAG laser is produced by frequency tripling the output to 355 nm and then to 118 nm in a Xe/Ar mixture. Together with a time-of-flight mass spectrometer, this radiation is used to selectively probe the gaseous fluxes of Ga, As, As2, and As4 during molecular beam epitaxy of III-V materials. The essential aspects of the method and details of calibration procedures to obtain relative fluxes are described. Cracking of the arsenic species does not occur in the laser/mass spectrometer, making relative species concentration measurements very reliable. Rapid data acquisition provides real time measurements of the fluxes of incident and scattered or desorbed materials during growth. Several basic examples are considered, including the thermal cracking of As4 on silicon and the desorption of arsenic and gallium species from GaAs during epitaxial growth. Recent work to correlate the flux determinations with reflection high energy electron diffraction (RHEED) oscillations during GaAs epitaxial growth is discussed.
Photoelectron spectra are reported for Cr(CO)3-, Mo(CO)3- and W(CO)3- anions prepared from the corresponding metal hexacarbonyls in a flowing afterglow ion source. The 488.0 nm spectra were obtained at an electron kinetic energy resolution of 5 meV using a newly constructed apparatus. The spectra exhibit transitions between the ground electronic states of the anions and the neutral molecules, and they show weak activity in the symmetric CO stretching, MC stretching, MCO bending and CMC bending vibrational modes. The observed vibrational structure indicates that the anions, like the neutral molecules, have C3v equilibrium geometries. Force constants estimated for the neutral M(CO)3 molecules from the fundamental vibrational frequencies measured here are consistent with stronger metal-ligand bonding in the coordinatively unsaturated complexes than in the corresponding hexacarbonyls. Franck-Condon analyses of the spectra indicate only small differences between the equilibrium bond lengths and bond angles of the anions and the corresponding neutral molecules. The electron affinity pattern observed among the three group VI metal tricarbonyls is compared with characteristic trends within triads of transition metal atoms, and within the coinage metal dimer series. This comparison, combined with the results of previously reported theoretical calculations, suggests that the extra electron in the M(CO)3- anions occupies an sp hybrid orbital. Electron affinities of 1.349 eV, 1.337 eV, and 1.859 eV (all +/- 0.006 eV) are obtained for Cr(CO)3, Mo(CO)3, and W(CO)3 respectively.
The photoelectron spectrum of Cr2- shows vibrational levels in the 1(Sigma) g+ ground state of the neutral molecule up to 7300 cm-1 above its zero point level. These data, obtained at an instrumental resolution of 5 meV (40 cm-1), reveal a panoramic view of the controversial ground state potential curve of Cr2. Low-lying vibrational levels are found to fit a Morse potential with (omega) e equals 479 +/- 2 cm-1 and (omega) e(chi) e equals 13.5 +/- 1.0 cm-1. This unusually large anharmonicity extrapolates to a dissociation asymptote of only 0.5 eV, considerably lower than the true 1.44 eV value. Between 4875 and 7320 cm-1 above the zero point level, we observe twenty peaks at 130 +/- 15 cm-1 intervals, which we assign as transitions from the ground electronic and vibrational state of the anion to high vibrational levels of the Cr2 ground state. Using an RKR inversion procedure, we have obtained a potential curve that fits all of the observed vibrational levels to within our experimental uncertainty. This potential curve is compared with the predictions of Goodgame and Goddard's modified GVB calculation. Transitions to highly excited vibrational levels of the Cr2 ground state are far more intense than would be expected for a direct photodetachment process, and are also strongly wavelength dependent. These non-Franck-Condon intensities are attributed to a resonance of the laser with one or more metastable states of the negative ion far above the electron detachment threshold. The electron affinity of Cr2 is measured to be 0.505 +/- 0.005 eV. An excited electronic state of Cr2 with a vibrational frequency of 580 +/- 20 cm-1 is observed 14,240 +/- 30 cm-1 above the ground state. For Cr2-, we obtain (omega) e equals 470 +/- 25 cm-1, (omega) e(chi) e equals 20 +/- 10 cm-1, and re equals 1.71 +/- 0.01 angstrom. Tentative state assignments of 1(Sigma) u+ or 3(Sigma) u+ for the excited Cr2 state, and 2(Sigma) u+ for the anion, are discussed. Preliminary results for Cr2H- and Cr2D- are also presented. The photoelectron spectra of these anions reveal the Cr-Cr and Cr-H stretching frequencies in the neutral molecules, and exhibit partially resolved rotational structure.