SOFIA has reached in the last two years its full operational capabilities and is producing now great science on typically three observing flights per week. The telescope is the backbone of the observatory and is working nearly perfectly. This may be the right time to have a look on the design history of the telescope and some of the major subsystems, which ensure the functionality under the harsh aero-acoustic environment inside the aircraft cavity. A comparison with SOFIA’s predecessor KAO gives insight in to the challenges of airborne telescope design. The paper describes the development of the optical subsystem, the telescopes structure, the telescope mount and the interface to the aircraft from the conceptual design up to the finally as-built telescope, and comments on their influence on the overall observatory performance.
The Stratospheric Observatory for Infrared Astronomy (SOFIA) will enable unique astronomical observations from visible to millimeter wavelengths. AIRES, a long-slit spectrograph with a mid-infrared slit viewing camera, would enable spectral imaging of gas-phase spectral features between 17 and 210 μm with resolving powers from ~60,000 to 5000. The Cryogenic Grating Spectrometer (CGS: AIRES' predecessor) which was flown on NASA's Kuiper Airborne Observatory (KAO) for 13 years, demonstrated the importance of this wavelength range. A 1997 proposal to develop AIRES was selected as the highest-ranked of 19 U.S. competitors for first-generation SOFIA science instruments. Funding was terminated in 2001 due to budget problems associated with an original under estimate and the advent of full cost accounting in NASA. Here we summarize AIRES' expected performance, its science potential, its status, and lessons learned. Highlighted are three successfully accomplished major technical developments: the world's largest monolithic cryogenic grating, cryogenic multiplexing amplifiers for far-infrared germanium photoconductor detectors, and an optical/mechanical design in a package suitable for installation on SOFIA. We show that AIRES would fill a unique role among other spectroscopic capabilities foreseen for space-borne missions. AIRES' capabilities remain a high but unfilled priority for SOFIA, and for the science community in general.
The 2.5 meter (m) effective diameter telescope on SOFIA - the Stratospheric Observatory for Infrared Astronomy - will operate in an open-port cavity which will be closed below operating altitudes by a cavity-door assembly. When
operating, the telescope will view the sky through an aperture defined by an aperture assembly (AA) with a nearly
rectangular opening extending 112 inches (2.84 m) in elevation (roll) and 129 inches (3.27 m) in cross-elevation. The
aperture will be servo-controlled in roll to track the telescope elevation (EL), and the aircraft heading will be adjusted to
maintain the telescope centered on the aperture in cross-elevation (XEL). An upper rigid door (URD) and lower
flexible door (LFD) move with the aperture to minimize the opening into the cavity containing the telescope. This paper
describes basic parameters of the door system, and estimates possible science impacts of its specification, configuration
and planned operation. Topics included are the geometry, expected aerodynamic disturbances, control system, gear life,
influences of radiative and diffraction effects on science instrument performance, testing, operational considerations,
and development status. As designed, the door system is expected not to limit the performance of science instruments or
observatory operational efficiency, but several potential concerns are considered. These include modulation of stray
and diffracted radiation, reliability, and maintainability.
The SOFIA telescope has a silicon carbide secondary mirror and a six degree-of-freedom secondary mirror mechanism. Each of these high-technology items represents a single-point failure mode, because both are essential for operation of the observatory, and neither has a spare. Reduced-performance, relatively inexpensive “backup” hardware can enable a large fraction of the planned SOFIA science observations, and so can help to assure a highly reliable flight program. Accordingly, we have developed an aluminum secondary mirror and derived design requirements for a backup secondary mirror mechanism that will meet minimum performance needs.
We describe the development of a cryogenic multiplexer for far-infrared (FIR) photoconductor detectors operating at moderate backgrounds. The device is called the SBRC 190. Its architecture and basic functions are based on the 1×32-channel CRC 696 CMOS device used on SIRTF. The SBRC 190 is designed to accommodate the higher backgrounds to be encountered on SOFIA and Herschel, to tolerates a wider range of backgrounds, to permit faster sampling, and to facilitate synchronization of sampling with chopping. Major design differences relative to the CRC 696 which have been incorporated in the SBRC 190 design are: (a) an AC coupled, capacitive feedback transimpedence unit cell, which minimizes input offset effects, thereby enabling low detector biases, (b) selectable feedback capacitors to enable operation over a wide range of backgrounds, and (c) clamp and sample-and-hold output circuits to improve sampling efficiency, which can be a concern at the relatively high readout rates required. A relationship between sampling efficiency and noise performance needed to achieve background-limited instrument performance (BLIP) is derived. Requirements for use on SOFIA, the basic circuit design, fabrication, and operation are discussed.
Testing of a 40 to 125 μm Ge:Sb photoconductor array for AIRES (Airborne Infra-Red Echelle Spectrometer) is described. The prototype array is a 2×24 module which can be close-stacked with other modules to provide larger two-dimensional formats. Collecting cones on a 0.08 inch pitch concentrate incident radiation into integrating cavities containing the detectors. The array is read out by two Raytheon SBRC 190 cryogenic multiplexers that also provide a CTIA (capacitive transimpedance amplifier) unit cell for each detector. We have conducted a series of tests to evaluate the array dark current, responsivity and detective quantum efficiency.
SOFIA will permit observations from the visible to mm wavelengths, and offer higher spectral and spatial resolution than any other facility at some wavelengths. Nine focal-plane instruments are being developed to exploit this capability during the first several years of SOFIA operation. These instruments are being built at universities, at research institutes in Germany, and at NASA's Goddard Space Flight Center. The broad wavelength span of SOFIA implies a wide variety of Science Instrument characteristics, including detector technologies, spectral definition techniques, and science objectives. Here we summarize the performance of the nine instruments in relatively uniform format to facilitate evaluation of feasibility of desired observations. For each instrument, three basic aspects are described: (1) spectral resolution or passbands (2) sensitivity for emission lines and/or continuum (3) angular resolution. Spectral resolution ranges from several hundred km/s down to 0.01 km/s; some of the instruments have several modes spanning several orders of magnitude within this range. Sensitivities for continuum and for emission line integrated fluxes are given in Janskies and W/m2 respectively, for specified integration time and S/N. For reference some Pogson magnitudes are also given at short (visible, near-IR) wavelengths, and some antenna temperature values are also given at sub-mm wavelengths. Angular resolution is expressed as the FWHM beam size in seconds of arc, as a function of wavelength. With this compilation of basic performance, any researcher may estimate the feasibility of potential observations with any of the first generation instruments. The performance summaries are available online at the SOFIA web site: http://SOFIA.arc.nasa.gov.
The SBRC 190 cryogenic readouts were developed for use in far-infrared arrays of Ge:Sb and Ge:Ga photoconductor detectors. The SBRC 190 provides an AC-coupled CTIA (capacitive trans-impedance amplifier) unit cell for each detector and multiplexes up to 32 detectors. This paper presents our test results characterizing and optimizing the performance of these novel devices. We discuss their basic behavior and investigate their performance in different clocking schemes.
In this paper we present the considerations for design and assembly of a stressed gallium doped germanium photoconductor array for the Airborne InfraRed Echelle Spectrometer on SOFIA. This 8 X 12 element array will cover the wavelength range from 125 to 210 micrometers . The considerations cover the aspects of the mechanical design for stressing the detectors in a uniform way, assembly of the components, contacting them electrically with minimized stray capacitance, and the layout of the light collecting cone assembly.
The basis for the pointing stability requirement in the SOFIA telescope is described. Fundamentally, it is desirable to retain the diffraction-limited image quality of the telescope to the shortest wavelengths not dominated by shear-layer seeing effects or intrinsic optical quality of the telescope. Image motion will blur the images, and may cause loss of signal and increased noise in science instruments. The expected diffraction and seeing limited image quality contributions are discussed, an analysis of the effects of image motion on observations is given, and examples related to the specification and to currently predicted performance for the SOFIA telescope are presented.
We have designed and prototyped an array of Ge:Sb photoconductors for use in AIRES, the Airborne InfraRed Echelle Spectrometer, on SOFIA. The 16 X 24 flight array will operate between 33 micrometers and 120 micrometers . In this paper we discuss the testing of a 3 X 3 prototype array and the resulting design of the flight array.
SOFIA will permit observations not possible from ground based telescopes, while retaining a number of their major advantages for observers. These include the opportunity to change focal plane instruments frequently, and continuous access to the instrument while observing. SOFIA is being designed to maximize the benefits of these features and to assure optimum performance of the instruments, within the constraints of available resources. This paper describes the top level optical, mechanical, and electronic interface parameters and configuration issues foreseen for Science Instruments on SOFIA.
NASA's Stratospheric Observatory for IR Astronomy (SOFIA) will enable unprecedented IR acuity at wavelengths obscured from the ground. To help open this new chapter in the exploration of the IR universe, we are developing the Airborne IR Echelle Spectrometer (AIRES) as a facility science instrument. Full funding was awarded for a four year development in October, 1997. The instrument is scheduled to come on-line with the observatory in the Fall of 2001. It will be used to investigate a broad range of phenomena that occur in the interstellar medium. AIRES will use a 1200 mm long, 76 degree blaze angle echelle to combine high resolution spectroscopy with diffraction-limited imaging in the cross-dispersion direction. Its three 2D detector arrays will prove good sensitivity over a decade in wavelength. An additional array will be used as a slit viewer for (lambda) <EQ 28 micrometers to image source morphology and to verify telescope pointing. Our scientific motivation, preliminary optical design and packaging, focal plane configuration, echelle prototyping, and cryostat layout are described.
The idea of interstellar communication with the aid of lasers and the possibility of manipulating natural lasers to this end make the search for lasing cosmic sources a part of the SETI program. Yet, apart from the very weak lasing in the 10 micrometers CO2 band in the Martian and Venusian atmospheres, only one case of possible natural lasing, in the 4.7 micrometers hydrogen Pf(beta) line from the Becklin-Neugenbauer source in Orion, has been reported but never confirmed. Given hundreds of maser sources detected during the 30 years after their first discover in 1965, the lack of detected natural lasers became a tantalizing puzzle. We undertook a search for high-gain hydrogen lasers in the far-infrared spectrum of MWC349, a peculiar star known as a unique source of mm/submm hydrogen masers. We used the facility cryogenic grating spectrometer onboard the Kuiper Airborne Observatory. An efficient criterion of lasing is elaborated using a set of nonlasing, spontaneous emission lines as a reference. The shortest wavelength line showing an excess of radiation in our search is H10(alpha) at 52 micrometers . We briefly discuss possible reasons for the lack of detectable lasers in the visual and near- infrared domains.