Proceedings Volume 4857

Airborne Telescope Systems II

Ramsey K. Melugin, Hans-Peter Roeser
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Proceedings Volume 4857

Airborne Telescope Systems II

Ramsey K. Melugin, Hans-Peter Roeser
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 3 March 2003
Contents: 8 Sessions, 39 Papers, 0 Presentations
Conference: Astronomical Telescopes and Instrumentation 2002
Volume Number: 4857

Table of Contents

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Table of Contents

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  • SOFIA Science Instruments I
  • SOFIA Science Instruments II
  • SOFIA Science Instruments III
  • SOFIA Science Instruments II
  • SOFIA Science Instruments III
  • SOFIA Science Instrument Technology
  • Lighter than Air Missions
  • Telescope Assembly
  • SOFIA Telescope Assembly I
  • SOFIA Telescope Assembly II
  • SOFIA Telescope Assembly I
  • Lighter than Air Missions
SOFIA Science Instruments I
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Performance summaries for SOFIA science instruments
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.
SOFIA water vapor monitor design
Robert Cooper, Thomas L. Roellig, Lunming Yuen, et al.
The SOFIA Water Vapor Monitor (WVM) is a heterodyne radiometer designed to determine the integrated amount of water vapor along the telescope line of sight and directly to the zenith. The basic technique that was chosen for the WVM uses radiometric measurements of the center and wings of the 183.3 GHz rotational line of water to measure the water vapor. The WVM reports its measured water vapor levels to the aircraft Mission Controls and Communication System (MCCS) while the SOFIA observatory is in normal operation at flight altitude. The water vapor measurements are also available to other scientific instruments aboard the observatory. The electrical, mechanical and software design of the WVM are discussed.
Characterization of FLITECAM: the first light camera for SOFIA
Amanda Kathryn Mainzer, Ian S. McLean, Ted Aliado, et al.
Preliminary test results are reported for FLITECAM, the First Light Camera for SOFIA. This instrument is designed to perform imaging from 1 to 5 μm over the entire 8 arcmin field of view of SOFIA with 0.47arcsec pixels. The detector is a 1024 × 1024 InSb array, and large refractive optics are used for collimation and re-imaging. FLITECAM also has a pupil-viewing mode optimized for 3.5 mm and can accommodate grisms for slit spectroscopy. The instrument has passed Critical Airworthiness Design Review and has received the first part of its certificate of conformity. Ground-based tests of the finished instrument are planned for later in 2002 at the Lick Observatory 3-m Shane Telescope to verify that the point spread function meets its required FWHM of 1 arcsec over the full field and wavelength range. FLITECAM will be used to test the image quality and background of the SOFIA telescope, as well as for science applications.
Forcast: the Faint Object Infared Camera for the SOFIA telescope
We report final design details and development progress for the Faint Object Infrared Camera for the SOFIA Telescope (FORCAST). FORCAST is a two-channel camera with selectable filters for continuum and line imaging in the 5-40 micron wavelength region. Simultaneous imaging will be possible in the two-channels--5-25 microns using a Si:As 256×256 blocked impurity band (BIB) detector array, and 25-40 microns using a Si:Sb BIB. FORCAST will sample 0.75 arcseconds per pixel allowing a 3.2'×3.2' instantaneous field-of-view in both channels simultaneously. Imaging will be diffraction limited for lambda > 15 microns. Since FORCAST operates in the wavelength range where the seeing is best from SOFIA, it will provide the highest spatial resolution possible from the airborne observatory. In addition to imaging, the FORCAST optical design provides for a simple upgrade to include spectroscopic observations using grisms mounted in the filter wheels. We report improvements to the optical system and progress in construction of this SOFIA facility instrument and its subsystems. FORCAST will be available for facility testing and astronomical observations at SOFIA first (f)light.
High-resolution mid-infrared spectroscopy with EXES and SOFIA
Matthew J. Richter, John H. Lacy, Qingfeng Zhu, et al.
The Stratospheric Observatory for Infrared Astronomy (SOFIA) will provide new opportunities for high spectral resolution observations in the mid-infrared. To take advantage of these opportunities, we are developing EXES, the Echelon-Cross-Echelle Spectrograph. EXES will operate from 4.5 microns to 28.5 microns and achieve a velocity resolution of 3 km/s for λ < 10 μm. EXES will be a versatile instrument with three spectroscopic modes: cross-dispersed with R~105; long-slit with R~104; and long-slit with R~3000. The unique aspect of EXES is the high-resolution capability provided by a 1 meter echelon grating and a 256 by 256 low-background Si:As IBC detector. Much of the design and operation of EXES has already been validated by the performance of a very similar ground-based instrument, the Texas Echelon-Cross-Echelle Spectrograph (TEXES). We present here a summary of the EXES design and current status; a brief description of ground-based, high spectral resolution, mid-infrared results; and a look ahead to the possible science using SOFIA and EXES.
SOFIA Science Instruments II
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The optical design of HIPO: a high-speed imaging photometer for occultations
HIPO (High-speed Imaging Photometer for Occultations) is a special-purpose science instrument for use on SOFIA (the Stratospheric Observatory For Infrared Astronomy). HIPO covers the spectral range from the atmospheric UV cutoff at 0.3 microns to the silicon detector limit at 1.1 microns. It is a dual channel imaging photometer using 1Kx1K Marconi CCD47-20 frame transfer CCD detectors. In addition to its science applications, HIPO will be used extensively during performance testing of the SOFIA observatory. The optical design of the instrument includes optimized focal reducing optics for both blue and red channels, pupil viewing optics, and Shack-Hartmann test optics. The imaging performance is excellent, insuring that the instrument will provide a faithful representation of the SOFIA telescope's PSF (Point Spread Function) during test observations.
Distributed framework for dyanmic telescope and instrument control
Troy J. Ames, Lynne Case
Traditionally, instrument command and control systems have been developed specifically for a single instrument. Such solutions are frequently expensive and are inflexible to support the next instrument development effort. NASA Goddard Space Flight Center is developing an extensible framework, known as Instrument Remote Control (IRC) that applies to any kind of instrument that can be controlled by a computer. IRC combines the platform independent processing capabilities of Java with the power of the Extensible Markup Language (XML). A key aspect of the architecture is software that is driven by an instrument description, written using the Instrument Markup Language (IML). IML is an XML dialect used to describe graphical user interfaces to control and monitor the instrument, command sets and command formats, data streams, communication mechanisms, and data processing algorithms. The IRC framework provides the ability to communicate to components anywhere on a network using the JXTA protocol for dynamic discovery of distributed components. JXTA (see http://www.jxta.org) is a generalized protocol that allows any devices connected by a network to communicate in a peer-to-peer manner. IRC uses JXTA to advertise a devices IML and discover devices of interest on the network. Devices can join or leave the network and thus join or leave the instrument control environment of IRC. Currently, several astronomical instruments are working with the IRC development team to develop custom components for IRC to control their instruments. These instruments include: High resolution Airborne Wideband Camera (HAWC), a first light instrument for the Stratospheric Observatory for Infrared Astronomy (SOFIA); Submillimeter And Far Infrared Experiment (SAFIRE), a Principal Investigator instrument for SOFIA; and Fabry-Perot Interferometer Bolometer Research Experiment (FIBRE), a prototype of the SAFIRE instrument, used at the Caltech Submillimeter Observatory (CSO). Most recently, we have been working with the Submillimetre High
Optical design and diffraction analysis for AIRES: an airborne infrared echelle spectrometer
The optical design is presented for a long-slit grating spectrometer known as AIRES (Airborne InfraRed Echelle Spectrometer). The instrument employs two gratings in series: a small order sorter and a large steeply blazed echelle. The optical path includes four pupil and four field stops, including two narrow slits. A detailed diffraction analysis is performed using GLAD by Applied Optics Research to evaluate critical trade-offs between optical throughput, spectral resolution, and system weight and volume. The effects of slit width, slit length, oversizing the second slit relative to the first, on- vs off-axis throughput, and clipping at the pupil stops and other optical elements are discussed.
SOFIA Science Instruments III
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Design and fabrication of a two-dimensional superconducting bolometer array for SAFIRE
The Submillimeter and Far-InfraRed Experiment (SAFIRE) on the SOFIA airborne observatory will employ a large-format, two-dimensional, close-packed bolometer array. SAFIRE is an imaging Fabry-Perot spectrometer operating at wavelengths between 100μm and 700μm. The array format is 16×32 pixels, using a 32-element multiplexer developed in part for this instrument. The low backgrounds achieved in spectroscopy require very sensitive detectors with NEPs of order 10-19 W/√Hz. An architecture which permits 512 pixels to be placed adjacent to each other in an area the size of a postage stamp, integrate them with multiplexers, and provide all the necessary wiring interconnections is a complex proposition, but can be achieved. Superconducting detectors can be close-packed using the Pop-Up Detector (PUD) format, and SQUID multiplexers operating at the detector base temperature can be intimately coupled to them. The result is a compact array, easily scalable to kilopixel arrays. We describe the PUD architecture, superconducting transition edge sensor bolometers we have manufactured and tested using the PUD architecture, and the electronics of SQUID multiplexed readouts. We show the design and assembly of the mechanical model of a 512-element bolometer array.
SOFIA Science Instruments II
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FIFI LS: a far-infared 3D spectral imager for SOFIA
FIFI LS is a far-infrared integral field spectrometer for SOFIA that maximizes observing efficiency by spectrally imaging fields in two medium velocity resolution bands simultaneously and nearly independently. Although the two observing bands, Red (110-210 microns) and Blue (42-110 microns), share some common fore-optics, the Field-Imaging Far-Infrared Line Spectrometer (FIFI LS) can observe diffraction-limited spectra at R = 1400 to 6500, depending on wavelength, with two separate Littrow mounted spectrometers. To further increase the observing efficiency, we employ an integral field technique that allows multiplexing spatially. This is achieved by utilizing slicer mirrors to optically re-arrange the 2D field into a single slit for a standard long slit spectrometer. Effectively, a 5 × 5 pixel spatial field of view is imaged to a 25 × 1 pixel slit and dispersed to a 25 × 16 pixel, 2D detector array. The detectors are two large format Ge:Ga arrays, axially stressed in the Red channel to achieve a longer wavelength response and slightly stressed in the Blue channel. Overall, for each of the 25 spatial pixels in each band, the instrument can cover a velocity range of approximately 1500 km/s with an estimated sensitivity of 2 × x 10-15 W Hz1/2 per pixel. This arrangement provides good spectral coverage with high responsivity. With this scheme FIFI LS will have advantages over single-slit spectrometers in detailed morphological studies of the heating and cooling of galaxies, star formation, the ISM under low-metalicity conditions as found in dwarf galaxies, active galactic nuclei and their environment, starbursts, and merging/interacting galaxies.
GREAT: the German Receiver for Astronomy at Terahertz Frequencies
Rolf Guesten, Ivan Camara, Paul Hartogh, et al.
GREAT - a heterodyne instrument for high-resolution spectroscopy aboard SOFIA is developed by a consortium of German research institutes. The first-light configuration will allow parallel observations in two far-infrared frequency bands. We will have a choice of back-ends, including a broad-band acousto-optical array and a high-resolution chirp transform spectrometer. We describe the structural and quasi-optical design of the receiver, update on the front-end and back-end developments and discuss the data acquisition system.
SOFIA Science Instruments III
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STAR: SOFIA terahertz array receiver
We present the concept for KOSMA's 16 element 1.9 THz heterodyne array STAR (SOFIA Terahertz Array Receiver) which is being developed for SOFIA. The instrument will consist of two interleaved sub-arrays of 8 pixels each. Together we will have a 4 × 4 pixel array with a beam spacing on the sky of approximately 1.5 times the beam size of 15 arcsec (FWHM). The receiver is mainly targeted at measuring the fine structure transition of ionized atomic carbon at 1.9 THz (158 microns). STAR's optics setup is modeled after the successful design used in KOSMA's SMART receiver. It will contain a K-mirror type beam rotator, a Martin-Puplett diplexer for LO coupling and an LO multiplexer using imaging Fourier gratings. Complete optical sub-assemblies will be machined monolithically as integrated optics units, to reduce the need for optical alignment. STAR will probably use waveguide mixers with diffusion cooled hot electron bolometers, which are being developed at KOSMA. The receiver backends will be KOSMA Array-AOSs. Local oscillator power will be provided by a backward wave oscillator (BWO), followed by a frequency tripler.
Far-infrared imaging spectroscopy with SAFIRE on SOFIA
The Submillimeter and Far-InfraRed Experiment (SAFIRE) on the SOFIA airborne observatory is an imaging Fabry-Perot spectrometer operating at wavelengths between 100μm and 700μm. SAFIRE’s key science goal is to investigate line emission in galaxies at wavelengths not visible from the ground, and to map the variation in this line emission in nearby galaxies. SOFIA will fly at an altitude where the atmosphere is mostly transparent, permitting SAFIRE to achieve a high point source sensitivity at most wavelengths. With a field of view of 160''×320'' at a spectral resolution of ~200km/s, when SAFIRE achieved first light in 2006, it will add substantial capability to the first light instrument complement of SOFIA. SAFIRE’s top priority observations will be to measure emission lines in the Galactic center, to map emission lines in nearby galaxies, and to understand the physics of the cores of ultraluminous galaxies from the local region to the high redshift universe through far-infrared fine-structure line emission.
Optics studies of the planned SOFIA instrument SPICA/LISA
Juergen Wolf, Alfred Krabbe, Michael Arnold
The proposed Spectral-Photometric Far-Infrared Camera (SPICA) will use the 2.5m SOFIA telescope to image astronomical objects at wavelengths between 20 and 220 mm. A low-resolution spectrometer channel will record simultaneously individual diffraction limited spectra from several tens to several hundreds of spatial positions on the sky. For both observing modes large format far-infrared arrays will be required. Their physical size would be in the range of 40 × 40 mm2 to 120 × 120 mm2. This extended geometry and the large fields-of-view are the major design drivers for the instrument optics. A concept study with a common collimator and three individual imagers showed that geometrical aberrations remain small compared to diffraction effects, even when standard manufacturing and alignment tolerances are included. An additional study also taking diffraction effects into account, showed the feasibility of a 40 - 70 mm spectrometer using a 64 × 64 element array and an image slicer to image a 26 × 46 square arcseconds sky field at a spectral resolution of ~30. The overall transmission of the spectrometer will be at the order of 20%. While the camera will study the morphology of objects, namely of their coldest matter components at 10 to 20 K, the spectrometer will determine the spectral energy distribution as well as broad band spectral features in galactic and extragalactic objects at the highest sensitivity.
SOFIA Science Instrument Technology
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Specification and design of the SBRC 190: a cryogenic multiplexer for far-infrared photoconductor detectors
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.
Characterization of SBRC 190: a multigain cryogenic readout multiplexer for IR detector arrays
Jam Farhoomand, David Sisson, Lunming Yuen, et al.
SBRC-190 readout multiplexer is a 1×32, multi-gain, capacitive transimpedance amplifier (CTIA) especially suitable for use with infrared detector arrays requiring low-bias levels--such as Ge:Ga far infrared detector arrays. The unit-cell design employs a feedback loop which keeps the bias across the detector constant and, therefore, prevents debiasing--a critical requirement for far infrared detectors. We have tested a number of these multiplexers at various cryogenic temperatures, down to 1.7K. In this presentation we will report the results of our tests and will discuss gain, uniformity, and read noise of the bare mux under correlated-double sampling at 4.2K.
Development and testing of a 1024x1024 pixel Si:As IBC detector for SOFIA-like applications
Kimberly A. Ennico, Thomas P. Greene, Craig R. McCreight, et al.
Airborne and space telescope astronomical observations in the 5-25 micron wavelength region are critical for understanding the physical conditions, composition, chemistry, and excitation of many environments in the interstellar medium, external galaxies, solar system objects, extra-solar systems, and stars. The scientific impact is particularly unique in the 5-8 micron and 14-25 micron regions which are inaccessible or poorly observed from ground-based observatories. Large format mid-infrared detectors sensitive over these wavelengths and operable under moderate backgrounds (~106 photons/s/pixel at R=2000, at 10 microns) are essential for efficient large-area survey imaging and for taking moderate resolution spectra over a large spectral range. Both SOFIA and passively cooled Explorer observatories could benefit from this technology. Current first-light SOFIA instruments use small-format mid-infrared focal plane arrays of sizes 256 × 256 pixels. With the collaboration of Raytheon Infrared Operations, NASA-Ames Research Center has developed and tested the first 1024 × 1024 mid-infrared device suitable for operating under moderate backgrounds: a combination of the ALADDIN III readout multiplexer, cryo-processed for 6 K operation, with Si:As IBC detector material designed for high QE. This device has exhibited low dark current, moderate noise levels, and > 200,000 electron linear well size at 6 K operation. We conclude with suggestions for future device development for optimal performance under moderate background, SOFIA- and low Earth orbit observing conditions.
FIFI LS: the optical design and diffraction analysis
We present the optical system of the Field-Imaging Far-Infrared Line Spectrometer (FIFI LS) for the SOFIA airborne observatory. The instrument is designed to allow diffraction limited integral field spectroscopy in the far infrared wavelength range 42 to 210 microns. Two parallel wavelength channels (42 - 110 microns and 110 - 210 microns) employ Littrow mounted diffraction gratings with anamorphic collimators. Mirror image slicers in each channel rearrange the 5 × 5 pixel field of view along the 1 × 25 entrance slit of the grating spectrograph. The spectral resolution varies in the range of R = 1400 - 6500, depending on observing wavelength. The optical components in the image slicer is comprised of several mirrors with physical dimensions on the order of a few tens of wavelength. Consequently diffraction effects are a serious concern in the design of the optical system. Substantial effort in modeling diffraction effects throughout the optical system and its impact upon the expected performance of the instrument have been made. The results of the scalar diffraction analysis carried out with a commercial software package has been confirmed by a full vectorial analysis, showing negligible dependence of the diffraction effects on the polarization properties of the electromagnetic field.
Performance of the SBRC 190: a cryogenic multiplexer for photoconductor arrays
Jessie L. Dotson, Christopher T. Koerber, Christopher G. Mason, et al.
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.
Evaluation of a 40- to 125-um Ge:Sb photoconductor array for airborne astronomy
Christopher Mason, Jessie L. Dotson, Christopher T. Koerber, et al.
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.
Lighter than Air Missions
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Long-duration flight of the TopHat experiment
Robert F. Silverberg, James Aguirre, Jeff Bezaire, et al.
The TopHat instrument was designed to operate on the top of a high altitude balloon. From this location, the experiment could efficiently observe using a clean beam with extremely low contamination from the far side lobes of the instrument beam. The experiment was designed to scan a large portion of the sky directly above it and to map the anisotropy of the Cosmic Microwave Background (CMB) and thermal emission from galactic dust. The instrument used a one-meter class telescope with a five-band single pixel radiometer spanning the frequency range from 150-600 GHz. The radiometer used bolometric detectors operating at ~250mK. Here, we report on the flight of the TopHat experiment over Antarctica in January, 2001 and describe the scientific goals, the operation, and in-flight performance.
The EDGE project
EDGE is a Long Duration Balloon (LDB) borne instrument designed to measure the large-scale anisotropy of the Cosmic Infrared Background (CIB). The goal is to use this signal as a new observational tool to measure the character of the spatial distribution of galaxies at the largest spatial scales. With a 6\arcmin\ beam mapping more than 400 square degrees of sky at 8 frequency bands between 250GHz and 1.5 THz the experiment can determine the variation of galaxy density on spatial scales ranging from >200h-1 Mpc, where dark matter variations are determined directly from Cosmic Microwave Background Radiation (CMBR) anisotropy, to <5h-1 Mpc where the distribution of dark matter and galaxies is determined from galaxy redshift surveys and the underlying dynamics of structure growth is non-linear. The instrument consists of a 1-meter class off-axis telescope and a Frequency Selective Bolometer (FSB) array radiometer. The FSB design provides the compact, multi-chromatic, high sensitivity focal plane needed for this measurement.
Concepts for a lightweight balloon telescope for solar observation
Sunrise is a bi-national US-German program for spectro-polarimetric high-resolution observations of the sun. The ob-servations will be done by a balloon telescope circulating in high altitudes around Antarctica. The paper describes de-sign concepts for the telescope in the balloon gondola. The designs are driven by the environment of the balloon (which is something between earthbound and space). Main design drivers for the telescope are light-weighting, thermal treat-ment of looking to the sun, stability and alignment of the optical elements during operation and handling and safety issues during lift-off and landing. The achievable weight and performance data are demonstrated by structural and ther-mal analyses results.
Feasibility study of a stratospheric-airship observatory
Douglas K. Griffin, Bruce Miles Swinyard, Sunil Sidher, et al.
This paper explores the concept of utilizing a long duration stratospheric airship as an astronomical observatory in the sub-millimetre wavelengths. In the first section of the paper, a conceptual description of the airship platform is presented along with the principles of operation of the platform. The results of a computer design code and trajectory simulation code are presented. These codes show that through the use of a modest power and propulsion system, the difficulty of constructing such a such a platform is greatly reduced. Finally, the results of a brief study into the accommodation and optical performance of a 3.5m class telescope and photometric and spectrographic instrument similar to the Herschel/SPIRE system within such an airship are presented. This study indicates that while the atmospheric absorption and emission characteristics impose some limitations on the spectrographic and photometric performance of the system in the 200μm to 1000μm band, the overall performance is more than adequate to render the concept viable and complementary to existing and planned ground, airborne and space based observatories.
Telescope Assembly
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Becoming reality: the SOFIA Telescope
After 4.5 years of development, the telescope of the Stratospheric Observatory For Infrared Astronomy, SOFIA is becoming reality. The telescope module was delivered at the end of August 2002 from Germany to Waco/Texas, where the integration into the aircraft will begin in fall 2002. Here I present a progress report and describe the recent achievements as well as the status of the telescope.
Evolution of the SOFIA Telescope system design: lessons learned during design and fabrication
The SOFIA Telescope System Design is very different from any other earthbound or space telescope design. The only somehow similar telescope is that of the Kuiper Airborne Observatory KAO dating about 40 years ago. In the course of the SOFIA program development an enormous number of concepts for the telescope were drafted (e.g. the author drafted at least 5 different concepts). The concepts were from very near to the KAO to very different. Now, after the finally chosen concept is realized, it is time 1. to look back on lessons learned during the realization, what was good, what was troublesome; and 2. to look from this viewpoint to the future of the SOFIA telescope itself as well as what should be taken in mind for the next telescope projects.
SOFIA primary mirror assembly: structural properties and optical performance
Hermann Bittner, Matthias Erdmann, Peter Haberler, et al.
The primary mirror assembly is the key opto-mechanical subsystem of the airborne SOFIA telescope. It consists of a 2.7-m primary mirror and a mirror support structure, the so-called primary mirror cell. The mirror is a monolithic ele-ment of Zerodur with a milled honeycomb structure on the backside. Despite of its size it has a mass of approx. 885 kg only. The mirror cell is a lightweight structure made from CFRP panels and profiles, bonded and riveted together with metallic inserts and joints. It provides an isostatic but stiff mounting of the mirror. The first natural frequencies are pre-dicted to be above 70 Hz for the whole 2000-kg assembly. The paper presents the actual structural properties of the primary mirror assembly determined in a modal survey test as well as the optical performance of the mirror mounted in the cell measured in horizontal and vertical orientation of the optical axis.
Development of the SOFIA silicon carbide secondary mirror
Michel Fruit, Pascal Antoine, Jean-Luc Varin, et al.
The SOFIA telescope is ajoint NASA-DLR project for a 2.5 m airborne Stratospheric Observatory for IR Astronomy to be flown in a specially adapted Boeing 747 SP plane, Kayser-Threde being resopinsible for the development of the Telescope Optics. The φ 352 mm Secondary Mirror is mounted ona chopping mechanism to allow avoidance of background noise during IR observations. Stiffness associated to lightness is a major demand for such a mirror to achieve high frequency chopping. This leads to select SIlicon Carbide for the mirror blank. Its development has been run by the ASTRIUM/BOOSTEC joint venture SiCSPACE, taking full benefit of the instrinsic properties of the BOOSTEC SiC-100 sintered material, associated to qualified processes specifically developed for space borne mirrors by ASTRIUM. Achieved performances include a low mass of 1.97 kg, a very high stiffness with a first resonant frequency of 1865 Hz and a measured optical surface accuracy of 39 nm rms, using Ion Beam Figuring. It is proposed here to present the major design features of the SOFIA Secondary Mirror, highlighting the main advantages of using Silicon Carbide, the main steps of its development and the achieved optomechanical performances of the developed mirror.
SOFIA secondary mirror Hindle test analysis
The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a NASA facility, nearing completion, consisting of an infrared telescope of 2.5 meter system aperture flying in a modified Boeing 747. Its Cassegrain secondary mirror has recently completed polishing. The SOFIA Project Office at Ames Research Center considered it important to perform an independent analysis of secondary mirror figure. The polishing was controlled by the standard test for a convex hyperboloid, the Hindle test, in a modified form with a meniscus lens partially reflecting on the concave face, rather than a fully reflecting mirror with a central hole. The spacing between this meniscus lens and the secondary mirror was controlled by three peripherally located spacing spheres. This necessitated special analysis to determine what the resulting curvature and conic constant of the mirror would be, if manufacturing imprecisions of the test set-up components were to be taken into account. This set-up was specially programmed, and the resulting hyperboloid calculated for the nominal case, and all extreme cases from the reported error limits on the manufacturing of the components. The results were then verified using the standard program CODE-V of Optical Research Associates. The conclusion is that the secondary mirror has a vertex radius of curvature of 954.05 mm ± .1 mm (design value: 954.13), and a conic constant of -1.2965 ± .001 (dimensionless, design value: -1.298). Such small divergences from design are to be expected, and these are within the refocusing ability of SOFIA, and would result in an acceptably small amount of spherical aberration in the image.
Image quality prediction for the SOFIA Telescope using actual mirror data
Hermann Bittner, Matthias Erdmann
The airborne Stratospheric Observatory for Infrared Astronomy SOFIA has a 2.7-m Cassegrain telescope with Nasmyth focus. The optical elements of the telescope are a 2.7-m diameter Zerodur parabolic primary mirror, mounted in a CFRP mirror support structure, a 0.35-m diameter convex silicon carbide secondary mirror, mounted on a chopping mechanism, and a plane dichroic tertiary mirror element. The nominal telescope focal length is 49.14 m. There are several distortion contributions to the total image quality. Among these are alignment errors, fabrication imperfections stemming from the manufacturing and polishing processes, static and dynamic mirror surface deformations caused by the mounting forces due to gravity and temperature effects and forces induced by aircraft vibra-tions and aerodynamic effects, and diffraction effects. The paper presents simulations for the image quality using wave-front performance data actually measured with the finished mirrors, where the primary was even mounted in its support structure. The influence of the static distortion contributions is also quantified.
SOFIA Telescope Assembly I
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Manufacturing and integration of the SOFIA suspension assembly
Eberhard Sust, Ulrich Weis, Eckhard Bremers, et al.
The Suspension Assembly is the most complex mechanical subsystem of the SOFIA telescope, responsible for suspending and positioning the telescope in the aircraft on the sky. It is a highly integrated system comprising of a vibration isolating system, a spherical hydraulic bearing, a spherical torque motor, a coarse drive and airworthiness relevant components like brakes, hard-stops etc. The components were manufactured under airworthiness standards by dedicated suppliers and integrated and commissioned in 2001/2002 at MAN Technologie in Augsburg. The paper describes the experience gotten during the manufacturing and integration process.
System integration and testing of the SOFIA Telescope
Walter Schubbach, Eberhard Sust, Martin Suess
System integration and testing of an airborne telescope like SOFIA is a complex process and needs to be done in three phases: 1. pre-assembling the telescope on ground; 2. integration of the telescope into the aircraft on ground; 3. in-flight commissioning and testing. Due to practical reasons (available time resources, costs) not every item can be fi-nally commissioned and tested under all environmental conditions before flight, and cooperation and joint activities between the aircraft people, the telescope people and the later users is necessary. The paper describes, how the work is shared in the three phases, what experience has been gotten so far and what is planned for the remaining activities.
Airworthiness verification of an airborne telescope in practice
Hartmut Dreger, Eckhard Bremers, Juergen Kuehn, et al.
The SOFIA Telescope is part of the outer hull of the pressurized passenger cabin of the SOFIA aircraft, in which the aircraft crew, the astronomers and their guests are located during flight. Therefore the telescope - including the science instrument - is an airworthiness relevant component of the observatory and has to fulfill airworthiness standards ac-cording the Federal Aviation Authority. The airworthiness issues were main drivers in the process of design, manufacturing, quality control, testing and documentation. The paper describes the experience gotten during this troublesome, exciting and costly job.
SOFIA tracking subsystem: results of assembly, operation, and calibration tests
Hermann Bittner, Matthias Erdmann, Juergen Schmolke, et al.
The SOFIA airborne telescope has a Tracking Subsystem for stellar acquisition, tracking, and pointing. The system has three high-performance imagers: the boresighted wide field (6 degrees FOV) and fine field imagers (70 arcminutes FOV), and the main-telescope-optics sharing focal plane imager (8 arcminutes FOV). The imagers are controlled by 3 CCD head controllers, an overall imager controller, and a tracker controller providing the tracking error signals from the objects observed by the imagers. There have been several test steps in the assembly, integration, and verification of the Tracking Subsystem. The paper presents the fully integrated system as actually built, the results of the thermal-vacuum and vibration tests of the fine field imager, the tested operational/functional S/W performance, as well as the results of the geometric and radiometric calibrations of the imagers.
SOFIA Telescope Assembly II
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User-relevant TA data architecture
Torsten Levin, Klaus Lattner, Hermann Bittner
For operation and monitoring, the SOFIA telescope assembly comprises a dedicated controls system. Basically this system appears to the user to consist of four major controller subsystems: Master Control Processor (MCP), Tracker Controller (TRC), Telescope Assembly Servo Control Unit (TASCU) and Secondary Mirror Controller (SMC). They are accessible via a single command driven interface link through the MCP. With the example of the subsystems MCP and TRC, commonality aspects in communication interface architecture and software protocol selection are discussed. The special requirements associated with the installation in an aircraft are considered as well as maintenance aspects for 20 years of operation. The paper covers user relevant hard-/software design aspects of the TA flight as well as ground support systems. The usage of the TA data structure definitions, provided in XML, throughout the whole observatory for interface description and implementation are explained.
Telescope control system of SOFIA
Karl Wandner
The telescope control system of an airborne observatory like SOFIA is the key component which realizes the commanding of the telescope by the flight crew and the observer. It has also to coordinate internally the complex optical and mechanical subsystems and to optimize their behavior under harsh aircraft environment. The paper describes the main features of the control system and the status of realization, implementation and testing.
Airborne pointing and pointing improvement strategy for SOFIA
Martin Suess, Karl Wandner, Hans Juergen Kaercher, et al.
Pointing is one of the most peculiar tasks in an airborne telescope. During design and realization on ground only half of the story can be solved. Final commissioning, tuning and testing can only be executed with the real telescope in the real aircraft during flight. The paper describes the commissioning and testing strategy for the pointing control on ground and in flight, and reports what has been done up to now at the pre-assembled telescope on ground, what is planned for the implementation 'in flight', and what strategy for further improvement is applied.
SOFIA Telescope Assembly I
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The SOFIA aircraft and its modification
The primary focus of this paper is to describe the development of a highly modified aircraft that carries a twenty ton telescope to the stratosphere and then loiters at this desired altitude to act as the observatory platform and dome. When the aircraft has reached its nominal cruise condition of Mach 0.84 in the stratosphere, a large cavity door opens (the dome opens), exposing a large portion of the interior of the fuselage that contains the telescope optics directly to the Universe. The topics covered in this paper include: the relevant criteria and the evaluation process that resulted in the selection of a Boeing 747-SP, the evolution of the design concept, the description of the structural modification including the analysis methods and tools, the aerodynamic issues associated with an open port cavity and how they were addressed, and the aeroloads/ disturbances imparted to the telescope and how they were measured in the wind tunnel and extrapolated to full size. This paper is complementary to a previous paper presented at the 2000 Airborne Telescope Systems conference which describes the challenges associated with the development of the SOFIA Telescope. For completeness, this paper also provides a brief overview of the SOFIA project including the joint project arrangement between NASA and DLR, a top level overview of the requirements, and finally the current project status.
Lighter than Air Missions
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Design issues for the SUNRISE Telescope
Hermann Bittner, Elke Schmidt, Klaus-Ruediger Haertel, et al.
SUNRISE is a balloon-borne instrument for spectro-polarimetric high-resolution observation of the solar atmosphere. It has a lightweight UV-VIS telescope of Gregory type with an aperture of 1 m, designed to be close to the VIS diffraction limit. The paper will first present the basic prescriptions of the optical design and the achievable performance. The re-quirements for the mechanisms in order to maintain the alignment over the range of environmental conditions will be derived. Secondly, the structural and thermal requirements will be discussed. Here, structural deflections due to gravity and residual thermal imbalances have to be taken into account. Preliminary structural and thermal designs will be out-lined.