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 present a progress report on the design and construction of the Field-Imaging Far-Infrared Line Spectrometer (FIFI LS) for the SOFIA airborne observatory. The design of the instrument is driven by the goal of maximizing observing efficiency, especially for observations of faint, extragalactic objects. Thus, FIFI LS utilizes an integral field technique that uses slicer mirrors to optically re- arrange the 2D field into a single slit for a long slit spectrometer. Effectively, a 5 X 5 pixel spatial field of view is imaged to a 25 X 1 pixel slit and dispersed to a 25 X 16 pixel, 2D detector array, providing diffraction- limited spatial and spectral multiplexing. In this manner, the instrument employs two parallel, medium resolution (R approximately 2000) grating spectrometers for simultaneous observations in two bands: a short wavelength band (42 to 110 micrometers ) and a long wavelength band (110 to 210 micrometers ). Overall, for each of the 25 spatial pixels, the instrument can cover a velocity range of approximately 1500 km/s around selected far-infrared lines with an estimated sensitivity of 2 X 10^-15 W Hz^1/2 per pixel. This arrangement provides good spectral coverage with high responsivity.

A consortium of German research laboratories has been established for the development of a modular dual-channel heterodyne instrument (GREAT: German Receiver for Astronomy at Terahertz Frequencies) for high-resolution spectroscopy aboard SOFIA. The receiver is scheduled to be available in time for SOFIA's very first astronomical mission in late 2002. The first-flight version will offer opportunities for parallel observations in two frequency bands. We will have a choice of backends, including an acousto-optic array (4 X 1 GHz) and a high-resolution chirp transform spectrometer.

The CAltech Submillimeter Interstellar Medium Investigations Receiver (CASIMIR) is a multichannel, heterodyne spectrometer being developed for the Stratospheric Observatory for Infrared Astronomy (SOFIA). It has a very high resolution, up to a million, over the submillimeter and far-infrared wavelength range of 150 to 600 micrometers , or 2.0 to 0.5 THz. CASIMIR is extremely well suited to the investigation of both the galactic and extragalactic warm, approximately 100 K, interstellar medium. A combination of advanced SIS and Hot Electron Bolometers receivers will be used to cover this frequency range with very high sensitivity. CASIMIR will use only solid state local oscillators, with quasioptical coupling to the mixers. We present a description of the instrument and its capabilities, including detailed discussions of the receivers, local oscillators and IF systems.

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 are developing a high spectral resolution grating spectrograph as a PI instrument for the Stratospheric Observatory for Infrared Astronomy. The Echelon-Cross- Echelle Spectrograph (EXES) will operate at 5.5 - 28.5 micrometers in three spectroscopic modes: R approximately 10⁵, 2 X 10⁴, and 4000. We use an echelon, a coarsely ruled, steeply blazed diffraction grating to achieve high resolution. Cross-dispersion is done with an echelle used at relatively low order. The detector is a 256 X 256 pixel Si:As IBC array. A very similar instrument, the Texas Echelon-Cross-Echelle Spectrograph (TEXES), has had a successful telescope run at the McDonald Observatory. We give here a progress report on the design of EXES and the status of TEXES.

As a facility class instrument on SOFIA, FLITECAM will be developed at the UCLA Infrared Imaging Detector Laboratory. Its primary purpose is to test the SOFIA telescope imaging quality from 1.0 to 5.5 microns, using a 1024 X 1024 InSb ALADDIN II array. Once the telescope test flights are finished, FLITECAM will be available to the science community. FLITECAM's field of view of 8' in diameter, with a plate scale of 0.47' per pixel, is one of the largest available for any facility camera. Grisms are available to produce moderate resolution of R approximately equals 1000 - 2000, depending on the slit width, with direct ruled ZnSe grisms. The detector readout electronics will be provided by Mauna Kea IR Inc. and is able to operate the detector array at all its planned operation modes, including occulations, telescope-nodding, high-speed shift-and-add, and optionally chopping at the longer wavelengths. Here we present our design approach to achieve those specifications. We also discuss the most important tests FLITECAM will carry out and give examples of science projects on SOFIA. For the latter, we present a preliminary list of filters which is expandable and open for discussions within the science community.

HOPI is a special-purpose science instrument for SOFIA that is designed to provide simultaneous high-speed time resolved imaging photometry at two optical wavelengths. We intend to make it possible to mount HOPI and FLITECAM on the SOFIA telescope simultaneously to allow data acquisition at two optical wavelengths and one near-IR wavelength. HOPI will have a flexible optical system and numerous readout modes, allowing many specialized observations to be made. The instrument characteristics required for our proposed scientific pursuits are closely aligned to those needed for critical tests of the completed SOFIA Observatory, and HOPI will be used heavily for these tests.

This paper will present test and performance data on the two largest monolithic, echelle gratings ever ruled. These cryogenically cooled gratings were manufactured by Hyperfine, Inc. and were developed for the Stratospheric Observatory For Infrared Astronomy (SOFIA).

We present a new type of phase grating, the Fourier grating, to be used as local oscillator beam multiplexer in heterodyne receivers. The device has been developed for the SOFIA Terahertz Array Receiver (STAR). In contrast to the binary phase gratings (Dammann gratings), which are being used in many array receivers, our gratings have a smooth surface structure without any sharp edges.

In this paper, we present the design for a 16-channel heterodyne array receiver for use on SOFIA. The array will be capable of using either hot-electron bolometers or membrane mounted Schottky diodes in efficient, low-cost waveguide mounts. Focal plane arrays will be constructed to target astrophysically important lines between approximately 1.9 and 3 THz. Due to the prevailing physical conditions in the interstellar medium, this frequency range is one of the richest in the FIR portion of the spectrum. An array receiver designed for this wavelength range will make excellent use of the telescope and the available atmospheric transmission, and will provide a new perspective on stellar, chemical, and galaxy evolution in the present as well as past epochs. The proposed system uses the most sensitive detectors available in an efficient optical system.

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.

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.

We are developing 2D 16 X 25 pixel detector arrays of both unstressed and stressed Ge:Ga photoconductive detectors for far-infrared astronomy from SOFIA. The arrays, based on earlier 5 X 5 detector arrays used on the KAO, will be for our new instrument, the Far Infrared Field Imaging Line Spectrometer (FIFI LS). The unstressed Ge:Ga detector array will cover the wavelength range from 40 to 120 micrometers , and the stressed Ge:Ga detector array from 120 to 210 micrometers . The detector arrays will be operated with multiplexed integrating amplifiers with cryogenic readout electronics located close to the detector arrays. The design of the stressed detector array and results of current measurements on several prototype 16 pixel linear arrays will be reported. They demonstrate the feasibility of the current concept.

Heterodyne receivers for applications in astronomy need quantum limited sensitivity. We have investigated phonon- cooled NbN hot electron bolometric mixers in the frequency range from 0.7 THz to 5.2 THz. The devices were 3.5 nm thin films with an in-plane dimension of 1.7 X 0.2 micrometers ² integrated in a complementary logarithmic spiral antenna. The best measured DSB receiver noise temperatures are 1300 K (0.7 THz), 2000 K (1.4 THz), 2100 K (1.6 THz), 2600 K (2.5 THz), 4000 K (3.1 THz), 5600 K (4.3 THz), and 8800 K (5.2 THz). The sensitivity fluctuation, the long term stability, and the antenna pattern were measured. The results demonstrate that this mixer is very well suited for GREAT, the German heterodyne receiver for SOFIA.

The Submillimeter Astronomy Investigation of Line Spectra is a balloon-borne experiment under study for a 100 day ultra- long duration balloon mission. The experiment would survey the galactic plane with 1 arc minute angular resolution and 1 km/sec velocity resolution in the important submillimeter lines of CII, NII, and OI. These tracers provide the structure and energetics of major components of the interstellar medium. This knowledge is crucial for understanding the life cycle of the Galactic gas and the processes of star formation and galactic evolution. This instrument's survey of large regions of the galactic plane complements both FIRST and SOFIA which will excel at pointed observations with higher angular resolution and broader spectral coverage. Details of the instrument design and observing strategy are presented.

We have developed the new balloon-borne telescope, Far Infrared Balloon-Borne Experiment (FIRBE), to survey the far-infrared radiations of star-forming regions. The primary mirror is an offset parabolid with a diameter of 50 cm (F/2) and telescope structure is made from Carbon Fiber Reinforced Plastics to lighten the whole telescope and hold the strain of image at the focal position minimum since its thermal contraction is very small. The telescope optics is off-axis system with on second mirror and no warm support structure in its optical path in order to reduce the infrared emission from the telescope structure itself.

In January 2000, an 80-cm F/1.5 Ritchey-Chretien solar telescope flew for 17 days suspended from a balloon in the stratosphere above Antarctica. The goal was to acquire long time series of high spatial resolution images and vector- magnetograms of the solar photosphere and chromosphere. Such observations will help to advance our basic scientific understanding of solar activity, in particular flares. Flying well above the turbulent layers of the Earth's atmosphere, the telescope should be able to operate close to its diffraction limited resolution of 0.2 arcsec, providing high resolution observations of small scale solar features. To achieve this goal we developed a platform for the optical telescope that is stable to nearly 10 arcsec. We also developed an image motion compensation system that stabilizes the solar image on the CCD focal plane to about 1 arcsec.

The terrestrial atmosphere is a great nuisance to astronomical observations of celestial objects--the attenuation due to the water-vapor absorption and the image- sharpness diluting effect caused by thermal turbulence. The alternatives to ground-based telescopes have been hitherto space-based or airplane-borne telescopes. Astronomical telescopes on airship-like structures could circumvent the limitations of the former alternatives. Starting with the airships of the early 20th century we present the modern airships of the 21st century, that will serve as future flying cranes for heavy and spacious loads or quasi- geostationary platforms for telecommunications at heights of 20 km. A 2 m telescope on such an aerostatic platform could serve as a first prototype for an astronomical stratospheric observatory.

The SOFIA telescope as the heart of the observatory is a major technological challenge. I present an overview on the astronomical and scientific requirements for such a big airborne observatory and demonstrate the impact of these requirements on the layout of SOFIA, in particular on the telescope design as it is now. Selected components of the telescope will be described in their context and functionality. The current status of the telescope is presented.

Telescopes are traditionally built and operated on earth. Others are built for operation in space. For both environments much of experience and data are available, which allow for a relatively straight-forward design. Not so an airborne telescope, which will be used not only for science but also for public outreach--a great idea, realized with SOFIA, the Stratospheric Observatory for Infrared Astronomy--with the consequence, that school classes will fly with the observatory. Therefore the telescope must be aircraft certified by the FAA, the Federal Aviation Administration, because it is part of the cabin pressure vessel. Also the environment for the telescope in the open cavity of the observatory aircraft is unfriendly compared with other telescope environments. It is surrounded by aircraft vibrations, high frequent excitations by air turbulence and temperature differences of 70 degree(s)C between the mirrors and the location of the science instruments. The paper explains the major design features of the SOFIA telescope, which are dominated by the harsh requirements.

This paper describes some of the many challenges involved in trying to obtain 0.2 arc-second (1 micro-radian) pointing stability for a large telescope mounted in an open port cavity on board an aircraft flying in the stratosphere, specifically the Stratospheric Observatory For Infrared Astronomy Project. It includes an overview of the SOFIA project including the joint project arrangement between NASA and DLR, the project status, a top overview of the science objectives and the resulting requirements.

Telescopes are traditionally operated on earth or in space. For both environments a lot of experience and data are available, which allow a proper and straightforward design. Not so an airborne telescope, which will be used on a transport aircraft, not only for science but also for public outreach, with school classes flying with the observatory. Therefore the telescope as a part of the aircraft must be certified by the `Federal Aviation Administration'. Because the telescope as a permanently mounted payload amends or even supersedes safety relevant parts of the aircraft, the rules applied to the TA are identical to the standard regulations for the basic airplane. However, these rules are obviously not tailored to airborne telescopes. The presentation explains the major rules applicable for the TA and the process to prove the compliance with these rules. Furthermore, the influence on design and manufacturing of telescope assemblies is shown.

The stratospheric observatory SOFIA will provide astronomers routine access to celestial objects at wavelength bands which are not observable from the ground due to atmospheric absorption. SOFIA comprises a 2.5 m entrance pupil diameter telescope, including all required control systems, installed in a Boeing 747SP aircraft to form an observatory which is operated at altitudes above 12.5 km. Since January 1997 the telescope system is being developed by an industrial team for the German and US space agencies DLR and NASA. Operating a high precision instrument within the extreme environment of the stratosphere as well as stringent restrictions on the telescope mass demand technical solutions that represent the cutting edge of the state of the art optical instrument design.

REOSC, SAGEM Group, has a significant contribution to the SOFIA project with the design and fabrication of the 2.7-m primary mirror and its fixtures as well as the M3 mirror tower assembly. This paper will primarily report the progress made on the primary mirror design and the first important manufacturing step: its lightweighting by machining pockets from the rear side of the blank.

One of the main requirements for the SOFIA Telescope Assembly is the achievement of a pointing stability of 0.2 arcsec. For ground based telescopes, this does not appear to be a major problem, caused by the use of heavy foundation to stabilize the telescopes to the ground. But since the SOFIA telescope is mounted into an aircraft, a high technology Suspension Assembly is required.

Supporting the Telescope structure is done via a Bearing Sphere in a Hydraulic bearing which is part of the SOFIA Suspension system. Frictionless bearing requires a sphere with varying stiffness in defined directions and a smooth and wear resistant surface giving also corrosion resistance against humidity and hydraulic fluid. Weight restrictions and through penetration by the Nasmyth Tube required a highly sophisticated design and manufacturing techniques. The 1.2 m sphere is cast from nodular cast iron, machined and coated by electrodeposition with a sulfamate nickel coating on the outer bearing sphere surfaces. The sphere represents a one of a kind construction unique in stiffness to mass ratio. The design as well as manufacturing philosophy of the sphere are presented.

Design of earthbound telescopes is normally based on conventional steel constructions. Several years ago thermostable CFRP Telescope and reflector structures were developed and manufacturing for harsh terrestrial environments. The airborne SOFIA TA requires beyond thermostability an excessive stiffness to mass ratio for the structure fulfilling performance and not to exceed mass limitations by the aircraft Boeing 747 SP. Additional integration into A/C drives design of structure subassemblies. Thickness of CFRP Laminates, either filament wound or prepreg manufactured need special attention and techniques to gain high material quality according to aerospace requirements. Sequential shop assembly of the structure subassemblies minimizes risk for assembling TA. Design goals, optimization of layout and manufacturing techniques and results are presented.

The challenging pointing stability requirement for the SOFIA telescope requires the application of sophisticated control concepts and high performance sensors and actuators. The pointing control concept involves a tracking system with optical cameras, a three-axis inertial stabilization loop, and the compensation of telescope deflections taking the secondary mirror control system into account. Special algorithms and techniques based on finite element calculations are applied for the online identification of telescope deflections and the design of the position control loop algorithm. New developments have been performed with regard to low noise fiber optic gyroscopes and spherical torquers for three degrees of freedom. The paper explains the control concept of the SOFIA telescope.

The Tracking Subsystem of the SOFIA telescope consists of three high performance imagers and a dedicated tracking control unit. There are two boresighted imagers for target acquisition and tracking, one with a wide (6 degrees) and one with a fine (70 arcmin) field-of-view, and one main- telescope-optics sharing imager with a narrow field-of-view (8 arcmin) for high performance tracking. From the recorded stellar images, tracking error signals are generated by the tracker controller. The tracker controller has several features to support various tracking schemes such as tracking the telescope as an inertial platform, on- axis/offset tracking, and limb tracking. The tracker has three modes, i.e. positioning, tracking and `override'. Special features are the handling of so-called areas-of- interest in the inertial reference frame and the external imager synchronization. The paper presents the design and functional/operational performance of the imagers and the tracking control unit.

To provide astronomers access to infrared wavelength unavailable from the ground the airborne telescope SOFIA is in development. This paper focuses on the image stability of the telescope, its modeling and simulation. The operation of the telescope under the harsh environmental conditions in the aircraft makes the prediction of the image stability during the design process necessary. For this purpose an integrated mathematical simulation model, which includes the optics, the structural dynamics and the control loops has been constructed. Because of the high relevance of the structural dynamics for image stability and control design, special attention is paid to the import and reduction of the finite element model of the telescopes mechanical structure. Different control approaches are considered for the attitude control and the compensation of the impact of the structural flexibility on the image motion. Additionally the secondary mirror servo-mechanism is utilized to optimize the image stability. Simulation results are shown.

The SOFIA telescope chopping secondary mirror is mounted on a Focus Centering Mechanism. This system is a novel type of parallel manipulator (hexapod) made of six linear actuators which provide active alignment and focus of the chopper unit with respect to the top ring frame. We describe the development of the compact high-precision linear actuator used for this hexapod mechanism. The paper reports the test results measured on the actuator prototype proving its submicron position accuracy capability as well as its high stiffness and force. The prototype was designed to be largely representative of the flight unit ones currently in the construction phase.

The software architecture for the SOFIA Mission Controls and Communications System (MCCS) must provide the initial operating capability for the observatory and support future needs over the planned twenty-year operational life. Two factors will drive the changing operational requirements of the observatory: (1) technology advances that will change the implementation of existing systems and (2) science advances that will define what the user will try to accomplish with the observatory. The MCCS software architecture must be able to sustain existing operational capability as the technology of the surrounding subsystems evolve, and it must be capable of providing new functions and features as new requirements are identified for the system. This paper describes how the MCCS software architecture provides a de-coupled infrastructure, using technologies such as XML and CORBA, to meet SOFIA's needs of an extensible and flexible command and data system with high mission reliability.

The Mission Controls and Communications System provides the communications infrastructure and mission operations framework to conduct science missions aboard the Stratospheric Observatory For Infrared Astronomy. There are several key philosophies that are driving the design and development of the MCCS. The two main design philosophies are to maximize the use of proven Commercial-Off-The-Shelf technologies to ensure cost effectiveness during development and to be scaleable to accommodate the changing operational and system requirements over the Observatory's expected 20 year operational lifetime.

The NASA Stratospheric Observatory for Infrared Astronomy (SOFIA) Observatory is based upon a refurbished and heavily modified Boeing 747 SP aircraft. The Observatory, which provides accommodations for the Deutsches Zentrum Fur Luftund Raumfahrt 2.5 m telescope, science investigator teams, scientific instruments, mission crew and support systems. The US contractor team has removed most of the aircraft original furnishings and designed a new Layout of Personnel Accommodations (LOPA) tailored to SOFIA's needs.

This paper presents an brief qualitative overview of potential implementations for SOFIA in the project's operational phase.

A study of the average upper atmospheric conditions has been carried out in order to optimize the scientific return from SOFIA. By examination of atmospheric data from satellite missions, we found that at typical SOFIA flight altitudes (between 37,000 and 45,000 ft), it can be an advantage to fly north, as the water vapor overburden and the frequency of cloud occurrence is less than if the flights were centered above Moffett Field, CA, which will be the base for SOFIA. It has also been shown that for certain science projects, the amount of time on target can be considerably extended.

This paper will summarize the stray-light study commissioned by USRA from BRO (Breault Research Organization) to estimate the level of dynamic background that might be observable at SOFIA's focal plane. This dynamic background is due to cavity and aircraft motions with respect to the inertially fixed telescope. BRO used their ASAP program to trace rays emitted from the Earth, aircraft engines, and telescope cavity to the focal plane through reflection and scatter off a number of surfaces (including Level 500 contaminated optics).

The epoch of supremacy of CCD technique allows us to elaborate a new approach to the problem of observations of any celestial body moving on the background of stars. Using the computerized star catalogues one can calculate several astrometric positions for any moving celestial object, the first and sometimes the second derivatives of its spherical coordinates by means of statistical treatment in a real time during CCD observations. The classic Laplace method for initial orbit determination may be successfully used now. These derivatives and also so called the Apparent Motion Parameters, namely topocentric angular velocity and acceleration, positional angle of motion and curvature of object's trajectory are the important additional values for an identification of the observed object. The algorithms and software were developed in Pulkovo observatory for the fast analysis of any CCD frame where the moving celestial objects were detected.

The far-infrared reflectance and scattering properties of telescope surfaces, surrounding cavity walls, and surfaces within focal-plane instruments can be significant contributors to background noise. Radiation from sources well off-axis, such as the earth, moon or aircraft engines may be multiply scattered by the cavity walls and/or surface facets of a complex telescope structure. The Non-Specular Reflectometer at NASA Ames Research Center was reactivated and upgraded, and used to measure reflectance and Bi- directional Reflectance Distribution Functions for samples of planned telescope system structural materials and associated surface treatments.

SAFIRE is a versatile imaging Fabry-Perot spectrograph covering 145 to 655 microns, with spectral resolving powers ranging over 5 - 10,000. Selected as a `PI' instrument for the airborne Stratospheric Observatory for Infrared Astronomy (SOFIA). SAFIRE will apply 2D pop-up bolometer arrays to provide background-limited imaging spectrometry. Superconducting transition edge bolometers and SQUID multiplexers are being developed for these detectors. SAFIRE is expected to be a `First Light' instrument, usable during the initial SOFIA operations. Although a PI instrument rather than a `Facility Class' science instrument, it will be highly integrated with the standard SOFIA planning, observation, and data analysis tools.

We describe the receiver concept for KOSMA's planned second generation SOFIA instrument STAR (SOFIA Terahertz Array Receiver). The receiver will contain a 4 X 4 element heterodyne mixer array for the frequency range from 1.7 to 1.9 THz (158 to 176 microns). Its main scientific goal is large scale mapping of the 158 micron fine structure transition of singly ionized carbon. The design frequency range covers this line out to moderate red shifts and also allows to observe a variety of other spectral lines.

Access to the cavity containing the SOFIA telescope will be severely limited to maintain mirror cleanliness. This will minimize mirror emissivity and extend the time between mirror cleaning/coating cycles, but precludes full access to the telescope for alignment of science instruments. Since there will be over 20 instrument change-outs per year, they must be efficient and trouble-free if SOFIA is to achieve its anticipated flight rate. A telescope assembly alignment simulator (TAAS) is being designed and built to enable verification of most mechanical, electrical, and optical interfaces between a science instrument and the telescope system. It is anticipated that an instrument will typically spend about a week on this simulator to complete its functional check-out and prepare for integration with the SOFIA telescope. This advance work on the simulator will enable the installation of science instruments onto the observatory in less than four hours. The current TAAS design and prototyping activities are described.

We are constructing a facility-class, mid/far-infrared camera for the Stratospheric Observatory for Infrared Astronomy (SOFIA). The Faint Object infraRed CAmera for the Sofia Telescope (FORCAST) is a two-channel camera with selectable filters for continuum imaging in the 5 - 8, 17 - 25 micron, and/or 25 - 40 micron regions. The design supports simultaneous imaging in the two-channels. Using the latest 256 X 256 Si:As and Si:Sb blocked-impurity-band detector array technology to provide high-sensitivity wide- field imaging. FORCAST will sample images at 0.75 arcsec/pixel and have a 3.2' X 3.2' instantaneous field- of-view. Imaging is diffraction limited for lambda > 15 microns.

The airworthiness of science instruments that will fly onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) has been a critical area of concern during the first two years of the observatory development program. The SOFIA platform is a 747SP aircraft which will be operated under Federal Aviation Regulations (FAR Part 25) with a level of review and documentation regarding safety that is new to the astronomical community.