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At least 937 records · Page 52

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NASA NTRS1980

Predicting the thermal/structural performance of the atmospheric trace molecules spectroscopy /ATMOS/ Fourier transform spectrometer

ATMOS is a Fourier transform spectrometer to measure atmospheric trace molecules over a spectral range of 2-16 microns. Assessment of the system performance of ATMOS includes evaluations of optical system errors induced by thermal and structural effects. In order to assess the optical system errors induced from thermal and structural effects, error budgets are assembled during system engineering tasks and line of sight and wavefront deformations predictions (using operational thermal and vibration environments and computer models) are subsequently compared to the error budgets. This paper discusses the thermal/structural error budgets, modelling and analysis methods used to predict thermal/structural induced errors and the comparisons that show that predictions are within the error budgets.

Miller, J. M.↗

NASA NTRS1981

Galactic civilizations - Population dynamics and interstellar diffusion

A model is developed of the interstellar diffusion of galactic civilizations which takes into account the population dynamics of such civilizations. The problem is formulated in terms of potential theory, with a family of nonlinear partial differential and difference equations specifying population growth and diffusion for an organism with advantageous genes that undergoes random dispersal while increasing in population locally, and a population at zero population growth. In the case of nonlinear diffusion with growth and saturation, it is found that the colonization wavefront from the nearest independently arisen galactic civilization can have reached the earth only if its lifetime exceeds 2.6 million years, or 20 million years if discretization can be neglected. For zero population growth, the corresponding lifetime is 13 billion years. It is concluded that the earth is uncolonized not because interstellar spacefaring civilizations are rare, but because there are too many worlds to be colonized in the plausible colonization lifetime of nearby civilizations, and that there exist no very old galactic civilizations with a consistent policy of the conquest of inhabited worlds.

Newman, W. I.↗

NASA NTRS1980

Design and breadboard evaluation of the SPS reference phase control system concept

Efficient operation of a very large phased array such as the proposed solar power satellite, requires precision focusing and pointing of the power beam; i.e., the power beam must have a planar wavefront directed precisely at the center of the target antenna (rectenna). To maintain such a power beam requires real-time phase compensation at each subaperture in order to adjust for structural deformations and other transitory factors. In the current solar power satellite (SPS) baseline, the spaceborne antenna (Spacetenna) is an active retrodirective array. A pilot signal transmitted from the center of the rectenna is phase-conjugated at each subaperture (power module) of the spacetenna, thereby assuring that the radiated composite wave is focused on the target. This scheme requires a large amount of precision electronic circuitry on the spacetenna. Specifically, pilot receivers must be located at each power module and an adaptive distribution network is required in order to provide a properly phased reference signal at each conjugator.

Hopkins, P. M.↗

NASA NTRS1981

Multifacet holographic optical elements

New types of holographic optical elements, combining the flexibility of computer generated holograms with the large space bandwidth product and high diffraction efficiency of interferometrically recorded volume phase holograms, are demonstrated. The optical elements are recorded by subdividing a volume hologram film surface into numerous small areas (facets), each of which is individually exposed under computer control. Each facet is used to produce a portion of the desired final wavefront. Three different optical elements are demonstrated.

Case, S. K.↗

NASA NTRS1982

Refraction at a curved dielectric interface - Geometrical optics solution

The transmission of a spherical or plane wave through an arbitrarily curved dielectric interface is solved by the geometrical optics theory. The transmitted field is proportional to the product of the conventional Fresnel's transmission coefficient and a divergence factor (DF), which describes the cross-sectional variation (convergence or divergence) of a ray pencil as the latter propagates in the transmitted region. The factor DF depends on the incident wavefront, the curvatures of the interface, and the relative indices of the two media. Explicit matrix formulas for calculating DF are given, and its physical significance is illustrated via examples.

Lee, S.-W.↗

NASA NTRS1982

Relative array analysis of upper mantle lateral velocity variations in Southern California

The many densely spaced stations of the Southern California Seismic Network, SCARLET, provide a unique opportunity to examine local variations in upper mantle velocities. By averaging the incoming teleseismic wavefronts over 100-km aperture subarrays, ray parameter dT/dDelta, azimuth, and average arrival time estimates were obtained for each event. These values are compared to whole array plane fit parameters with relative array diagrams and representations of net subarray delays. The conducted study confirms the existence of a high-velocity body at depth beneath the Transverse Ranges and a low-velocity region under the Salton Trough. An uppermost mantle anomaly beneath the San Bernardino Mountains is well established both by this study and from local data. The relation of this body to the deeper structure is investigated.

Walck, M. C.↗

NASA NTRS1982

Plasma ignition for laser propulsion

For a specific optical system a pulsed carbon dioxide laser having an energy output of up to 15 joules was used to initiate a plasma in air at one atmosphere pressure. The spatial and temporal development of the plasma were measured using a multiframe image converter camera. In addition the time dependent velocity of the laser supported plasma front which moves opposite to the direction of the laser pulse was measured in order to characterize the type of wavefront developed. Reliable and reproducible spark initiation was achieved. The lifetime of the highly dense plasma at the initial focal spot was determined to be less than 100 nanoseconds. The plasma front propagates toward the laser at a variable speed ranging from zero to 1.6 x 1,000,000 m/sec. The plasma front propagates for a total distance of approximately five centimeters for the energy and laser pulse shape employed.

Askew, R. F.↗

NASA NTRS1983

Coherent scattering of a spherical wave from an irregular surface

The scattering of a spherical wave from a rough surface using the Kirchhoff approximation is considered. An expression representing the measured coherent scattering coefficient is derived. It is shown that the sphericity of the wavefront and the antenna pattern can become an important factor in the interpretation of ground-based measurements. The condition under which the coherent scattering-coefficient expression reduces to that corresponding to a plane wave incidence is given. The condition under which the result reduces to the standard image solution is also derived. In general, the consideration of antenna pattern and sphericity is unimportant unless the surface-height standard deviation is small, i.e., unless the coherent scattering component is significant. An application of the derived coherent backscattering coefficient together with the existing incoherent scattering coefficient to interpret measurements from concrete and asphalt surfaces is shown.

Fung, A. K.↗

NASA NTRS1982

Gravity deflections of lightweighted mirrors

Two 20-in-diameter lightweight fused-silica mirrors, one having a single arch configuration and the other a double arch configuration, were tested interferometrically at their centers of curvature in both a face-up and face-down support mode. By subtracting the wavefront errors in these two support modes, the gravity deflections were determined in spite of some residual figure error. It is found that the single arch mirror, supported horizontally on three points, has about half the total deflection of the double arch design. The single arch mirror is twice as stiff azimuthally as the double arch, but radially the double arch has four times the stiffness of the single arch. If a more uniform azimuthal support were provided for the double arch, its deflections would be expected to improve significantly.

Anderson, D.↗

NASA NTRS1983

Deformable subreflector computed by geometric optics

Using a Cassegrainian geometry, the 64-meter antenna with its distorted paraboloidal reflecting surface is forced to produce a uniform phase wavefront by a pathlength-compensating subreflector. First, the computed distortion vectors at the joints or nodes of the main reflector structure supporting the surface panels are best fitted to a paraboloid. Second, the resulting residual distortion errors are used to determine a compensating subreflector surface by ray tracing using geometric optics principles. Third, the totally corrected subreflector surface is defined by the normal directions and distances to the surface of the original symmetric hyperboloid for the purpose of evaluation. Finally, contour maps of distortions of the paraboloid reflector and the compensating subreflector are presented. A field-measured check of the subreflector in focused position as computed by the described methodology is also presented for the antenna position at horizon look with the geometry at 45 degrees elevation.

Katow, M. S.↗

NASA NTRS1983

Fused silica mirror evaluation for the Shuttle Infrared Telescope Facility (SIRTF)

The SIRTF optics are intended for operation at 20 K (or less); it will be extremely inconvenient, expensive, and time consuming if it becomes necessary to accomplish all of the optical element testing, assembly, and alignment at comparable temperatures. The thermal strain behavior, including potential anisotropies and inhomogeneities, of a mirror substrate between room temperature and 20 K thus becomes a major factor in the selection of the substrate material, structural configuration, and joining methods for lightweight structures. With support from Space Projects, NASA Ames Research Center, an optical figure evaluation of a 0.65-meter, lightweight, fused silica mirror at a low-temperature goal of 20 K is being conducted. The design details of a thermal shroud, provisions for extracting heat from the low-conductivity mirror, and wavefront error sources other than the mirror surface are discussed and preliminary test results presented.

Barnes, W. P., Jr.↗

NASA NTRS1984

Wave scheduling - Decentralized scheduling of task forces in multicomputers

Decentralized operating systems that control large multicomputers need techniques to schedule competing parallel programs called task forces. Wave scheduling is a probabilistic technique that uses a hierarchical distributed virtual machine to schedule task forces by recursively subdividing and issuing wavefront-like commands to processing elements capable of executing individual tasks. Wave scheduling is highly resistant to processing element failures because it uses many distributed schedulers that dynamically assign scheduling responsibilities among themselves. The scheduling technique is trivially extensible as more processing elements join the host multicomputer. A simple model of scheduling cost is used by every scheduler node to distribute scheduling activity and minimize wasted processing capacity by using perceived workload to vary decentralized scheduling rules. At low to moderate levels of network activity, wave scheduling is only slightly less efficient than a central scheduler in its ability to direct processing elements to accomplish useful work.

Van Tilborg, A. M.↗

NASA NTRS1984

Adaptive optics

An estimation of the costs and potential resolution and sensitivity benefits of adaptive optics systems is presented. The effects of atmospheric disturbances on the peak-to-peak wavefront errors are assessed. Definitions are included of the maximum permissible controlled patch of entrance pupil of the system, the permissible response time, the permissible field radius, and the required magnitude and frequency of a reference star. It is pointed out that adaptive optics system for visible of near-visible wavelengths is likely to be very expensive, whereas operation at 2-3 microns under best seeing conditions is expected to be useful and have reasonable cost.

Woolf, N. J.↗

NASA NTRS1986

Passive Element Shapes Antenna Radiation Pattern

Parasitic waveguide element suppresses ground-reflected multipath radiation. Small section of waveguide placed in front of phased-array antenna operating at 850 MHz modifies radiation field by reducing power density parallel to and below ground plane without significantly affecting primary radiation at angles greater than 20 degrees above horizontal. Acts as parasitic radiator that shifts phase of portion of wavefront by approximately 180 degrees, causing partial cancellation in ground plane, thereby reducing radiation intensity in that direction by about 10 dB. Produces corresponding reduction in multipath radiation scattered from ground. Waveguide technique also used to decouple two nearby antennas.

Bonebright, M. E.↗

NASA NTRS1985

Impact of chopping on image quality in the SIRTF telescope

The initial Phase-A concept for the Space Infrared Telescope Facility (SIRTF), which was established in 1981, is concerned with a cryogenically cooled, 85 cm diameter, f/24 Ritchey-Chretien telescope, followed by a Multiple Instrument Chamber (MIC), containing six scientific instruments. In 1982, the Phase-A concept was reviewed with the aim to assess the technical readiness for the next phase of development. Various areas of concern were subsequently investigated in three parallel studies by industry. Two of the arising questions are considered in the present paper, taking into account the system and technology implications of achieving diffraction-limited resolution at 2 microns, and the limitations on the size of the imaging field under this condition. The conducted study takes into account an evaluation of the different methods of chopping. Attention is given to the telescope parameters, symmetric chopping, asymmetrical chopping, focus and alignment errors, the wavefront-error budget, and image quality.

Bottema, M.↗

NASA NTRS1986

15 meter multiple mirror telescope design study

Taking as a starting point the existing Multiple Mirror Telescope (MMT), a concept for a larger and more advanced instrument has been developed. It makes use of four 7.5-m diameter paraboloidal glass primaries of the honeycomb sandwich type being developed by the University of Arizona. These are mounted quite close together in a square configuration, with their axes coaligned. Separate optical configurations are provided, for optical and infrared applications. To minimze telescope emissivity in the thermal infrared at the combined focus, all the beam combining and streering optics that follow the tertiary mirrors are enclosed in a large central dewar and cooled with liquid nitrogen. The diffraction-limited resolution at the combined focus of 0.11 arcsec at 10 micrometers wavelength is equivalent to that of a 20.5 meter filled aperture. Diffraction-limited resolution should be routinely achievable at 10 and 20 micrometers, if active correction of large-scale wavefront errors is implemented.

Angel, J. R. P.↗

NASA NTRS1985

Planetary searches using optical astrometric interferometers

The detectability of planets is discussed, and observations made with an astrometric interferometer (AI) at Mount Wilson are presented. Planets are detectable as a result of their periodic perturbation of the position of a central observable star. This detection requires relative astrometric accuracies of about 3 x 10 to the -4th for Jovian planets and about 3 x 10 to the -7th arcsec for terrestrial planets. The principles governing astrometric interferometry are explained, and it is conjectured that a two-color optical AI on earth should be capable of about 0.0001 arcsec rms accuracy for stars brighter than magnitude 7-10; small one-color space systems should achieve at least 0.00001 arcsec. Owing to atmospheric turbulence, two-color systems permit correction every few milliseconds for the random wavefront tilt. A 3.4-meter baseline interferometer on Mount Wilson has demonstrated arcsec rms fluctuations of about 0.02 for 1-sec samples. Space-based optical astrometric interferometry is also discussed.

Staelin, D. H.↗

NASA NTRS1986

Optical processing and Space Station automation

Optical information processing research aimed at Space Station automation applications is reviewed. The emphasis of the NASA Ames Research Center program is on intelligent optical pattern recognition and optical control processing. Attention is given to the primary functions of an overall scene understanding system: distortion-invariant optical feature generation, dimensionality reduction, object classification, and contextual information processing. A method of using synthetic discriminant functions to facilitate learning in a high-speed optical correlator is described. A discussion is presented of candidate analog and digital architectures for the optical implementation of state-estimation algorithms needed for the control of high-dimension dynamic systems. The multivariate system chosen for the optical control technology demonstration - a segmented, adaptive mirror and interferometrically based wavefront sensor - is also described.

Ennis, D. J.↗