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At least 379 records · Page 21

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NASA NTRS1981

Dish concentrators for solar thermal energy - Status and technology development

Comparisons are presented of point-focusing, or 'dish' solar concentrator system features, development status, and performance levels demonstrated to date. In addition to the requirements of good optical efficiency and high geometric concentration ratios, the most important future consideration in solar thermal energy dish concentrator design will be the reduction of installed and lifetime costs, as well as the materials and labor costs of production. It is determined that technology development initiatives are needed in such areas as optical materials, design wind speeds and wind loads, structural configuration and materials resistance to prolonged exposure, and the maintenance of optical surfaces. The testing of complete concentrator systems, with energy-converting receivers and controls, is also necessary. Both reflector and Fresnel lens concentrator systems are considered.

Jaffe, L. D.↗

NASA NTRS1982

The swing to concentrator arrays

Objectives and progress in both low concentration ratio (6 to 10) and high concentration ratio ( 100) array developments are summarized. Problems encountered include: thermal control, maldistribution of concentrated sunlight, current busing, and optical surface degradation. The potential advantages over planar arrays are an order of magnitude reduction in per unit cost of power plus increased immunity to radiation damage.

Miller, J. L.↗

NASA NTRS1982

LANDSAT-D investigations in snow hydrology

The sample LANDSAT-4 TM tape (7 bands) of NE Arkansas/Tennessee area was received and displayed. Snow reflectance in all 6 TM reflective bands, i.e. 1, 2, 3, 4, 5, and 7 was simulated, using Wiscombe and Warren's (1980) delta-Eddington model. Snow reflectance in bands 4, 5, and 7 appear sensitive to grain size. One of the objectives is to interpret surface optical grain size of snow, for spectral extension of albedo. While TM data of the study area are not received, simulation results are encouraging. It also appears that the TM filters resemble a "square-wave" closely enough to permit assuming a square-wave in calculations. Integrated band reflectance over the actual response functions was simulated, using sensor data supplied by Santa Barbara Research Center. Differences between integrating over the actual response functions and the equivalent square wave were negligible.

Dozier, J.↗

NASA NTRS1983

Precision machining technology and metrology for AXAF study

The optical surfaces for the Advanced X-ray Astrophysical Facility (AXAF), was studied. The applicability of precision machining technology to the manufacture of the AXAF objective mirrors and the proposed and alternative methods for manufacturing and testing; (1) the AXAF technology mirrors and (2) the AXAF high resolution mirror assembly were reviewed. Surface shape and smoothness metrology for grazing incidence X-ray surfaces and the feasibility of applying heterodyne surface profilometry to nonflat surfaces were investigated. Three sets of scattering flats with known surface profiles and microtopographic character produced by precision machining and polished precision machine surfaces were analyzed.

Sanger, G. M.↗

NASA NTRS1983

Snow reflectance from thematic mapper

Calculations of snow reflectance in all 6 TM reflective bands (i.e., 1,2,3,4,5, and 7) using a delta Eddington model show that snow reflectance in bands 4,5, and 7 is sensitive to grain size. Efforts to interpret the surface optical grain size for the spectral extension of albedo are described. Results show the TM data include spectral channels suitable for snow/cloud discrimination and for snow albedo measurements that can be extended throughout the solar spectrum. Except for band 1, the dynamic range is large enough that saturation occurs only occasionally. The finer resolution gives much better detail on the snowcovered area and might make it possible to use textural information instead of the snowline as an index to the amount of snow melt runoff.

Dozier, J.↗

NASA NTRS1982

Generation of off-axis aspherics

A method for precise generation of optical surfaces is described. The work is turned about one axis, while a grinding head or cutting tool is gradually moved across by an arm turning about a second axis. This geometry can be used to directly generate spherical surfaces or aspherical surfaces presently termed 'hulahoids'. The properties of these surfaces are analyzed to determine how well large paraboloids and their off-axis segments can be generated. Very close approximations are possible. For example, 1.5-m segments of a 10-m paraboloid differ by only 0.6 microns peak-to-peak from the best fit hulahoid when the focal ratio is f/2, and 60 microns when it is f/0.4. For accurate generation the cutting tool may need numerically controlled travel of 1 mm or less, with the position set as a function of the swing arm angle.

Angel, J. R. P.↗

NASA NTRS1983

Space Station Control Requirements and Flywheel System Weights for Combined Momentum and Energy Storage

The specifications of the flywheel system for momentum storage and vehicle torquing are somewhat dependent upon the attitude control requirements of the space station in orbit. As a ground rule, the flywheel system will be sized large enough to provide all attitude maneuvers, if practical, to avoid or minimize turning on the reaction control system (RCS). The RCS, whenever used, expels expensive mass and tends to contaminate optical surfaces of the vehicle. The vehicle rate and acceleration specifications of 0.10 deg/sec and 0.01 deg/square sec are tentative, and may be reduced if lesser values are more practical for flywheel design. For local vertical attitude hold, the average attitude error should be zero, and not the classical 1 degree, since control moment gyro (CMG) gimbal angles provide an exact reference feedback for gravity gradient momentum. Docking presents a problem for docking transients and attitude alignment which will require use of the RCS.

Elam, F. M.↗

NASA NTRS1983

Spectral characterization of the LANDSAT thematic mapper sensors

Data collected on the spectral characteristics of the LANDSAT-4 and LANDSAT-4 backup thematic mapper instruments, the protoflight (TM/PF) and flight (TM/F) models, respectively, are presented and analyzed. Tests were conducted on the instruments and their components to determine compliance with two sets of spectral specifications: band-by-band spectral coverage and channel-by-channel within-band spectral matching. Spectral coverage specifications were placed on: (1) band edges--points at 50% of peak response, (2) band edge slopes--steepness of rise and fall-off of response, (3) spectral flatness--evenness of response between edges, and (4) spurious system response--ratio of out-of-band response to in-band response. Compliance with the spectral coverage specifications was determined by analysis of spectral measurements on the individual components contributing to the overall spectral response: filters, detectors, and optical surfaces.

Markham, B. L.↗

NASA NTRS1984

Thermal contacts between metal and glass for use at cryogenic temperatures

Thermal contacts designed to cool a 50-cm diam fused-silica mirror to 5 K without serious distortion of the optical surfaces were tested. Rubber cement and copper-filled rubber cement were found to be well suited for maintaining thermal contact between the fused-silica and copper straps. A variety of other adhesives proved to be unreliable because they spalled the glass. The thermal resistances of the rubber cement and copper-filled rubber cement were determined. For the copper-filled rubber cement, the thermal resistances varied from 570 K/W at 60 K to 15,000 K/W at 10 K.

Obrien, K.↗

NASA NTRS1984

Vacuum-deposited optical coatings experiment (A0138-4)

The stability of various vacuum deposited optical coatings exposed to the space environment were analyzed. A wide range of optical components manufactured by vacuum deposition, such as metallic and multidielectric reflective coatings in the UV range, metal dielectric interference filtes in the UV and IR ranges, narrow-bandpass filters int he near-UV and visible ranges, selective metallic mirrors in the range from 1500 to 2500 A. antireflective and reflective IR coatings, beam splitters in the visible and IR ranges, and optical surface reflection (OSR) coatings were developed. Many of these components were incorporated into scientific and technical experiments flown on balloons and rockets as well as on Symphonie, Meteosate, OTS, D2-B, TIROS n, and others. It appears that these components operate successfully in flight.

Malherbe, A.↗

NASA NTRS1984

Spectral Characterization of the LANDSAT Thematic Mapper Sensors

Data collected on the spectral characteristics of the LANDSAT-4 and LANDSAT-4 backup thematic mapper instruments, the protoflight (TM/PF) and flight (TM/F) models, respectively, are presented and analyzed. Tests were conducted on the instruments and their components to determine compliance with two sets of spectral specifications: band-by-band spectral coverage and channel-by-channel within-band spctral matching. Spectral coverage specifications were placed on: (1) band edges - points at 50% of peak response, (2) band edge slopes - steepness of rise and fall-off of response, (3) spectral flatness - evenness of response between edges, and (4) spurious system response - ratio of out-of-band response to in-band response. Compliance with the spectral coverage specifications was determined by analysis of spectral measurements on the individual components contributing to the overall spectral response: filters, detectors, and optical surfaces.

Markham, B. L.↗

NASA NTRS1984

Preparation and cleanliness verifications of a space simulation chamber for contamination sensitive test specimens

Contamination sensitive optical devices were to be evaluated under thermal vacuum conditions. Test specifications called for an extremely clean chamber environment, ambient temperature chamber walls, and optical surface temperatures at approximately - 30 C. Chamber preparation included the replacement of diffusion pumps with cryopumps and the cleaning of the chamber walls to reduce contamination levels. Chamber cleaning using vacuum bake was tried and abandoned as time consuming and ineffective. Chemical cleaning and cleanliness verification methods were developed. Gas chromatograph analysis techniques were used extensively throughout the cleaning process to verify cleanliness levels prior to evacuating the chamber. A chamber cleanliness verification test using a thermoelectric quartz crystal microbalance and witness samples was performed to demonstrate that an appropriate cleanliness level was achieved.

Koch, W. R.↗

NASA NTRS1985

Snow Reflectance from Thematic Mapper

Dozier, J.↗

NASA NTRS1984

Aspheric concave grating spectrographs

The design of aspheric concave gratings for high-resolution spectroscopes is examined theoretically, using the requirements of the proposed Far-UV Spectroscopic Explorer (effective area 100 sq cm and lambda/delta lambda = 30,000 in the 90-120-nm range) as a concrete example and taking the limitations of current fabrication technology (straight grooves only and maximum density 6000 lines/mm) into account. The total-path function derived by Namioka (1961) for an ellipsoidal grating is subjected to aberration analysis to obtain the optical surface best suited to the design requirements; expressions for corrections to the conventional stigmatic Rowland-circle mount are obtained and shown to produce significant performance improvements; and the design advantages of coma-corrected non-Rowland geometries are explored. Ray tracings and deviation contours are provided.

Cash, W. C., Jr.↗

NASA NTRS1986

Effects of impurities on silicon solar-cell performance

Model analyses indicate that sophisticated solar cell designs (back surface fields, optical reflectors, surface passivation, and double layer antireflective coatings) can produce devices with conversion efficiencies above 20%. To realize this potential, the quality of the silicon from which the cells are made must be improved; and these excellent electrical properties must be maintained during device processing. As the cell efficiency rises, the sensitivity to trace contaminants also increases. For example, the threshold Ti impurity concentraion at which cell performance degrades is more than an order of magnitude lower for an 18% cell than for a 16% cell. Similar behavior occurs for numerous other metal species which introduce deep level traps that stimulate the recombination of photogenerated carriers in silicon. Purification via crystal growth in conjunction with gettering steps to preserve the large diffusion length of the as grown material can lead to the production of devices with efficiencies above 18%, as verified experimentally.

Hopkins, R. H.↗

NASA NTRS1986

Role of scattering distribution functions in spacecraft contamination control practices

A method for spacecraft optical surface contamination monitoring based on the bidirectional reflectance distribution function (BRDF) and the bidirectional transmittance distribution function (BTDF) is described. In the experimental set up, BRDF/BTDF measurements were made at 0.6328 microns using a 35-mW He-Ne laser light source. A correlation of the second order between BRDF and cleanliness levels was observed. It is suggested that bidirectional scattering distribution functions measured on witness mirrors can give information about contamination in clean rooms or vacuum chambers, and that they can be adopted to establish contamination control criteria.

Carosso, P. A.↗

NASA NTRS1986

Impurity effects in silicon for high efficiency solar cells

Model analyses indicate that sophisticated solar cell designs including, e.g., back surface fields, optical reflectors, surface passivation, and double layer antireflective coatings can produce devices with conversion efficiencies above 20 percent (AM1). To realize this potential, the quality of the silicon from which the cells are made must be improved; and these excellent electrical properties must be maintained during device processing. As the cell efficiency rises, the sensitivity to trace contaminants also increases. For example, the threshold Ti impurity concentration at which cell performance degrades is more than an order of magnitude lower for an 18-percent cell. Similar behavior occurs for numerous other metal species which introduce deep level traps that stimulate the recombination of photogenerated carriers in silicon. Purification via crystal growth in conjunction with gettering steps to preserve the large diffusion length of the as-grown material can lead to the production of devices with efficiencies aboved 18 percent, as has been verified experimentally.

Hopkins, R. H.↗

NASA NTRS1986

A technique for the optical analysis of deformed telescope mirrors

The NASTRAN-ACCOS V programs' interface merges structural and optical analysis capabilities in order to characterize the performance of the NASA Goddard Space Flight Center's Solar Optical Telescope primary mirror, which has a large diameter/thickness ratio. The first step in the optical analysis is to use NASTRAN's FEM to model the primary mirror, simulating any distortions due to gravitation, thermal gradients, and coefficient of thermal expansion nonuniformities. NASTRAN outputs are then converted into an ACCOS V-acceptable form; ACCOS V generates the deformed optical surface on the basis of these inputs, and imaging qualities can be determined.

Bolton, John F.↗