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At least 379 records · Page 21

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NASA NTRS1982

Throughput of diffraction-limited field optics systems for infrared and millimetric telescopes

Telescopes for submillimeter wavelengths have point spread functions some millimeters or centimeters in diameter, but the detectors may be only fractions of a millimeter in size. Thus a field aperture and collecting optics are needed. Optimizing the aperture by a calculation of the effects of diffraction on signal and resolution as a function of size of the collecting aperture is shown. Calculations are compared to experimental results from observations of Mars at submillimeter wavelengths.

Hildebrand, R. H.↗

NASA NTRS1982

Microwave edge diffraction by features in Saturn's rings - Observations with Voyager 1

Classical edge diffraction patterns are formed at centimeter wavelengths by several features of Saturn's rings. These patterns were discovered in 3.6- and 13-centimeter radio signals from Voyager 1 during occultation by the rings. The observed shapes are in agreement with theoretical patterns computed for screens of perfectly abrupt edges having large but finite opacity. Comparison with models in which the opacity at the edge tapers to zero from a finite value sets a new bound of less than about 200 meters on the microwave edge thickness. Certain features of the data suggest a smaller upper bound of about 130 meters on the edge thickness.

Marouf, E. A.↗

NASA NTRS1982

Inversion of the anomalous diffraction approximation for variable complex index of refraction near unity

The Fymat analytic inversion method for retrieving a particle-area distribution function from anomalous diffraction multispectral extinction data and total area is generalized to the case of a variable complex refractive index m(lambda) near unity depending on spectral wavelength lambda. Inversion tests are presented for a water-haze aerosol model. An upper-phase shift limit of 5 pi/2 retrieved an accurate peak area distribution profile. Analytical corrections using both the total number and area improved the inversion.

Smith, C. B.↗

NASA NTRS1983

Sound diffraction at wall impedance discontinuities in a circular cylinder, investigated using Wiener-Hopf technique

Rigorous solutions are presented for sound diffraction in a circular cylinder with axial discontinuities of the wall admittance (or impedance). Analytical expressions are derived for the reflection and the transmission coefficients for duct modes. The results are discussed quantitatively in the limits of small admittance shifts (delta) and of low frequencies (ka). One of the results is the low frequency behavior of the reflection coefficient R(o) sub 00 of the fundamental mode. For the mode of a hardwall duct reflected from the junction with a softwall duct, (R(o) sub oo yields - (1-square root of (ka) square root of (2/i delta)); this result is in contrast to the frequency dependence of the reflection from the open end of a hardwall duct, for which R(o) sub oo yields - 1-(ka) squared/2 .

Cho, Y. C.↗

NASA NTRS1983

X-ray diffraction analysis of Nb-3Ge and NbGe alloys

Of all the A-15 samples of NbGe alloy examined, DT 094 is unique in that it was at least 99% pure A-15 phase. Also its diffraction peaks were noisy as if there were about a one percent compositional variation on this phase. DT 094, however, was only a large fragment of the drop tube drop, and thus its small sample size may have reduced the intensity, thus enhancing fluctuations enough to explain some of the loss of peak resolution.

Davis, J. H.↗

NASA NTRS1983

A radio-frequency analysis of paraboloidal antennas located near diffracting fences

The cylindrical waves associated with the Sommerfeld solution for a diffracting half-plane are applied in the evaluation of the effects of secondary radiation on the focal-region-fields of paraboloids. A cross-correlation between the computed focal-region-fields and the feed-antenna-fields is then introduced to determine the modified radiation pattern of paraboloidal antennas. The half-plane solution is adapted to estimate the effects of a polygonal fence on 9-meter S-band and 3-meter S- and Ku-band antennas at the Merrit Island, Fl Spacecraft Tracking and Data Network Station.

Schmidt, R. F.↗

NASA NTRS1983

Diffraction-limited spatial resolution of circumstellar shells at 10 microns

A new spatial array instrument provided diffraction-limited mid-infrared intensity profiles of the type-M supergiant stars alpha Orionis and alpha Scorpii, both of which are known to exhibit excess 10 microns radiation due to the presence of circumstellar dust shells. In the case of alpha Ori, there is a marked asymmetry in the dust distribution, with peak intensity of dust emission a distance of 0.9 inches from the star.

Bloemhof, E. E.↗

NASA NTRS1983

Diffraction coupled phase-locked semiconductor laser array

A new monolithic, diffraction coupled phase-locked semiconductor laser array has been fabricated. Stable narrow far-field patterns (approximately 3 deg) and peak power levels of 1 W have been obtained for 100-micron-wide devices with threshold currents as low as 250 mA. Such devices may be useful in applications where high power levels and stable radiation patterns are needed.

Katz, J.↗

NASA NTRS1984

Diffraction-limited spatial resolution of circumstellar dust shells at 10 microns

A new spatial array instrument has provided diffraction-limited mid-infrared intensity profiles of the type M supergiant stars Alpha Orionis and Alpha Scorpii, both of which are known to exhibit excess 10 micron radiation due to the presence of circumstellar dust shells. In the case of Alpha Ori, there is a marked asymmetry in the dust distribution, with peak intensity of dust emission a distance of 0.9 sec from the star.

Bloemhof, E. E.↗

NASA NTRS1984

The evaluation of a deformable diffraction grating for a stigmatic EUV spectroheliometer

Two crucial areas in the development of the high resolution EUV spectroheliometer are the elastically deformable toroidal diffraction grating and the open-structure imaging pulse counting detector system. The optical performance of a high frequency toroidal grating at EUV wavelengths was verified with a 3600 groove/mm master grating on a spherical concave blank. An open-structure (256 x 1024) pixel, multi-anode microchannel array was developed and tested as a pulse counting detector system. These two systems verify the proof of concept of the spectroheliometer design.

Timothy, J. G.↗

NASA NTRS1985

Application of Modern Aperture Integration (AI) and Geometrical Theory of Diffraction (GTD) Techniques for Analysis of Large Reflector Antennas

The application of aperture integration (AI) and geometrical theory of diffraction (GTO) techniques to analyze large reflector antennas is outlined. The following techniques were used: computer modeling, validation of analysis and computer codes, computer aided design modifications, limitation on the conventional aperture integration (AIC) method, extended aperture integration (AIE) method, the AIE method for feed scattering calculations, near field probing predictions for 15 meter model, limitation on AIC for surface tolerance effects, aperture integration on the surface (AIS) method, and AIC and GTD calculations for compact range reflector.

Rudduck, R. C.↗

NASA NTRS1984

Response characteristics of laser diffraction particle size analyzers - Optical sample volume extent and lens effects

The response characteristics of laser diffraction particle sizing instruments were studied theoretically and experimentally. In particular, the extent of optical sample volume and the effects of receiving lens properties were investigated in detail. The experimental work was performed with a particle size analyzer using a calibration reticle containing a two-dimensional array of opaque circular disks on a glass substrate. The calibration slide simulated the forward-scattering characteristics of a Rosin-Rammler droplet size distribution. The reticle was analyzed with collection lenses of 63 mm, 100 mm, and 300 mm focal lengths using scattering inversion software that determined best-fit Rosin-Rammler size distribution parameters. The data differed from the predicted response for the reticle by about 10 percent. A set of calibration factor for the detector elements was determined that corrected for the nonideal response of the instrument. The response of the instrument was also measured as a function of reticle position, and the results confirmed a theoretical optical sample volume model presented here.

Hirleman, E. D.↗

NASA NTRS1984

Change of calibration of diffraction-based particle sizers in dense sprays

A calibration procedure for diffraction-based particle-sizing instruments using multielement detectors is briefly reviewed. The effects of multiple scattering due to dense sprays on the calibration of these instruments are then examined over a range of sprays with Sauter mean diameters from 20 to 55 microns. An empirical scheme to correct for changes in calibration due to dense sprays or other scattering media is presented.

Dodge, L. G.↗

NASA NTRS1985

Vector diffraction analysis of reflector antennas with mesh surfaces

Reflector antennas with mesh surfaces are used extensively in many satellite and ground antenna systems. A strip-aperture modeling of commonly used mesh surfaces is presented which provides considerable versatility in characterizing the mesh cells. The mesh transmission coefficients are constructed using a Floquet-modal expansion in conjuction with two dominant aperture modes. To account for the mesh local coordinates, the Eulerian angle transformation is invoked to obtain the total induced current on the curved reflector surface. General formulas are presented to show how the solid surface induced current is modified due to the transmission through the mesh. The effects of a variety of mesh configurations on both the co-polar and cross-polar patterns of reflector antennas are studied by numerically evaluating the vector diffraction integral using the Jacobi-Bessel expansion. For some special cases, a comparison is made with the results of the commonly used wire-grid formulation. Many of the numerical data are tailored to the dimensions of a conceptually designed mesh deployable offset reflector of the land mobile satellite system (LMSS).

Rahmat-Samii, Y.↗

NASA NTRS1985

Diffraction of a baffled dipole - Frequency dependence

Experimental results are presented which confirm the diffraction effects predicted by the MacDonald (1915) form of the Green's function for the baffled field in the theory of trailing edge noise. Using this Green's function, the acoustic intensity of noise generated by flow over a trailing edge has been determined to be proportional to the 5th-power of the flow velocity.

Mcinerny, S. A.↗

NASA NTRS1985

Energy dispersive X-ray diffraction in the diamond anvil, high-pressure apparatus - Comparison of synchrotron and conventional X-ray sources

The use of both conventional fixed-anode X-ray sources and synchrotron radiation to carry out energy-dispersive X-ray diffraction experiments at high pressure in a diamond anvil cell, is discussed. The photon flux at the sample and at the detector for the two cases are compared and the results are presented in graphs. It is shown that synchrotron radiation experiments can be performed with nearly two orders of magnitude increase in data rate if superior detectors and detector electronics are available.

Spain, I. L.↗

NASA NTRS1984

Planar techniques for fabricating X-ray diffraction gratings and zone plates

The state of current planar techniques in the fabrication of Fresnel zone plates and diffraction gratings is reviewed. Among the fabrication techniques described are multilayer resist techniques; scanning electron beam lithography; and holographic lithography. Consideration is also given to: X-ray lithography; ion beam lithography; and electroplating. SEM photographs of the undercut profiles obtained in a type AZ 135OB photoresistor by holographic lithography are provided.

Smith, H. I.↗

NASA NTRS1985

X-ray diffraction studies of phase transformations in heavy-metal fluoride glasses

Powder X-ray diffraction and differential scanning calorimetry studies of the crystallization properties of five ZrF4-based glass compositions have indicated that the crystalline phase in Zr-Ba-La-Pb fluoride glass is beta-BaZrF6; no such identification of crystal phases was obtainable, however, for the other glasses. Reversible polymorphic phase transformations occur in Zr-Ba-La-Li and Zr-Ba-La-Na fluoride glasses, upon heating to higher temperatures.

Bansal, N. P.↗