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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Steps toward eight-meter honeycomb mirror blanks. I Rationale and approach

The best form for 8-m telescope mirrors is discussed. It is concluded that honeycomb sandwich structure can meet the most exacting demand goals for subarcsecond image quality in optical and infrared astronomy. These structures are light, have good stiffness against gravitational and wind loading, and have low thermal inertia. The latter permits them to be operated at the ambient air temperature to avoid local seeing problems. Borosilicate and aluminosilicate glasses are adequate materials for ventilated honeycombs, which do not require low expansion materials.

Angel, J. R. P.↗
NASA NTRS1986

Coding for Electronic Mail

Scheme for coding facsimile messages promises to reduce data transmission requirements to one-tenth current level. Coding scheme paves way for true electronic mail in which handwritten, typed, or printed messages or diagrams sent virtually instantaneously - between buildings or between continents. Scheme, called Universal System for Efficient Electronic Mail (USEEM), uses unsupervised character recognition and adaptive noiseless coding of text. Image quality of resulting delivered messages improved over messages transmitted by conventional coding. Coding scheme compatible with direct-entry electronic mail as well as facsimile reproduction. Text transmitted in this scheme automatically translated to word-processor form.

Rice, R. F.↗
NASA NTRS1984

Human factors of visual displays

Several human factors issues in visual displays are addressed in this report. They are as follows: (1) the importance of luminance range and contrast; (2) uniformity of visual displays; (3) image quality; (4) color contrast; and (5) dot matrix fonts.

Snyder, H. L.↗
NASA NTRS1984

Study of alternate optical and fine guidance sensor designs for the space infrared telescope facility (SIRTF)

A unique optical design was developed that compensates for the coma degraded images caused by field chopping in SIRTF. The conic constants of a Cassegrain telescope were altered to compensate for the coma induced by the secondary mirror tilt. The modulation transfer function is essentially independent of secondary mirror tilt, and diffraction limited image quality is maintained over a several arcminute field during chopping. With an untilted secondary mirror, the coma compensated (CC) design has a smaller field than the unchopped Ritchey-Chretien design; but use of relay optics, such as the inverted Cassegrain design developed for the fine guidance sensor (FGS), can increase the CC telescope's field size. A reactionless secondary mirror chopper mechanism that uses superconducting magnets was studied. The heart producing elements are confined to a reaction plate that is not directly viewed by the IR focal plane. A design was also developed for a low moment of inertia, reticulated HIP beryllium secondary mirror consistent with blank fabrication technology and optical finishing requirements.

Wissinger, A.↗
NASA NTRS1984

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

A second toroidal replica grating was found to have an aspect ratio within 10% of the desire value. The aspect ratio of replica toroidal gratings is coming out somewhat low. Whether this is caused by some relaxation in the epoxy or some residual error in the setup when the replication is made from the deformable submaster grating will be determined. The first imaging tests at EUV wavelengths near 500 A were carried out. The initial results are most encouraging with blur sizes of the order of, or less than, 40 microns. Further focussing and additional photographic tests will be carried out, and preparations made for the first photoelectric measurements of the image quality using the (256 x 1024)-pixel MAMA detection system. The visible-light (256 x 1024)-pixel MAMA detector tube has completed initial evaluation.

Timothy, J. G.↗
NASA NTRS1984

The spatial evolution of t he directional wave spectrum in the Southern Ocean: Its relation to extreme waves in Agulhas Current

An experiment using the Shuttle Imaging Radar-B (SIR-B) to monitor certain properties of the ocean wave directional spectrum and to track the long swell systems as they propagate northward to encounter the Agulhas near the southeastern coast of Africa is discussed. The experiment is designed around the unique capability of SIR-B to overcome key limitations of the Seasat synthetic aperture radar data set, and to extend the existing Seasat results into new areas. Ocean wave systems will be tracked. The variable-incidence-angle capability to examine wave imaging quality will be utilized. Doppler current measurements will be attempted. An effort will be made to verify that the lower range-to-velocity ratio of SIR-B will lead to the improved response of azimuth-traveling wave systems.

Beal, R. C.↗
NASA NTRS1985

Test and evaluation of the 2.4-micron photorefractor ocular screening system

An improved 2.4-m photorefractor ocular screening system was tested and evaluated. The photorefractor system works on the principal of obtaining a colored photograph of both human eyes; and, by analysis of the retinal reflex images, certain ocular defects can be detected such a refractive error, strabismus, and lens obstructions. The 2.4-m photorefractory system uses a 35-mm camera with a telephoto lens and an electronic flash attachment. Retinal reflex images obtained from the new 2.4-m system are significantly improved over earlier systems in image quality. Other features were also improved, notably portability and reduction in mass. A total of 706 school age children were photorefracted, 211 learning disabled and 495 middle school students. The total students having abnormal retinal reflexes were 156 or 22 percent, and 133 or 85 percent of the abnormal had refractive error indicated. Ophthalmological examination was performed on 60 of these students and refractive error was verified in 57 or 95 percent of those examined. The new 2.4-m system has a NASA patent pending and is authorized by the FDA. It provides a reliable means of rapidly screening the eyes of children and young adults for vision problems. It is especially useful for infants and other non-communicative children who cannot be screened by the more conventional methods such as the familiar E chart.

Richardson, J. R.↗
NASA NTRS1984

Glancing incidence EUV telescopes - Analysis and comparison

Steps in the derivation of generalized surface equations describing either a Wolter or Wolter-Schwarzschild type II extreme ultraviolet telescope are outlined. These surface equations are incorporated in an optical design program where first-order design parameters are varied to produce a number of different series of type II telescopes. These series are then ray traced to compare and analyze the geometric image quality for different designs in terms of minimum rms angular blur and field curvature. Results of an analysis illustrating the dependence of effective area on first-order design parameters are also presented. The differences and similarities between equivalent Wolter and Wolter-Schwarzschild designs are discussed.

Davila, P. M.↗
NASA NTRS1985

Doppler parameter estimation for spaceborne synthetic-aperture radars

Problems in the determination of Doppler parameters for spaceborne synthetic-aperture radar (SAR) data processing are examined. The degradations in image quality due to errors in these parameters are summarized. It is shown that these parameters can be estimated using accurate spacecraft ancillary data. In cases where such data are not available, two techniques are proposed to estimate these parameters using the coherent radar return. These techniques were tested with the Seasat SAR data and the test results demonstrate that the accuracies achieved exceed the system performance requirements. Possible applications of these techniques in other areas of SAR data utilization are briefly discussed.

Li, F.-K.↗
NASA NTRS1985

Preliminary results on the various U.V. straylight sources for the VWFC onboard SL 1

Technical difficulties encountered using the Very Wide Field Camera (VWFC) during the Spacelab 1 Shuttle mission are reported. The VWFC is a wide low resolution (5 arcmin half-half width) photographic camera, capable of operating in both spectrometric and photometric modes. The bandpasses of the photometric mode of the VWFC are defined by three Al + MgF2 interference filters. A piggy-back spectrograph attached to the VWFC was used for observations in the spectrometric mode. A total of 48 astronomical frames were obtained using the VWFC, of which only 20 were considered to be of adequate quality for astronomical data processing. Preliminary analysis of the 28 poor-quality images revealed the following possible defects in the VWFC: darkness in the spacing frames, twilight/dawn UV straylight, and internal UV straylight. Improvements in the VWFC astronomical data processing scheme are expected to help identify and eliminate UV straylight sources in the future.

Viton, M.↗
NASA NTRS1985

Structural Characterization and Gas Reactions of Small Metal Particles by High Resolution In-situ TEM and TED

A commercial electron microscope with flat-plate upper pole piece configuration of the objective lens and top entry specimen introduction was modified to obtain 5 x 10 to the minus 10th power mbar pressure at the site of the specimen while maintaining the convenience of a specimen airlock system that allows operation in the 10 to the 10th power mbar range within 15 minutes after specimen change. The specimen chamber contains three wire evaporation sources, a specimen heater, and facilities for oxygen or hydrogen plasma treatment to clean as-introduced specimens. Evacuation is achieved by dural differential pumping, with fine entrance and exit apertures for the electron beam. With the microscope operating at .000001 mbar, the first differential pumping stage features a high-speed cryopump operating in a stainless steel chamber that can be mildly baked and reaches 1 x 10 to the minus 8th power mbar. The second stage, containing the evaporation sources and a custom ionization gauge within 10 cm from the specimen, is a rigorously uncompromised all-metal uhv-system that is bakable to above 200 C throughout and is pumped with an 80-liter ion pump. Design operating pressures and image quality (resolution of metal particles smaller than 1 nm in size) was achieved.

Heinemann, K.↗
NASA NTRS1985

Cryogenic optical systems and instruments; Proceedings of the Meeting, San Diego, CA, August 23, 24, 1984

Systems considerations for the Shuttle Infrared Telescope Facility (SIRTF) are discussed, taking into account SIRTF telescope engineering and trade studies, implications of orbital inclination for SIRTF design and operations, the approaching of the natural background limit with the SIRTF, a long life feasibility study for SIRTF, and the impact of chopping on image quality in the SIRTF telescope. Cryogenic systems and coolers for space missions are considered, giving attention to the Infrared Astronomical Satellite (IRAS) hardware flight performance, the design of the superfluid helium dewar for the Cosmic Background Explorer (COBE), Infrared Telescope (IRT) system cryogenic performance, the solidification of a cryogen by pumping, the production of superfluid helium using a Joule-Thomson Expander, and open cycle He-3 cooling to 0.18 K for space operation. Other topics explored are related to cryogenic optical instruments, facilities, and components, and to supporting technology for cryogenic infrared telescopes in space.

Melugin, R. K.↗
NASA NTRS1986

An ultrahigh vacuum multipurpose specimen chamber with sample introduction system for in situ transmission electron microscopy investigations

A commercial transmission electron microscope (TEM), with flat-plate upper pole piece configuration of the objective lens, and top-entry specimen introduction was modified by introducing an ultrahigh vacuum (UHV) specimen chamber for in situ TEM experimentation. The pumping and design principles and special features of this UHV chamber, which makes it possible to obtain 5 x 10 to the -10th mbar pressure at the site of the specimen, while maintaining the airlock system that allows operation in the 10 to the -10th mbar range within 15 min after specimen change, are described. Design operating pressures and image quality (resolution of metal particles smaller than 1 nm in size) were achieved. Schematic drawings and design dimensions are included.

Heinemann, K.↗
NASA NTRS1986

A data compression technique for synthetic aperture radar images

A data compression technique is developed for synthetic aperture radar (SAR) imagery. The technique is based on an SAR image model and is designed to preserve the local statistics in the image by an adaptive variable rate modification of block truncation coding (BTC). A data rate of approximately 1.6 bit/pixel is achieved with the technique while maintaining the image quality and cultural (pointlike) targets. The algorithm requires no large data storage and is computationally simple.

Frost, V. S.↗
NASA NTRS1985

Hubble Space Telescope satellite

The Hubble Space Telescope, named for the American astronomer Edwin Powell Hubble, will be the largest and most powerful astronomical instrument ever orbited. Placed above the obscuring effects of the earth's atmosphere in a 600-km orbit, this remotely-controlled, free-flying satellite observatory will expand the terrestrial-equivalent resolution of the universe by a factor of seven, or a volumetric factor of 350. This telescope has a 2.4-m primary mirror and can accommodate five scientific instruments (cameras, spectrographs and photometers). The optics are suitable for a spectral range from 1100 angstrom to 1 mm wavelength. With a projected service life of fifteen years, the spacecraft can be serviced on-orbit for replacement of degraded systems, to insert advanced scientific instruments, and to reboost the telescope from decayed altitudes. The anticipated image quality will be a result of extremely precise lambda/20 optics, stringent cleanliness, and very stable pointing: jitter will be held to less than 0.01 arcsecond for indefinite observation periods, consistent with instrument apertures as small as 0.1 arcsecond.

Mitchell, R. E.↗
NASA NTRS1986

Technology and the Hubble Space Telescope

The components of the Hubble Space Telescope which is a 13.1 m long, 4.27 m in diameter, and weighs 11,000 kg are described. The telescope will be placed in a 600-km circular orbit of 28.5 deg inclination and it has an expected service life of 15 years. The optical telescope assembly contains the hyperbolic Ritchey-Chretin type primary and secondary mirrors and the optical control system. The telescope is to have a field of view of 28 arcmin in diameter. The use of the fine guidance sensors for pointing stability is examined. The optical and scientific system of the telescope are contained in the support system module. A wide-field/planetary camera, ESA-furnished faint object camera, the high-speed photometer, faint-object spectrograph, high resolution spectrograph, and ESA-furnished solar arrays are located on the telescope. The maintenance and refurbishment of the telescope are discussed. The precise lambda/20 optics, stringent cleanliness, and stable pointing of the telescope produce good image quality for astronomical and solar system science studies.

Mitchell, R. E.↗
NASA NTRS1986

Analysis of a new class of grazing incidence spectroscopic telescope

The throughput and imaging properties of one of a new class of grazing incidence spectroscopic telescope are examined with a Monte Carlo ray tracing technique. The results are compared with Wolter Schwarzschild type II telescopes of similar size. The image quality of this telescope is comparable, and the control of the off-axis light is superior to the Wolter Schwarzschild design.

Green, J. C.↗
NASA NTRS1986

Upgrading telescopes by active pupil wavefront correction

Exit pupil correction of the Large Deployable Reflector's (a proposed IR to sub-mm space telescope) segmented primary can be done by reimaging it onto a like segmented surface at the exit pupil. This allows the primary to be more flexible, the adaptive element to be smaller, and the supporting structure to be cheaper than if all correction were performed at a stiffly supported primary. Piston, tilt, and decenter errors of an annulus of the primary and the equations for the required corrections are considered. To verify these, the perturbations with spline functions in the lens design program are simulated. Strehl ratios used to measure image quality show that a piston error of 1 mm is fully corrected over a 5 arcmin field for an f/10 system with a 0.7 n.a. primary at 30 micrometers. Limits of correction are also shown for tilt and decenter errors of segments. Tolerances are given for tilt and decenter errors of the remaining optics also.

Stacy, J. E.↗