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NASA NTRS1986

A grazing incidence extreme ultraviolet spectrometer for use with a diverging beam

Attention is given to the 100-350 A performance of a grazing incidence spectrometer configuration that accepts a diverging beam for the conduct of moderate resolution spectroscopy. Assuming a focal ratio of 10 for the diverging beam, the first optic of the configuration is an off-axis ellipsoid that is used at grazing incidence to refocus the diverging beam and increase the focal ratio. A detailed study is made of the performance of a plane, straight-groove, variable-line-space grating; the aberrations are evaluated analytically and verified by geometrical ray tracing. System resolution is a function of telescope image quality. An optimum design is found by varying six parameters and maximizing the average resolution over the bandpass. The spectrometer can be located far from the optical axis, and fed by a small grazing incidence pick-off mirror.

Hurwitz, Mark↗

NASA NTRS1986

OSAC analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) telescope

An investigation is made of the sensitivity of the image quality for the proposed FUSE telescope to mirror misalignments and a wide spatial frequency range of figure errors. Representative figure error data was obtained for the analysis from measurements made on the SEUTS (Solar Extreme Ultraviolet Telescope Spectrograph) telescope mirrors. The tolerancing analysis was carried out with the aid of the Optical Surface Analysis Code (OSAC) program.

Saha, Timo T.↗

NASA NTRS1986

Narrow band imagery in the 8-14 micron spectral region

The problem of deconvolution of the point source response function (PSF) for two-dimensional imaging infrared arrays is addressed. A variety of phenomena can lead to PSF degradation, ranging from atmospheric turbulence to the array itself. The maximum entropy algorithm is used to demonstrate that substantial image quality improvements are possible in the case of a PSF which is degraded by electronic rather than optical smearing. This result implies that IR arrays need not have perfect or even very good pixel-pixel crosstalk rejection ratios for use in astronomy.

Goebel, J. H.↗

NASA NTRS1986

Evaluation of gratings for the Extreme Ultraviolet Explorer

Extensive grating calibration facilities have been developed at the Space Sciences Laboratory at Berkley, which are now being used for the evaluation of the gratings for the spectrometer on the Extreme Ultraviolet Explorer. Measurements of efficiency scattering and imaging quality can be made at wavelengths from 44A to 2500A.

Mrowka, Stan↗

NASA NTRS1988

SIR-B stereo-radargrammetry of Australia

Results are reported from an ongoing program of radargrammetric experimentation with SIR-B data. Six stereo models of orbits over Australia were processed in an analytical stereo plotter. Height accuracies were up to + or - 25 m or 1.8 times the range resolution. This is better than in previous SIR-B stereo work, probably due to better image quality. Some inconsistencies were encountered between look angle geometries and height measurement accuracies. While general observations about earlier SIR-B stereo results were confirmed, inconsistencies with theoretical expectations could not be fully explained.

Leberl, F.↗

NASA NTRS1988

Imaging extreme ultraviolet spectrometer employing a single toroidal diffraction grating - The initial evaluation

The design and test results of a high-efficiency EUV imaging spectrometer employing a concave toroidal grating which is illuminated at normal incidence in a Rowland circle mounting and has only one reflecting surface are discussed. Photographic tests and initial photoelectric tests with a two-dimensional pulse-counting detector system demonstrate the excellent image quality of the grating at wavelengths near 600 A. Applications of the spectrometer for the study of nonsolar objects and solar regions (including the chromosphere, transition region, and corona) are considered.

Huber, M. C. E.↗

NASA NTRS1988

Spatial compression of Seasat SAR imagery

The results of a study of techniques for spatial compression of synthetic-aperture-radar (SAR) imagery are summarized. Emphasis is on image-data volume reduction for archive and online storage applications while preserving the image resolution and radiometric fidelity. A quantitative analysis of various techniques, including vector quantization (VQ) and adaptive discrete cosine transform (ADCT), is presented. Various factors such as compression ratio, algorithm complexity, and image quality are considered in determining the optimal algorithm. The compression system requirements are established for electronic access of an online archive system based on the results of a survey of the science community. The various algorithms are presented and their results evaluated considering the effects of speckle noise and the wide dynamic range inherent in SAR imagery. The conclusion is that although the ADCT produces the best signal-to-distortion-noise ratio for a given compression ratio, the two-level tree-searched VQ technique is preferred due to its simplicity of decoding and near-optimal performance.

Chang, C. Y.↗

NASA NTRS1989

Catadioptric Optics for laser Doppler velocimeter applications

In the design of a laser velocimeter system, attention must be given to the performance of the optical elements in their two principal tasks: focusing laser radiation into the probe volume, and collecting the scattered light. For large aperture applications, custom lens design and fabrication costs, long optical path requirements, and chromatic aberration (for two color operation) can be problematic. The adaptation of low cost Schmidt-Cassegrain astronomical telescopes to perform these laser beam manipulation and scattered light collection tasks is examined. A generic telescope design is analyzed using ray tracing and Gaussian beam propagation theory, and a simple modification procedure for converting from infinite to near unity conjugate ratio operation with image quality near the diffraction limit was identified. Modification requirements and performance are predicted for a range of geometries. Finally, a 200-mm-aperture telescope was modified for f/10 operation; performance data for this modified optic for both laser beam focusing and scattered light collection tasks agree well with predictions.

Dunagan, Stephen E.↗

NASA NTRS1989

Context dependent prediction and category encoding for DPCM image compression

Efficient compression of image data requires the understanding of the noise characteristics of sensors as well as the redundancy expected in imagery. Herein, the techniques of Differential Pulse Code Modulation (DPCM) are reviewed and modified for information-preserving data compression. The modifications include: mapping from intensity to an equal variance space; context dependent one and two dimensional predictors; rationale for nonlinear DPCM encoding based upon an image quality model; context dependent variable length encoding of 2x2 data blocks; and feedback control for constant output rate systems. Examples are presented at compression rates between 1.3 and 2.8 bits per pixel. The need for larger block sizes, 2D context dependent predictors, and the hope for sub-bits-per-pixel compression which maintains spacial resolution (information preserving) are discussed.

Beaudet, Paul R.↗

NASA NTRS1989

Optical design for SOFIA

A preliminary first-order optical design for the Stratospheric Observatory for Infrared Astronomy (SOFIA) is presented. This is a Cassegrain design with a 3 meter diameter, approximately f/1 primary mirror. Phenomena limiting the image quality of the telescope are divided into 'seeing', optics, and guidance. An error budget is presented for these categories and specific effects contributing to each. The seeing effects from the shear layer between the telescope cavity and the external air are expected to be dominant. Results are presented on the necessary thermal, optical, structural and guidance requirements to maintain contributions of these phenomena below that of the shear-layer seeing.

Davis, Paul K.↗

NASA NTRS1989

Comparison of thermal (FLIR) and television images

The human eye is sensitive to electromagnetic radiation in the 0.4 to 0.7 micron band (light). Thermal imaging (TI) systems are sensitive to heat radiation in the infrared band (3-5 or 8-14 microns) and are capable of transforming the distribution of relative temperatures in a scene into a visible TV image. The present experiment was designed to investigate the impact of the difference between TIs and regular TV images on the detection and identification of natural and man-made targets. Parallel TV and TI videotapes were recorded during helicopter flights. Fifteen subjects who viewed both the TV and the TI images (separately), were asked to detect predefined targets and to identify features pointed out to them by the experimenter. In general, performance with TVs was superior to performance with TIs in terms of response times and errors. However, subjects required significantly less time to detect man-made objects with TIs than with TVs. The correlation between the performance of the same task with the two kinds of images was very low. The results are discussed in terms of image quality and in terms of humans' internal representations of natural categories.

Brickner, Michael S.↗

NASA NTRS1990

Site comparison for optical visibility statistics in southern Arizona

One of the best locations in the continental United States for astronomical telescopes is southern Arizona. The mountains surrounding Tucson have clear skies 80 percent of the year, with image quality generally better than 2 seconds on peaks. Two of the existing observatory sites in this area are being considered as locations for one of the three Atmospheric Visibility Monitoring (AVM) observatories. These sites are Mount Lemmon and Mount Hopkins. A comparison of the characteristics of each of the sites is made here to identify the more desirable of the two locations. It is recommended that Mount Lemmon be selected as the Arizona site for this project.

Cowles, K. A.↗

NASA NTRS1990

High spatial resolution studies of galaxies in the far IR: Observations with the KAO, and the promise of SOFIA

NASA, in collaboration with the West German Science Ministry (BMFT), plans a larger airborne telescope as a successor to the Kuipper Airborne Observatory (KAO) that will achieve these goals. The Stratospheric Observatory for Infrared Astronomy (SOFIA) is entering the final stages of Phase B review with targeted new start early in the next decade. SOFIA is a 2.7 m diameter telescope that is carried in a Boeing 747SP. In addition to having 3 times the spatial resolution of the KAO, and 10 times the light gathering power, it will incorporate improvements over the KAO in lower optical emissivity and better telescope tracking stability. The thin primary mirror will equilibrate quickly to ambient temperature at an altitude which, accompanied by airflow improvements across the telescope cavity, will result in better image quality. The sensitivity of SOFIA will allow us to see a large number of typical bright galactic HII regions in local group galaxies. The spatial resolution of 8 seconds (full width half maximum Airy disk) at 100 microns will allow these regions to be measured independently, if they are distributed similarly to those in our own galaxy. At this spatial resolution, the disks of normal galaxies will be easily resolved out to distances of several hundred Mpc. This portion of space includes many of the superluminous galaxies discovered by the Infrared Astronomy Satellite (IRAS), and this spatial scale is relevant for studies of the morphology of regions of interaction among the majority of these galaxies that are members of colliding pairs.

Lester, Dan F.↗

NASA NTRS1990

IAL SPACE: A test laboratory for the ISO cryogenic payload

The ESA Infrared Space Observatory (ISO) satellite is a 3 axes pointed platform designed to make accurate pointed observations of astronomical objects and sources in the wavelength range between 2.5 and 200 microns. ISO is composed of a service module and a payload module which is a large cylindrical vacuum vessel. The vessel is in fact a cryostat (capacity of 2250 l of liquid He II) which contains the telescope and the four focal scientific instruments. The latter being cooled up to a temperature less than 4 K. The qualification of the payload requires the measurement respectively of: the image quality of the telescope through wave front error (WFE) measurements; and the optical alignment of the scientific instruments with respect to the telescope axis and the telescope focus, and this under cryogenic conditions. Consequently, since 1988, the FOCAL 5 IAL Space facility has been upgraded in order to perform the cryogenic optical tests of the ISO optical subsystems.

Cucchiaro, A.↗

NASA NTRS1990

Hypermedia = hypercommunication

New hardware and software technology gave application designers the freedom to use new realism in human computer interaction. High-quality images, motion video, stereo sound and music, speech, touch, gesture provide richer data channels between the person and the machine. Ultimately, this will lead to richer communication between people with the computer as an intermediary. The whole point of hyper-books, hyper-newspapers, virtual worlds, is to transfer the concept and relationships, the 'data structure', from the mind of creator to that of user. Some of the characteristics of this rich information channel are discussed, and some examples are presented.

Laff, Mark R.↗

NASA NTRS1991

Adaptive optics and interferometry

Adaptive optics and interferometry, two techniques that will improve the limiting resolution of optical and infrared observations by factors of tens or even thousands, are discussed. The real-time adjustment of optical surfaces to compensate for wavefront distortions will improve image quality and increase sensitivity. The phased operation of multiple telescopes separated by large distances will make it possible to achieve very high angular resolution and precise positional measurements. Infrared and optical interferometers that will manipulate light beams and measure interference directly are considered. Angular resolutions of single telescopes will be limited to around 10 milliarcseconds even using the adaptive optics techniques. Interferometry would surpass this limit by a factor of 100 or more. Future telescope arrays with 100-m baselines (resolution of 2.5 milliarcseconds at a 1-micron wavelength) are also discussed.

Beichman, Charles A.↗

NASA NTRS1991

Ultraviolet imaging of globular clusters and galaxies

The techniques of UV imaging and its scientific potential are reviewed for studies of advanced stellar evolution, the star formation history and interstellar medium of galaxies, active nuclei, and the evolutionary state of systems observed at high redshift. Early Hubble Space Telescope imaging is expected to emphasize such programs since its image quality is excellent by earlier standards for the UV, despite the presence of spherical aberration.

O'Connell, Robert W.↗

NASA NTRS1992

Bandwidth compression of the digitized HDTV images for transmission via satellites

This paper investigates a subband coding scheme to reduce the transmission bandwidth of the digitized HDTV images. The HDTV signals are decomposed into seven bands. Each band is then independently encoded. The based band is DPCM encoded and the high bands are encoded by using nonuniform Laplacian quantizers with a dead zone. By selecting the dead zone on the basis of energy in the high bands an acceptable image quality is achieved at an average of 45 Mbits/sec (Mbps) rate. This rate is comparable to some very hardware intensive schemes of transform compression or vector quantization proposed in the literature. The subband coding scheme used in this study is considered to be of medium complexity. The 45 Mbps rate is suitable for transmission of HDTV signals via satellites.

Al-Asmari, A. KH.↗