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Lesh, J. R.

Publications and source records attributed to Lesh, J. R..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Phased-Antenna-Array Conical Scanning

Antenna pointing faster than mechanical scanning. Three antenna phased array connected to receiving signal-processing system through two phase-shifting networks. Two networks simultaneously steer phased array in two slightly-different beam directions; one for scanning, one for tracking. Technique has many uses in military and civilian radar, principally in tracking aircraft, balloonborne weather instruments, and other moving signal sources or reflectors.

Lesh, J. R.↗
NASA NTRS1984

Microwave Radiation Detector

Direct photon detector responds to microwave frequencies. Method based on trapped-ion frequency-generation standards proposed to detect radio-frequency (RF) radiation at 40.5 GHz. Technique used for directdetection (RF) communication, radar, and radio astronomy.

Lesh, J. R.↗
NASA NTRS1983

Capacity limit of the noiseless, energy-efficient optical PPM channel

It is shown that even though the capacity per photon can be made to increase without bound, the capacity per channel use (for best power efficiency) is always less than 2 nats per symbol. Furthermore, it approaches 2 nats per symbol as the bandwidth expansion factor goes to infinity. Previously announced in STAR as N82-20121

Lesh, J. R.↗
NASA NTRS1982

Optical communications research to demonstrate 2.5 bits/detected photon

The transmission of information by optical signals over a space channel with a power efficiency of 2.5 bits/detected photon markedly increases the amount of information that can be transmitted to satellites. An account is given of the research program at the Jet Propulsion Laboratory that is attempting to demonstrate that optical signals can be used to transmit information over a space channel with this power efficiency. It is noted, however, that the ability to attain 2.5 bits/detected photon (or higher) depends heavily on the validity of the mathematical models used in the performance analysis. Therefore, verification of the channel dark current noise models is a crucial first step. Another prerequisite is a high-brightness, single-spatial mode laser emitter. It is believed that single spatial model devices with power outputs of about 1W can be achieved by coherently combining a number of GaAs lasers in what effectively amounts to a phased array.

Lesh, J. R.↗
NASA NTRS1982

Bandwidth limitations on noiseless optical channel capacity

The channel capacity of an optical communications link utilizing direct photon detection can be substantially larger than the heterodyne detection quantum limit. In the limit of a noiseless channel the capacity per received photon can even be infinite. In most communications systems the real constraint is to deliver a certain throughput capacity measured in nats/second. The current investigation has the objective to show that due to physical limitations on the minimum time resolution which can be achieved, and for any given throughput capacity in nats/second, there exists an optimum PPM word size which maximizes the 'power efficiency capacity' expressed in nats/photon. Such an optimization will allow the system-designer to minimize the amount of power needed to achieve the desired throughput without violating his bandwidth constraint.

Butman, S. A.↗
NASA NTRS1982

Capacity limit of the noiseless, energy-efficient optical PPM channel

It is shown that even though the capacity per photon can be made to increase without bound, the capacity per channel use (for best power efficiency) is always less than 2 nats per symbol. Furthermore, it approaches 2 nats per symbol as the bandwidth expansion factor goes to infinity.

Lesh, J. R.↗
NASA NTRS1982

Special types of B stars

The properties of the Beta Cephei stars are examined with emphasis on spectral types, luminosities, periods, and light and velocity variations. Typical members of the group and their position in the observational and theoretical HR diagram are discussed. Those B stars with anomalous spectra (chemically peculiar) are considered including hot subdwarfs, extreme helium, weak helium, strong helium, helium variable, magnetic and mercury-manganese stars.

Lesh, J. R.↗
NASA NTRS1982

Power efficient optical communications for space applications

Optical communications technology promises substantial size, weight and power consumption savings for space to space high data rate communications over presently used microwave technology. These benefits are further increased by making the most efficient use of the available optical signal energy. This presentation will describe the progress to date on a project to design, build and demonstrate in the laboratory an optical communication system capable of conveying 2.5 bits of information per effective received photon. Such high power efficiencies will reduce the need for photon collection at the receiver and will greatly reduce the requirements for optical pointing accuracy, both at the transmitter as well as the receiver. A longer range program to demonstrate even higher photon efficiencies will also be described.

Lesh, J. R.↗
NASA NTRS1980

Velocity fields and spectrum peculiarities in Beta Cephei stars

The acquisition of short wavelength spectra of Beta Cephei variable stars from the International Ultraviolet Explorer is reported. A total of 122 images of 10 variable stars and 3 comparison stars were obtained. All of the images were observed in the high dispersion mode through a small aperture. The development of image processing methods is also briefly discussed.

Lesh, J. R.↗
NASA NTRS1980

Mode identification in Beta Cephei stars

The essential observational characteristics related to mode identification are summarized. Major emphasis is placed on the following: both light and velocity amplitudes; typical periods in both light and radial velocity; the light curve for Beta Cephei stars in comparison to the classical Cepheids and RR Lyrae stars; the van Hoof effect with respect to the radial velocity curves in many Beta Cephei stars; and the line profiles of many Beta Cephei stars.

Aizenmen, M. L.↗
NASA NTRS1979

Ultraviolet photometry with the Astronomical Netherlands Satellite /ANS/ Observations of Beta Canis Majoris variables

The paper deals with detailed ANS observations of three Beta Canis Majoris variables: Xi-1 CMa, HD 61068 (whose discovery is reported here) and 15 CMa. Light curves at five ultraviolet wavelengths are presented, and the periods and amplitudes are discussed. The ultraviolet colors are used to derive temperatures and temperature variations, which are compared with the MK spectral types. The anomalously high luminosity found for Xi-1 CMa on the basis of certain line strengths is also discussed.

Lesh, J. R.↗
NASA NTRS1979

Distribution of astronomical sources in the second Equatorial Infrared Catalogue

Measurements of infrared (2.7-micron) source positions and flux densities have been derived based on an additional 60.6 hours of satellite observations beyond those considered in the preparation of the Equatorial Infrared Catalogue No. 1 (EIC-1). These data have been processed together with the EIC-1 data to produce EIC-2. The new catalog differs from EIC-1 as follows: there are 1278 sources; there is a larger percentage of unidentified sources; there are increased numbers of sources identified with Two-Micron Sky Survey sources, AFGL sources, AGK3 stars and SAO stars.

Nagy, T. A.↗
NASA NTRS1978

Ultraviolet photometry from the Orbiting Astronomical Observatory. XXVIII - Ultraviolet light curves for Alpha Lupi and BW Vulpeculae

Photometric data from the Wisconsin Experiment Package on OAO-2 have been used to construct light curves at three ultraviolet wavelengths for Alpha Lup and at seven wavelengths for BW Vul. Both stars are well-known variables of the Beta Cephei (Beta Canis Majoris) type. The light curves for Alpha Lup are in good agreement with the radial-velocity period. A temperature variation of 400-500 K is derived. The BW Vul light curves confirm recent ephemerides based on a secularly varying period and show a stillstand near light maximum at some wavelengths. Both stars exhibit increasing light amplitude at the shortest ultraviolet wavelengths. There is little evidence for cycle-to-cycle variations on a time scale of the order of 1 day.

Lesh, J. R.↗
NASA NTRS1978

The observational status of the Beta Cephei stars

The Beta Cephei stars are a group of variables of spectral type B, whose light and radial velocity periods lie between about three and seven hours. An up-to-date summary of the observational data for individual stars considered to be members of the Beta Cephei group is provided. The general properties of Beta Cephei stars as determined from a comparison between theory and observation are considered, taking into account location in the HR diagram and evolutionary state, physical properties, statistics and galactic distribution, and period-luminosity relation and pulsation mode. Recent trends concerning the observation of Beta Cephei stars are examined, giving attention to a search for new variables, ultraviolet observations from spacecraft, and high-resolution studies of profile variations.

Lesh, J. R.↗
NASA NTRS1978

Development of the Equatorial Infrared Catalogue from satellite data

More than 40,000 infrared measurements of stellar sources have been obtained since November, 1976 during the ongoing process of compiling an Equatorial Infrared Catalogue. Because of the problem of eliminating spurious sources, which has affected earlier space surveys, an extensive effort is being made to verify the sources by means of (1) repetitive observations by satellite sensors, (2) cross correlation with a large data base developed from ground-based and space surveys at other wavelengths, and (3) investigation of a significant subset of the sources with a ground-based infrared telescope. As sources are verified, they are transferred from a working list to a screened preliminary version of the catalogue. The catalogue comprises the only survey of a significant area of the sky that has been accomplished (or is presently planned) with positional accuracies of a few arc seconds at a wavelength of not less than 2 microns.

Heinsheimer, T. F.↗