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At least 433 records · Page 24

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NASA NTRS1989

Maximum entropy deconvolution of the optical jet of 3C 273

The technique of maximum entropy image restoration is applied to the problem of deconvolving the point spread function from a deep, high-quality V band image of the optical jet of 3C 273. The resulting maximum entropy image has an approximate spatial resolution of 0.6 arcsec and has been used to study the morphology of the optical jet. Four regularly-spaced optical knots are clearly evident in the data, together with an optical 'extension' at each end of the optical jet. The jet oscillates around its center of gravity, and the spatial scale of the oscillations is very similar to the spacing between the optical knots. The jet is marginally resolved in the transverse direction and has an asymmetric profile perpendicular to the jet axis. The distribution of V band flux along the length of the jet, and accurate astrometry of the optical knot positions are presented.

Evans, I. N.↗

NASA NTRS1990

A photoelectric astrometric telescope using a Ronchi ruling

A new generation of photoelectric astrometric telescopes promises significant advances for both semiglobal and differential astrometry. Some of these new telescopes have a Ronchi ruling at the prime focal plane. The images move relative to the ruling, behind which photomultipliers or similar detectors view the modulated transmitted light. When several stars are viewed simultaneously, this technique exploits correlations in the atmospheric seeing to significantly improve differential astrometric precision. This article presents some general design and analysis criteria for this type of telescope and describes the particular reflector telescope. This has a 29-cm aperture and has met a 4-milli-arc-second differential precision per 5-minute observation. The system is designed for a program of improved annual parallax and solar-system object measurements.

Buffington, Andrew↗

NASA NTRS1990

A Correlation Between Changes in Solar Luminosity and Differential Radius Measurements

Solar luminosity variations occurring during solar cycle 21 can be attributed in large part to the presence of sunspots and faculae. Nevertheless, there remains a residual portion of the luminosity variation distinctly unaccounted for by these phenomena of solar activity. At the Santa Catalina Laboratory for Experimental Relativity by Astrometry (SCLERA), observations of the solar limb are capable of detecting changes in the solar limb darkening function by monitoring a quantity known as the differential radius. These observations are utilized in such a way that the effects of solar activity are minimized in order to reveal the more fundamental structure of the photosphere. The results of observations made during solar cycle 21 at various solar latitudes indicate that a measurable change did occur in the global photospheric limb darkening function. It is proposed that the residual luminosity change is associated in part with this change in limb darkening.

Kroll, R. J.↗

NASA NTRS1991

Spacewatch search for near-Earth asteroids

The objective of the Spacewatch Program is to develop new techniques for the discovery of near-earth asteroids and to prove the efficiency of the techniques. Extensive experience was obtained with the 0.91-m Spacewatch Telescope on Kitt Peak that now has the largest CCD detector in the world: a Tektronix 2048 x 2048 with 27-micron pixel size. During the past year, software and hardware for optimizing the discovery of near-earth asteroids were installed. As a result, automatic detection of objects that move with rates between 0.1 and 4 degrees per day has become routine since September 1990. Apparently, one or two near-earth asteroids are discovered per month, on average. The follow up is with astrometry over as long an arc as the geometry and faintness of the object allow, typically three months following the discovery observations. During the second half of 1990, replacing the 0.91-m mirror with a larger one, to increase the discovery rate, was considered. Studies and planning for this switch are proposed for funding during the coming year. It was also proposed that the Spacewatch Telescope be turned on the sky, instead of having the drive turned off, in order to increase the rate of discoveries by perhaps a factor of two.

Gehreis, Tom↗

NASA NTRS1990

Search for near-Earth asteroids with the spacewatch camera

The Spacewatch Program began as an engineering development of the discovery of near-earth asteroids and comets, with the firs observations starting in 1983, using an RCA charge coupled device (CCD) that has 320 x 512 pixels. Extensive software was written to scan the sky and learn how to do astrometry for precise orbits of newly discovered objects. The merit of the concept was proven, with many observations reported and comets recovered. The precision of astrometric observations for comets and asteroids was improved. For efficient scanning and discovery of new objects, a Tektronix 2048 x 2048 CCD was obtained and is now permanently on the Spacewatch Telescope. Other aspects of the Spacewatch Program are briefly discussed.

Gehrels, T.↗

NASA NTRS1990

Calibration and operation of a large space-based optical interferometer

The on-orbit calibration of the optics, structure, and control systems of the CSI Focus Mission Interferometer (FMI) is described. The calibration involves the estimation and propagation of both positional and rotational parameters and the propagation of both positional and rotational parameters at the nanometer/nanoradian level. It is shown that, given a nanometer class metrology system to monitor positional changes of critical optical elements, this calibration procedure should enable the FMI to perform 50 picoradian astrometry. The same Kalman filter that implements the initializing calibration of the interferometer baselines and internal pathlengths will also participate in the astrometric measurements of stellar positions.

Laskin, R. A.↗

NASA NTRS1991

Using image area to control CCD systematic errors in spaceborne photometric and astrometric time-series measurements

The effect of some systematic errors for high-precision time-series spaceborne photometry and astrometry has been investigated with a CCD as the detector. The 'pixelization' of the images causes systematic error in astrometric measurements. It is shown that this pixelization noise scales as image radius r exp -3/2. Subpixel response gradients, not correctable by the 'flat field', and in conjunction with telescope pointing jitter, introduce further photometric and astrometric errors. Subpixel gradients are modeled using observed properties of real flat fields. These errors can be controlled by having an image span enough pixels. Large images are also favored by CCD dynamic range considerations. However, magnified stellar images can overlap, thus introducing another source of systematic error. An optimum image size is therefore a compromise between these competing factors.

Buffington, Andrew↗

NASA NTRS1991

Infrared polarization images of star-forming regions. I - The ubiquity of bipolar structure

The inefficiency of the stellar formation process leads rather generally to high residual dust densities, and so to the existence of infrared reflection nebulosity (IRN), in regions of star formation. Polarization images of several star-forming regions with mass outflows (GSS 30, S255, GL 5180, GL 2591, GGD 27, and NGC 7538) presented here: (1) establish the universality of bipolarity and of shell or cavity structure in the IRN consistent with that of CO outflow; (2) identify the source of the mass outflow in each case; (3) show that the opening angle near this central source is large; and (4) demonstrate several instances of multiple shells, probably arising from episodic mass loss. Astrometry of 2.2-micron sources with arcsecond accuracy identifies the illuminating source of each IRN uniquely with a compact H II region or a bright IR source. The polarization images provide strong evidence for large-scale dust toroids around each of these sources. The density and mass of these disks are estimated from the extinction through the disk.

Tamura, M.↗

NASA NTRS1990

Microwave and optical lunar transponders

The scientific areas which used data from the Lunar Laser Ranging Experiment, collected from measurements to the Apollo 11, 14, and 15 and Lunakhod 2, include lunar science (i.e., studies of variations in the lunar angular orientation from that for uniform rotation, lunar tidal displacements, and the lunar mass distribution), geodynamics, astrometry, and gravitational physics. This paper argues that the placement of microwave and optical transponders on the moon would improve the accuracy of laser range measurements by nearly two orders of magnitude and would simplify the measurements. The K-band microwave transponders would be operated at the lunar base and at two remote sites on the moon surface, yielding much improved lunar libration and tidal displacement measurements. A two-wavelength laser transponder also would be operated at the lunar base, allowing accurate tropospheric propagation corrections to be made. This would introduce major improvements in measurements of the lunar orbit and of the earth's rotation, and in tests of general relativity.

Bender, P. L.↗

NASA NTRS1992

Study Of Space-Based Optical Interferometer

Report discusses calibration and operation of conceptual Focus Mission Interferometer (FMI), consisting of component instruments mounted at widely separated locations on large truss structure in orbit 1,400 km above Earth. Includes six telescopes in linear array. Outputs combined in pairlike fashion so FMI operates as three distinct two-telescope interferometers. Accurate enough for submilliarcsecond astrometry.

Redding, David C.↗

NASA NTRS1991

Studies of asteroids, comets, and Jupiter's outer satellites

Observational, theoretical, and computational research was performed, mainly on asteroids. Two principal areas of research, centering on astrometry and photometry, are interrelated in their aim to study the overall structure of the asteroid belt and the physical and orbital properties of individual asteroids. Two highlights are: detection of CN emission from Chiron; and realization that 1990 MB is the first known Trojan type asteroid of a planet other than Jupiter. A new method of asteroid orbital error analysis, based on Bayesian theory, was developed.

Bowell, Edward↗

NASA NTRS1991

Cometary spectroscopy and imaging

The objective is to investigate the spectroscopic and morphological characteristics of comets and selected minor planets over a wide range of heliocentric distances as they may suggest or constrain models of cometary processes, their formation environments and evolution. Direct images of all observable comets and 300-800 nm spectra of the brighter ones are obtained on a monthly basis with a charge coupled device spectrograph-camera. The direct images may be used for astrometry, photometry, and studies of coma and tail morphology. In some cases, anisotropic dust emission can provide information on the nucleus spin vector. Spectra may provide data on strengths of the principle emission for comparison of gas/dust ratios of a large sample of comets. Long integrations of minor planets in comet-like and nearby orbits are made to search for faint comae.

Larson, Stephen M.↗

NASA NTRS1991

Astrometric observations of comets and asteroids and subsequent orbital investigations

The transformation of the Oak Ridge astrometry from a photographic program to a charge coupled device (CCD) program can now be considered complete, and the number of observations being made is now approaching an order of magnitude greater than in the old photographic program. Astrometric observations with the 1.5-m reflector were made at the Oak Ridge Observatory, and the progress, accomplishments, and projected accomplishments are presented.

Marsden, B. G.↗

NASA NTRS1991

Visible and infrared investigations of planet-crossing asteroids and outer solar system objects

The project is supporting lightcurve photometry, colorimetry, thermal radiometry, and astrometry of selected asteroids. Targets include the planet-crossing population, particularly Earth approachers, which are believed to be the immediate source of terrestrial meteorites, future spacecraft targets, and those objects in the outer belt, primarily the Hilda and Trojan populations, that are dynamically isolated from the main asteroid belt. Goals include the determination of population statistics for the planet-crossing objects, the characterization of spacecraft targets to assist in encounter planning and subsequent interpretation of the data, a comparison of the collisional evolution of dynamically isolated Hilda and Trojan populations with the main belt, and the determination of the mechanism driving the activity of the distant object 2060 Chiron.

Tholen, David J.↗

NASA NTRS1991

Palomar Planet-Crossing Asteroid Survey (PCAS) January 1990 - February 1991

Several of our near-Earth asteroid (NEA) discoveries made relatively close approaches to the Earth allowing successful radar observations to be made. Detection of echoes from these asteroids provides otherwise unavailable information about their physical properties. Also, our role in alerting radar astronomers about new candidate asteroids makes it possible for them to acquire critical astrometry which improves the orbital elements of the new discovery.

Helin, Eleanor F.↗

NASA NTRS1992

The Telecommunications and Data Acquisition Report

Archival reports on developments in programs managed by the Jet Propulsion Laboratory's (JPL's) Office of Telecommunications and Data Acquisition (TDA) are published in the TDA Progress Report. In the search for extraterrestrial intelligence (SETI), the TDA Progress Report reports on implementation and operations for searching the microwave spectrum. In solar system radar, it reports on the uses of the Goldstone Solar System Radar for scientific exploration of the planets, their rings and satellites, asteroids, and comets. In radio astronomy, the areas of support include spectroscopy, very long baseline interferometry, and astrometry. These three programs are performed for NASA's Office of Space Science and Applications (OSSA), with the Office of Space Operations funding DSN operational support.

Posner, Edward C.↗

NASA NTRS1990

Maximum likelihood estimates of polar motion parameters

Two estimators developed by Jeffreys (1940, 1968) are described and used in conjunction with polar-motion data to determine the frequency (Fc) and quality factor (Qc) of the Chandler wobble. Data are taken from a monthly polar-motion series, satellite laser-ranging results, and optical astrometry and intercompared for use via interpolation techniques. Maximum likelihood arguments were employed to develop the estimators, and the assumption that polar motion relates to a Gaussian random process is assessed in terms of the accuracies of the estimators. The present results agree with those from Jeffreys' earlier study but are inconsistent with the later estimator; a Monte Carlo evaluation of the estimators confirms that the 1968 method is more accurate. The later estimator method shows good performance because the Fourier coefficients derived from the data have signal/noise levels that are superior to those for an individual datum. The method is shown to be valuable for general spectral-analysis problems in which isolated peaks must be analyzed from noisy data.

Wilson, Clark R.↗

NASA NTRS1992

The AGK3U - An updated version of the AGK3

An account is given of an updated AGK3 with improved proper motions derived from the Palomar 'Quick V' survey. It is judged that the astrometric potential of large-scale Schmidt plates can be realized via the subplate technique, followed by integration with the results of classical photographic astrometry. The new proper motions have a total 2D mean error of 0.82 arcsec/cy. Extensive quality-assurance double-checking has been performed at each stage of this work.

Bucciarelli, B.↗