Engineering topics

Dunham, E. W.

Publications and source records attributed to Dunham, E. W..

At least 19 records

NASA NTRS2000

The Prediction and Observation of the 1997 July 18 Stellar Occultation by Triton: More Evidence for Distortion and Increasing Pressure in Triton's Atmosphere

We used CCD (charge coupled device) astrometric data to predict where the occultation path of the star Tr 176 was located, on July 18, 1997. It could be seen from northern Australia and the southern section of North America. We set up an array of portable and mixed telescopes which had high-speed photometric equipment to observe the occultation. Goals included the following: (1) mapping the central flash; (2) obtaining light curves for the signal-to-noise ratio; (3) acquiring light curves from Triton's disk. We combined these with data from others to find the radius and geometry of the half-light surface of the atmosphere, as well as the equivalent-isothermal temperature latitudes below the occultation on Triton.

Elliot, J. L.↗

NASA NTRS1997

The Kepler Mission: A Mission to Determine the Frequency of Inner Planets Neat the Habitable Zone of a Wide Range of Stars

The surprising discovery of giant planets in inner orbits around solar-like stars has brought into question our understanding of the development and evolution of planetary systems, including our solar system. To make further progress, it is critical to detect and obtain data on the frequency and characteristics of Earth-class planets. The Kepler Mission is designed to be a quick, low-cost approach to accomplish that objective. Transits by Earth-class planets produce a fractional change in stellar brightness of 5 x 10(exp -5) to 40 x 10(exp -5) lasting for 4 to 16 hours, From the period and depth of the transits, the orbit and size of the planets can be calculated. The proposed instrument is a one-meter aperture photometer with a 12 deg field-of-view (FOV). To obtain the required precision and to avoid interruptions caused by day-night and seasonal cycles, the photometer will be launched into a heliocentric orbit. It will continuously and simultaneously monitor the flux from 80,000 dwarf stars brighter than 14th magnitude in the Cygnus constellation. The mission tests the hypothesis that the formation of most stars produces Earth-class planets in inner orbits. Based on this assumption and the recent observations that 2% of the stars have giant planets in inner orbits, several types of results are expected from the mission: 1. From transits of Earth-class planets, about 480 planet detections and 60 cases where two or more planets are found in the same system. 2. From transits of giant planets, about 160 detections of inner-orbit planets and 24 detections of outer-orbit planets. 3. From the phase modulation of the reflected light from giant planets, about 1400 planet detections with periods less than a week, albedos for 160 of these giant planets, and densities for seven planets.

Borucki, W. J.↗

NASA NTRS1997

The Kepler Mission: A Mission to Determine the Frequency of Inner Planets Near the Habitable Zone of a Wide Range of Stars

The surprising discovery of giant planets in inner orbits around solar-like stars has brought into question our understanding of the development and evolution of planetary systems, including our solar system. To make further progress, it is critical to detect and obtain data on the frequency and characteristics of Earth-class planets. The Kepler Mission is designed to be a quick, low-cost approach to accomplish that objective. Transits by Earth-class planets produce a fractional change. in stellar brightness of 5 x 10(exp -5) to 40 x 10(exp -5) lasting for 4 to 16 hours. From the period and depth of the transits, the orbit and size of the planets can be calculated. The proposed instrument is a one-meter aperture photometer with a 12 deg. field-of-view (FOV). To obtain the required precision and to avoid interruptions caused by day-night and seasonal cycles, the photometer will be launched into a heliocentric orbit. It will continuously and simultaneously monitor the flux from 80,000 dwarf stars brighter than 14th magnitude in the Cygnus constellation. The mission tests the hypothesis that the formation of most stars produces Earth-class planets in inner orbits. Based on this assumption and the recent observations that 2% of the stars have giant planets in inner orbits, several types of results are expected from the mission: 1. From transits of Earth-class planets, about 480 planet detections and 60 cases where two or more planets are found in the same system. 2. From transits of giant planets, about 160 detections of inner-orbit planets and 24 detections of outer-orbit planets. 3. From the phase modulation of the reflected light from giant planets, about 1400 planet detections with periods less than a week, albedos for 160 of these giant planets, and densities for seven planets.

Borucki, W. J.↗

NASA NTRS1995

Test of CCD Precision Limits for Differential Photometry

Results of tests to demonstrate the very high differential-photometric stability of CCD light sensors are presented. The measurements reported here demonstrate that in a controlled laboratory environment, a front-illuminated CCD can provide differential-photometric measurements with reproducible precision approaching one part in 10(exp 5). Practical limitations to the precision of differential-photometric measurements with CCDs and implications for spaceborne applications are discussed.

Robinson, L. B.↗

NASA NTRS1995

Test of CCD Precision Limits for Differential Photometry

Results of tests to demonstrate the very high differential-photometric stability of CCD light sensors are presented. The measurements reported here demonstrate that in a controlled laboratory environment, a front-illuminated CCD can provide differential-photometric measurements with reproducible precision approaching one part in 105. Practical limitations to the precision of differential-photometric measurements with CCDs and implications for spaceborne applications are discussed.

Borucki, W. J.↗

NASA NTRS1995

Jet-like features near the Nucleus of Chiron

Considered as a comet, the object 2060 Chiron is unusual in two respects: it exhibits outbursts at very large distances from the Sun, and its nucleus is much larger than that of any other known comet. It is, however, similar in size to the recently discovered Kuiper-belt objects - a population of objects with orbits beyond Neptune, which are a possible source of short-period comets. This has led to the conjecture that Chiron is related to these objects, but its chaotic orbit has brought it much closer to the Sun. Here we report observations of a recent stellar occultation by Chiron which permit the identification of several features associated with Chiron's coma. The observation of discrete, jet-like features provides evidence that the coma material originates from just few, small active areas, rather than from uniform sublimations, and that the particles in at least one of these features have radii greater than 0.25 microns. The observations also suggest the presence of material in the plane of Chiron's orbit and are consistent with a gravitationally bound coma. Finally, the present data, and those from a previous occultation, constrain the radius of Chiron to lie between 83 and 156 km.

Elliot, J. L.↗

NASA NTRS1991

Pluto-Charon stellar occultation candidates - 1990-1995

A search to identify stars that might be occulted by Pluto or Charon during the period 1990-1995 and part of 1996 is studied. This search was made with an unfiltered CCD camera operated in the strip scanning mode, and it reaches an R magnitude of approximately 17.5 - about 1.5 mag fainter than previous searches. Circumstances for each of the 162 potential occultations are given, including an approximate R magnitude of the star, which allows estimation of the signal-to-noise ratio (S/N) for observation of each occultation. The faintest stars in the list would yield an S/N of about 20 for a 1 s integration when observed with a CCD detector on an 8 m telescope under a dark sky. The astrometric precision (+/- 0.2 arcsec, with larger systematic errors possible for individual cases) is insufficient to serve as a final prediction for these potential occultations, but is sufficient to identify stars deserving of further, more accurate, astrometric observations. Statistically, about 32 of these events to be observable somewhere on earth are expected. The number of events actually observed will be substantially smaller because of clouds and the sparse distribution of large telescopes. Finder charts for each of the 91 stars involved are presented.

Dunham, E. W.↗

NASA NTRS1991

Pluto-Charon Stellar Occultation Candidates: 1990-1995

We have carried out a search to identify stars that might be occulted by Pluto or Charon during the period 1990-1995 and part of 1996. This search was made with an unfiltered CCD camera operated in the strip scanning mode, and it reaches an R magnitude of approximately 17.5-about 1.5 mag fainter than previous searches. Circumstances for each of the 162 potential occultations are given, including an approximate R magnitude of the star, which allows estimation of the signal-to-noise ratio (S/N) for observation of each occultation. The faintest stars in our list would yield an S/N of about 20 for a 1 S integration when observed with a CCD detector on an 8 m telescope under a dark sky. Our astrometric precision (+/- 0.2 arcsec, with larger systematic errors possible for individual cases) is insufficient to serve as a final prediction for these potential occultations, but is sufficient to identify stars deserving of further, more accurate, astrometric observations. Statistically, we expect about 32 of these events to be observable somewhere on Earth. The number of events actually observed will be substantially smaller because of clouds and the sparse distribution of large telescopes. Finder charts for each of the 91 stars involved are presented.

Dunham, E. W.↗

NASA NTRS1989

Image quality on the Kuiper Airborne Observatory. I - Results of the first flight series

The NASA Kuiper Airborne Observatory (KAO) was flown three times during June and July, 1984 in order to study the causes of the poor seeing obtained with the 0.9-m telescope. High-speed pressure and temperature sensors were placed in the telescope cavity. Several thousand stellar images were recorded under various flight and optical configurations. It is found that the long-exposure image size is affected by telescope tracking errors, imperfect optics, poor optical alignment, telescope and instrument vibration, thermal fluctuations in the telescope cavity, and density fluctuations in the shear layer that forms the boundary between the cavity air and outside air. Possible ways to improve the quality of the images are discussed.

Elliot, J. L.↗

NASA NTRS1989

Pluto's atmosphere

Airborne CCD photometer observations of Pluto's June 9, 1988 stellar occultation have yielded an occultation lightcurve, probing two regions on the sunrise limb 2000 km apart, which reveals an upper atmosphere overlying an extinction layer with an abrupt upper boundary. The extinction layer may surround the entire planet. Attention is given to a model atmosphere whose occultation lightcurve closely duplicates observations; fits of the model to the immersion and emersion lightcurves exhibit no significant derived atmosphere-structure differences. Assuming a pure methane atmosphere, surface pressures of the order of 3 microbars are consistent with the occultation data.

Elliot, J. L.↗

NASA NTRS1986

Photometry of Phoebe

The nonsynchronous rotational period and light curve for Phoebe are analyzed. The variations in CCD chip pixel response, the intensity of the images, the sky background level, and the atmospheric effects and color response of the detector are examined. Consideration is also given to brightness variations and the solar phase function at small phase angles (0.2-1.2 deg) of the light curve. The Phoebe surface is compared with asteroid surfaces by fitting the data to a linear phase function. It is determined that the Phoebe's sidereal rotational period is refined to 9.282 + or - 0.015 hr, the light curve in V filter exhibits 20 percent brightness variations with weaker higher-order harmonics, and the solar phase coefficient at angles less than 1.2 deg is 0.180 + or 0.035 mag/deg. The Phoebe data at small phase angles correlated well with a phase function for C-type asteroids.

Kruse, S.↗

NASA NTRS1986

Portable high speed photometry systems for observing occultations

Ring orbit studies for Uranus have been particularly fruitful because of the ability, through occultations, to obtain data of high spatial resolution at the rate of 1 to 2 times per year. The occultation program at M.I.T. involves: (1) identifying the scientific questions that can be answered by occultation events, (2) predicting the zone of visibility for the useful events, (3) maintaining and improving a set of portable high-speed photometric systems, (4) obtaining the observations, and (5) reducing the data and interpreting the results. Two stellar occultations by Uranus occurred in May 1985, and were observed with a network of large telescopes that encompassed the Northern and Southern Hemispheres. Portable quartz-oscillator time standards were used at all observatories, and were calibrated before and after each event. Observations obtained form Cerro Tololo and McDonald Observatories of the 4 May and 24 May 1985 occultations by the Uranian rings clearly show a companion to the delta ring on both the immersion and emersion traces. The occultation by Ceres that occurred on 12 November 1984 was observed. Preliminary results give a mean diameter of 933 + or - 10 km, which implies a geometric albedo, Pv = 0.07 + or - 0.01 and a density 2.7 + or - 0.3 gm cu/cm. Hence Ceres is likely composed of silicate material throughout.

Elliot, J. L.↗

NASA NTRS1986

Signal-to-noise ratios for possible stellar occultations by Pluto

Successful observation of a stellar occultation by Pluto would make it possible to obtain information regarding the presence of an atmosphere at the planet. If an atmosphere is found, the determination of its structure might be feasible. Two or more occultation chords could make it possible to obtain the precise diameter of Pluto. Mink and Klemola (1985) have presented a list of ten stars which might be occulted by Pluto during the period 1985-1990. In this paper, BVR photometry of the occultation candidate stars are presented, and a calculation is conducted of the S/N which could be achieved with a 1-m telescope. It is pointed out that from a photometric standpoint, the best potential occultation (after that of P2) would be that of P8. However, even the faintest stars on the list should give interesting results if their occultations by Pluto can be observed with a 1-m telescope and sensitive photometric equipment.

Bosh, A. S.↗

NASA NTRS1985

The 1983 June 15 occultation by Neptune. I - Limits on a possible ring system

Observations on 15 June 1983 of an occultation of a star by Neptune from Mauna Kea, Mount Stromlo, Siding Spring, and the Kuiper Airborne Observatory show no evidence for equatorial rings between 25,300 and 200,000 km (R/N/ = 25,269 km). Within most of this region, the upper limit on the optical depth along the line of sight, for rings broader than 6 km , is 0.04, which corresponds to a normal optical depth of 0.016. These results rule out a Neptunian ring system similar to that of Saturn or Uranus, but not a system of low optical depth similar to the Jovian rings. The data show no features that appear likely to have been caused by material in the equatorial plane of Neptune near the Roche limit.

Elliot, J. L.↗

NASA NTRS1985

The 1983 June 15 occultation by Neptune. II - The oblateness of Neptune

The oblateness and radius of Neptune were determined from an analysis of photoelectric observations of the June 15, 1983 occultation by Neptune at six stations, combined with the results of the Apr. 7, 1968 Neptune occultation of BD - 17 deg 4388. The oblateness is 0.0191 + or - 0.0017 and the equatorial radius is 25,268 + or - 12 km at the level probed by the occultation. The results are consistent with recent determinations of Neptune's rotation period and J2 (the second-order gravitational harmonic coefficient) and suggest that Neptune is far less centrally condensed than Uranus. Temperature profiles for Neptune's upper atmosphere were also derived from these data.

French, R. G.↗

NASA NTRS1985

A high-speed, dual-CCD imaging photometer

The SNAPSHOT is a high-speed, dual-CCD imaging photometer, with special capability for occultation observations. It is capable of recording occultation light curves in the presence of bright, nonuniform background illumination, such as occultations of stars by Saturn's rings or by comets. Such events are difficult or impossible to observe with conventional photometers. The fundamental idea that makes high time-resolution observations possible is to reduce the data rate by digitizing only those pixels in subframes containing the objects of interest.

Dunham, E. W.↗

NASA NTRS1984

Exploring the Solar System with Stellar Occultations

By recording the light intensity as a function of time when a planet occults a relatively bright star, the thermal structure of the upper atmosphere of the planet can be probed. The main feature of stellar occultation observations is their high spatial resolution, typically several thousand times better than the resolution achievable with ground-based imaging. Five stellar occultations have been observed. The main results of these observations are summarized. Stellar occultations have been observed on Uranus, Mars, Pallas, Neptune and the Jovian Ring.

Elliot, J. L.↗

NASA NTRS1984

Image Quality on the KAO

Three flights of the Kuiper Airborne Observatory (KAO) were taken to study the cause of the point spread function (PSF) degradation. The preliminary conclusions are: (1) the KAO point spread function decreases in size with elapsed time at altitude; (2) image motion is occasionally comparable to the size of the image, but is usually a sub-arcsecond effect; and (3) star images show small scale internal structural changes on a 200 microsecond timescale.

Dunham, E. W.↗