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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
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NASA NTRS1995

GAIA and beyond: The perspective from the milliarcseconds of Hubble and Hipparcos

Astrometry with the Hubble space telescope and Hipparcos satellite advanced the optical astrometric accuracies to the milliarcsec level. The global astrometric interferometer for astrophysics (GAIA) satellite and other proposed optical interferometry space missions would advance the optical astrometric accuracy to better than 10 microarcsec. A catalog figure of merit is defined which allows a quantitative comparison to be carried out for astrometric catalogs. Using two specific astrophysical problems, the level of the expected scientific contribution from a GAIA-type mission is assessed. The two problems are: the age of the globular clusters in relation to the age of the Universe, and the initial mass function compared with fluctuations in the star formation rate with time.

Hemenway, Paul D.↗
NASA NTRS1996

Combinations of Earth Orientation Measurements: SPACE94, COMB94, and POLE94

A Kalman filter has been used to combine independent measurements of the Earth's orientation taken by the space-geodetic observing techniques of lunar laser ranging, satellite laser ranging, very long baseline interferometry, and the Global Positioning System. Prior to their combination, the data series were adjusted to have the same bias and rate, the stated uncertainties of the measurements were adjusted, and data points considered to be outliers were deleted. The resulting combination, SPACE94, consists of smoothed, interpolated polar motion and UT1-UTC values spanning October 6, 1976, to January 27, 1995, at 1-day intervals. The Kalman filter was then used to combine the space-geodetic series comprising SPACE94 with two different, independent series of Earth orientation measurements taken by the technique of optical astrometry. Prior to their combination with SPACE94, the bias, rate and annual term of the optical astrometric series were corrected, the stated uncertainties of the measurements were adjusted, and data points considered to be outliers were deleted. The adjusted optical astrometric series were then combined with SPACE94 in two steps: (1) the Bureau International de l'Heure (BIH) optical astrometric series was combined with SPACE94 to form COMB94, a combined series of smoothed, interpolated polar motion and UT1-UTC values spanning January 20, 1962, to January 27, 1995, at 5-day intervals, and (2) the International Latitude Service (ILS) optical astrometric series was combined with COMB94 to form POLE94, a combined series of smoothed, interpolated polar motion values spanning January 20, 1900, to January 21, 1995, at 30.4375-day intervals.

Gross, Richard S.↗
NASA NTRS2003

Potential Refinement of the ICRF

The analysis and data used for the ICRF represented the state of the art in global, extragalactic, X/S band microwave astrometry in 1995. The same general analysis method was used to extend the ICRF with subsequent VLBI data in a manner consistent with the original catalog. Since 1995 there have been considerable advances in the geodetic/astrometric VLBI data set and in the analysis that would significantly improve the systematic errors, stability, and density of the next realization of the ICRS when the decision is made to take this step. In particular, data acquired since 1990, including extensive use of the VLBA, are of higher quality and astrometric utility because of changes in instrumentation, schedule design, and networks as well as specifically astrometric intent. The IVS (International VLBI Service for Geodesy and Astrometry) continues to devote a portion of its observing capability to systematic extension of the astrometric data set. Sufficient data distribution exists to select a better set of defining sources. Improvements in troposphere modeling will minimize known systematic astrometric errors while accurate modeling and estimation of station effects from loading and nonlinear motions will permit the reintegration of the celestial reference frame, terrestrial reference frame and Earth orientation parameters though a single VLBI solution. The differences between the current ICRF and the potential next realization will be described.

Ma, Chopo↗
NASA NTRS2000

Observational Model for Precision Astrometry with the Space Interferometry Mission

The Space Interferometry Mission (SIM) is a space-based 10-m baseline Michelson optical interferometer operating in the visible waveband that is designed to achieve astrometric accuracy in the single digits of the microarcsecond domain. Over a narrow field of view SIM is expected to achieve a mission accuracy of 1 microarcsecond. In this mode SIM will search for planetary companions to nearby stars by detecting the astrometric "wobble" relative to a nearby reference star. In its wide-angle mode, SIM will provide 4 microarcsecond precision absolute position measurements of stars, with parallaxes to comparable accuracy, at the end of its 5-year mission. The expected proper motion accuracy is around 3 microarcsecond/year, corresponding to a transverse velocity of 10 m/ s at a distance of 1 kpc. The basic astrometric observable of the SIM instrument is the pathlength delay. This measurement is made by a combination of internal metrology measurements that determine the distance the starlight travels through the two arms of the interferometer, and a measurement of the white light stellar fringe to find the point of equal pathlength. Because this operation requires a non-negligible integration time, the interferometer baseline vector is not stationary over this time period, as its absolute length and orientation are time varying. This paper addresses how the time varying baseline can be "regularized" so that it may act as a single baseline vector for multiple stars, as required for the solution of the astrometric equations.

TILE OBSERVATIONS↗
NASA NTRS2004

SIM spectral characteristics and accuracy analysis

The Space Interferometry Mission (SIM) will perform global astrometry (full sky), local wide-angle (15 degree) and narrow-angle (1 degree) observations to search extra-solar planets, and can calibrate stellar and galactic evolution theories. The astrometric accuracy of the SIM mission depends on spectral characteristics of the optics, detectors and targets. This paper will discuss the photometric throughput of the SIM instrument, and analyze effects of wavefront errors, optical mismatches and control biases as a function of wavelength. The color dependence models of the instrument optics including mirrors, lenses, field-stop and beam-splitter are presented. The performances of different detectors with a variety of coatings are compared. A model of the SIM fringe spectrometer is created. For early and late types of stars, brightness dependency errors are analyzed for different combinations of optics and detectors. Visibility loss due to imperfect optics is investigated in detail. Based on the models of instrument and estimated visibilities, the astrometric accuracies for various kinds of stars are evaluated. It is important to emphasize that not only light sources, mirrors, lenses, field stop and detectors are all wavelength dependent, but also fringe visibility loss, wavefront error, optics control error, etc. are all a function of wavelengths. For the first time the estimate of SIM performance is based on spectral analysis of all factors above, rather than monochromatic approximations of detected fringes, or simply adopted constants. This paper summarizes the astrometric accuracies for a wide range of stars and various combinations of optical design and detector configurations. It has been verified that SIM has astrometric accuracy of about 4 μas for targets with different spectra.

Shao, Michael↗
NASA NTRS2020

Status of NASA’s stellar astrometry testbeds for exoplanet detection: Science and technology overview

Accurate measurement of exoplanetary masses is a critical step in addressing key aspects of NASA's science vision. Measuring masses of Earth-analogs around FGK stars out to 10 pc requires sub-microarcsecond astrometric accuracy, which is not within the capabilities of current instrumentation. Thus, new technology will be required to build an astrometric instrument capable of achieving such performance. This will immediately empower the possibility for dedicated astrometric missions, and perhaps most enticing, it will enable astrometric observing modes to be added (with relatively low cost and impact) to any mission boasting a sufficiently stable direct imaging platform. In this paper, we provide an overview of the scientifi?c goals and technology utilized on two of NASA's astrometry testbeds dedicated to advancing stellar astrometry for exoplanet detection. The ?first one, located at the Jet Propulsion Laboratory (JPL), is dedicated to imaging stellar astrometry on sparse ?fields. The goal of this testbed is to mature the Diffractive Pupil (DP) technology to TRL-5, demonstrating high-?fidelity performance in a relevant environment. This testbed operates in a vacuum tank at the High Contrast Imaging Testbed (HCIT) at JPL. The second testbed, located at NASA Ames Research Center, is dedicated to advancing narrow angle relative astrometry to detect exoplanets around nearby binary stars. The key technology in this testbed is a DP specially designed to measure the angle between two sources on the sky. This testbed operates in air and aims to bring this technology to TRL-4.

Guyon, Olivier↗
NASA NTRS2021

Active Wide-Field Sky Simulator Design for the JPL Astrometry Testbed

Exoplanet detection through astrometry will provide mass measurements that are key to our understanding of exoplanet demographics. Astrometric surveys are also critical pre-cursor observations for identifying targets for future direct imaging missions, such as the HabEx and LUVOIR mission concepts. However, Earth-mass planets orbiting Sun-like stars within 10 pc only cause a 0.3 uas astrometric signal, which is about two orders of magnitude smaller than the current state-of-the-art. In order to advance the technologies needed for future astrometry instruments, a testbed capable of simulating and measuring equivalent Earth-like exoplanet astrometric signals was created. The testbed has a stable light source that simulates a field of stars and can introduce a sub-uas astrometric signal onto a central artificial target star. This is done by illuminating an array of pinholes lithographically printed onto a glass substrate and translating, with picometer resolution, a target-star pinhole located on a centrally cut flexure feature. To detect this motion, a diffractive pupil is used to calibrate the limiting optical distortions of an Offner relay. Presented in this paper are the requirements, design, and implementation of the wide-field astrometry testbed’s light source.

Bendek, Eduardo↗
NASA NTRS1980

Project Orion: A Design Study of a System for Detecting Extrasolar Planets

A design concept for a ground based astrometric telescope that could significantly increase the potential accuracy of astrometric observations is considered. The state of current techniques and instrumentation is examined in the context of detecting extrasolar planets. Emphasis is placed on the direct detection of extrasolar planets at either visual or infrared wavelengths. The design concept of the imaging stellar interferometer (ISI), developed under Project Orion, is described. The Orion ISI employs the state-of-the-art technology and is theoretically capable of attaining 0.00010 arc sec/yr accuracy in relative astrometric observations.

Black, D. C.↗
NASA NTRS1988

Discovery and study of planetary systems using astrometry from space

The intellectual issues that must be confronted in any attempt to discover planetary systems are examined along with the implications that these issues have for the performance of instruments built for that task. The progress that potentially can be made with precise astrometric measurements is examined. Particular attention is given to the Astrometric Telescope Facility, based on the Multichannel Astrometric Photometer.

Levy, E. H.↗
NASA NTRS1992

Potential of long-baseline infrared interferometry for narrow-angle astrometry

Narrow-angle astrometry has many astrophysical applications, from the measurement of parallaxes to the search for planets around nearby stars. Ground-based long-focus telescopes with photoelectric detectors have achieved accuracies of about 2-3 milliarcsec in 1 h. This accuracy is limited primarily by the atmosphere, and is consistent with models of atmospheric turbulence. However, applying these turbulence models to observations with long-baseline interferometers yields astrometric errors that are far smaller than can be achieved with long-focus telescopes. The predictions for the ultimate accuracy of ground-based narrow-angle astrometry using long-baseline IR (2.2 micron) stellar interferometers are very promising. With the excellent seeing at a high altitude site like Mauna Kea, the atmospheric limit for a 1 h astrometric measurement is expected to be of the order of 10 microarcsec for 1 h of integration. This two-order-of-magnitude improvement over conventional measurements is due to two effects. One is that a long-baseline IR interferometer can find useful reference stars very near an arbitrary target star, so that the atmospherically-induced motions of both stars are highly correlated. The second is that the baseline length can be much larger than the separation of the stellar beams in the turbulent atmosphere, resulting in a reduction in astrometric error with increasing baseline length.

Shao, M.↗
NASA NTRS1994

The in-flight calibration of the Hubble Space Telescope fine guidance sensors, 2 (a success story)

The Hubble Space Telescope's fine guidance sensors (FGS's) are unique in the performance levels being attempted; spacecraft control and astrometric research with accuracies better than 3 milli-arcseconds (mas) are the ultimate goals. This paper presents a review of the in-flight calibration of the sensors, describing both the algorithms used and the results achieved to date. The work was done primarily in support of engineering operations related to spacecraft pointing and control and secondarily in support of the astrometric science calibration effort led by the Space Telescope Astrometry Team. Calibration items of principal interest are distortion, sensor magnification, and relative alignment. An initial in-flight calibration of the FGS's was performed in December 1990; this calibration has been used operationally over the past few years. Followup work demonstrated that significant, unexpected temporal variations in the calibration parameters are occurring; provided good characterization of the variation; and set the stage for a distortion calibration designed to achieve the full design accuracy for one of the FGS's. This full distortion calibration, using data acquired in January 1993, resulted in a solution having single-axis residuals with a standard deviation of 2.5 mas. Scale and alignment calibration results for all of the FGS's have been achieved commensurate with the best ground-based astrometric catalogs (root-mean-square error approximately 25 mas). A calibration monitoring program has been established to allow regular updates of the calibration parameters as needed.

Welter, Gary L.↗
NASA NTRS1996

Astrometry of Single-Chord Occultations: Application to the 1993 Triton Event

This paper outlines a method for reducing astrometric data to derive the closest approach time and distance to the center of an occultation shadow for a single observer. The method applies to CCD frames, strip scans or photographic plates and uses a set of field stars of unknown positions to define a common coordinate system for all frames. The motion of the occulting body is used to establish the transformation between this common coordinate system and the celestial coordinate system of the body's ephemeris. This method is demonstrated by application to the Tr6O occultation by Triton on 1993 July 10 UT. Over an interval of four nights that included the occultation time, 80 frames of Triton and Tr6O were taken near the meridian with the U.S. Naval Observatory (USNO) 61-inch astrometric reflector. Application of the method presented here to these data yields a closest approach distance of 359 +/- 133 km (corresponding to 0.017 +/- 0.006 arcsec) for the occultation chord obtained with the Kuiper Airborne Observatory (KAO). Comparison of the astrometric closest approach time with the KAO light-curve midtime shows a difference of 2.2 +/- 4.1 s. Relative photometry of Triton and Tr6O, needed for photometric calibration of the occultation light curve, is also presented.

Olkin, Catherine B.↗
NASA NTRS2004

Preliminary Parallaxes of 40 L and T Dwarfs from the US Naval Observatory Infrared Astronometry Program

We present preliminary trigonometric parallaxes and proper motions for 22 L dwarfs and 18 T dwarfs measured using the ASTROCAM infrared imager on the US naval Observatory (USNO) 1.55 m Strand Astrometric Reflector. The results presented here are based on observations obtained between 2000 September and 2002 November; about half of the objects have an observational time baseline of t 1:3 yr and half t 2:0 yr. Despite these short time baselines, the astrometric quality is sufficient to produce significant new results, especially for the nearer T dwarfs. Seven objects are in common with the USNO optical CCD parallax program for quality control and seven in common with the European Southern Observatory 3.5 m New Technology Telescope parallax program. We compare astrometric quality with both of these programs. Relative to absolute parallax corrections are made by employing Two Micron All Sky Survey and/or Sloan Digital Sky Survey photometry for reference-frame stars. We combine USNO infrared and optical parallaxes with the best available California Institute of Technology (CIT) system photometry to determine MJ , MH, and MK values for 37 L dwarfs between spectral types L0 and L8 and 19 T dwarfs between spectral types T0.5 and T8 and present selected absolute magnitude versus spectral type and color diagrams, based on these results. Luminosities and temperatures are estimated for these objects. Of special interest are the distances of several objects that are at or near the L-T dwarf boundary so that this important transition can be better understood. The previously reported early to mid T dwarf luminosity excess is clearly confirmed and found to be present at J, H, and K. The large number of objects that populate this luminosity-excess region indicate that it cannot be due entirely to selection effects. The T dwarf sequence is extended to MJ 16:9 by 2MASS J041519 0935, which, at d 5:74 pc, is found to be the lluminous LOG (L=L )pa

L DWARFS↗
NASA NTRS2004

Baseline Monitoring for Astrometry in Interferometry

One of the science goals of NASA's Navigator program is ground-based narrow-angle astrometry for extra-solar planet detection, which could be done as part of the proposed Outrigger Telescopes Project. The narrow-angle measurement process, which would use the outrigger telescopes, starts with the determination of the conventional interferometer astrometric baseline, determined from wide-angle astrometry of Hipparcos stars. A baseline monitor system would be employed at each outrigger telescope. This system monitors the pivot point of each telescope - the end point of the astrometric baseline - to measure telescope imperfections that would cause the baseline to vary with telescope rotation. The baseline monitor includes azimuth and elevation cameras that monitor runout along the azimuth and elevation axes of the telescopes. In conjunction with the baseline monitor system, a pivot monitor camera in the dual-star module is used to register the laser metrology corner-cube reflector to the telescope pivot, tying the narrow-angle baseline, which applies to the narrow-angle astrometric measurement, to the wide-angle baseline. In this paper we present the proposed designs for the baseline monitor and pivot-point camera.

pivot point↗
NASA NTRS2015

High Precision Focal Plane Astrometry… and possibilities for the HDST

The calibration techniques we are developing for WFIRST can be also used on HDST, turning it into a revolutionary astrometric instrument. Estimated astrometric accuracy is 10’s of nano arcseconds.Two examples of what such a capability allows us to do: 1)Parallaxes of galactic neighborhood out to 100 Mpc 2) Astrometric characterization of sun like stars out to 50 p.

Shao, Mike↗
NASA NTRS1995

Orbiting Stellar Interferometer: An Overview

The Orbiting Stellar Interferometer (OSI) is a concept for a first-generation space interferometer with astrometric and imaging capabilities and is responsive to the recommendations of the Astronomy and Astrophysics Survey Committee for an astrometric interferometry mission. OSI is a triple Michelson interferometer with articulating siderostats and optical delay lines. The design uses a 7m maximum baseline and aperture diameters of 33 cm; the targeted astrometric performance is a wide-field accuracy of 5µas for 20-mag objects. The instrument would also be capable of synthesis imaging with a resolution of 10mas. Laser metrology is used to relax structural requirements thereby reducing cost. The currently envisaged flight system fits into an Atlas II shroud, for insertion into a 900 km sun-synchronous orbit.

['OSI↗
NASA NTRS1974

Application of very long baseline interferometry to Astrometry and Geodesy: effects of frequency standard instability on accuracy

The accuracy of geodetic and astrometric information obtained from very long baseline interferometry (VLBI) observations is dependent upon the stability of the frequency standard, or clock, used at each site of VLBI array. The sensitivities of two hydrogen maser frequency standards of different design to pressure, temperature, and magnetic field variations were measured; and, for one of the standards, sensitivity was found to be severe enough to degrade the information content of VLBI measurements. However, the effect on the geometric and astrometric information of such clock instabilities, with time scales of hours or greater, can be sharply reduced through the use of differencing techniques.

Clark, T. A.↗