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Journal articles on the topic 'Astrometric'

Author: Grafiati
Published: 4 June 2021
Last updated: 10 March 2023

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1

Klioner, S. A. "Relativistic astrometry and astrometric relativity." Proceedings of the International Astronomical Union 3, S248 (October 2007): 356–62. http://dx.doi.org/10.1017/s174392130801956x.

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AbstractThe interplay between modern astrometry and gravitational physics is very important for the progress in both these fields. Below some threshold of accuracy, Newtonian physics fails to describe observational data and the Einstein's relativity theory must be used to model the data adequately. Many high-accuracy astronomical techniques have already passed this threshold. Moreover, modern astronomical observations cannot be adequately modeled if relativistic effects are considered as small corrections to Newtonian models. The whole way of thinking must be made compatible with relativity: this starts with the concepts of time, space and reference systems.An overview of the standard general-relativistic framework for modeling of high-accuracy astronomical observations is given. Using this framework one can construct a standard set of building blocks for relativistic models. A suitable combination of these building blocks can be used to formulate a model for any given type of astronomical observations. As an example the problem of four dimensional solar system ephemerides is exposed in more detail. The limits of the present relativistic formulation are also briefly summarized.On the other hand, high-accuracy astronomical observations play important role for gravitational physics itself, providing the latter with crucial observational tests. Perspectives for these astronomical tests for the next 15 years are summarized.
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2

Tubbs, R., N. M. Elias, R. Launhardt, S. Reffert, F. Delplancke, A. Quirrenbach, T. Henning, and D. Queloz. "ESPRI data-reduction strategy and error budget for PRIMA." Proceedings of the International Astronomical Union 3, S248 (October 2007): 132–33. http://dx.doi.org/10.1017/s1743921308018887.

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AbstractThe Exoplanet Search with PRIma (ESPRI) will use the PRIMA dual-feed astrometric capability on the Very Large Telescope Interferometer (VLTI) to perform astrometric detections of extra-solar planets. We present an overview of our data-reduction strategy for achieving 10-μarcsecond accuracy narrow-angle astrometry using the PRIMA instrument. We discuss the error budget for astrometric measurements, and those aspects of our strategy which are designed to minimise the astrometric measurement errors.
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3

Seidelmann, P. K. "Space Astrometry and the HST Wide Field/Planetary Camera." Symposium - International Astronomical Union 141 (1990): 347–54. http://dx.doi.org/10.1017/s0074180900087040.

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The launch of the Hipparcos spacecraft marked the beginning of space astrometry. Hopefully, this will be followed in the near future by the launch of the Hubble Space Telescope, which is not primarily an astrometric instrument, but has astrometric capabilities which will be described in this paper. In addition, there are plans and proposals for future astrometric spacecraft. These include the launch of a radio antenna, which combined with Earth-based antennae would provide a very, very long base line interferometer (Levy, 1986, 1988). There are proposals for launching optical interferometers, such as POINTS (Reasenberg et al 1988). There are also proposals by York and Gatewood (Gatewood et al., 1986; Gatewood 1987, 1989) for launching astrometric instruments using gratings and detectors. Thus, the future holds the prospects for a whole new capability in the field of astrometry.
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4

Sakai, N. "SYNERGY BETWEEN OPTICAL (GAIA) AND RADIO (VLBI)ASTROMETRIC RESEARCHES." Revista Mexicana de Astronomía y Astrofísica Serie de Conferencias 52 (October 5, 2020): 9–11. http://dx.doi.org/10.22201/ia.14052059p.2020.52.03.

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Optical and radio astrometry have become significant for mapping the Milky Way. We introduce an example of synergy between optical and radio astrometry on a research of the Galactic spiral arms. Kinematics and spatial distribution of star and gas indicate a new and complex picture of the Galactic spiral arms. Synergy of astrometric study at multi-wavelength would be enhanced thanks to future infrared astrometric projects (e.g.,Small Jasmine; GaiaNIR) in 2020-2030s.
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Schilbach, E., Wenjing Jin, M. Crézé, P. D. Hemenway, I. I. Kumkova, I. K. Platais, S. Röser, C. Turon, and J. J. Wang. "Commission 24: Photographic Astrometry: (Astrometrie Photographique)." Transactions of the International Astronomical Union 24, no. 1 (2000): 48–59. http://dx.doi.org/10.1017/s0251107x00002571.

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The scope of scientific interests of about 130 Commission members is much more complex than one may expect from the title of the commission. Besides traditional topics like the compilation of astrometric catalogues and the construction of an inertial reference system, more and more investigations performed by our members have been dedicated to the astrophysical interpretation of observations including among others astrometric data.
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6

Fey, Alan L. "Limits on Astrometric Accuracy." International Astronomical Union Colloquium 180 (March 2000): 20–28. http://dx.doi.org/10.1017/s0252921100000063.

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AbstractA general overview of the various factors limiting the accuracy to which astrometric observations can be made is presented. Since this is a very broad topic, it is limited primarily to radio astrometry of extragalactic objects.
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7

Hobbs, David, Berry Holl, Lennart Lindegren, Frédéric Raison, Sergei Klioner, and Alexey Butkevich. "Determining PPN γ with Gaia's astrometric core solution." Proceedings of the International Astronomical Union 5, S261 (April 2009): 315–19. http://dx.doi.org/10.1017/s1743921309990561.

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AbstractThe ESA space astrometry mission Gaia, due for launch in early 2012, will in addition to its huge output of fundamental astrometric and astrophysical data also provide stringent tests of general relativity. In this paper we present an updated analysis of Gaia's capacity to measure the PPN parameter γ as part of its core astrometric solution. The analysis is based on small-scale astrometric solutions taking into account the simultaneous determination of stellar astrometric parameters and the satellite attitude. In particular, the statistical correlation between PPN γ and the stellar parallaxes is considered. Extrapolating the results to a full-scale solution using some 100 million stars, we find that PPN γ could be obtained to about 10−6, which is significantly better than today's best estimate from the Cassini mission of 2 × 10−5.
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8

Platais, I., A. L. Fey, S. Frey, S. G. Djorgovski, C. Ducourant, Ž. Ivezić, A. Rest, C. Veillet, R. F. G. Wyse, and N. Zacharias. "Deep Astrometric Standards." Proceedings of the International Astronomical Union 3, S248 (October 2007): 320–23. http://dx.doi.org/10.1017/s1743921308019455.

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AbstractThe advent of next-generation telescopes with very wide fields-of-view creates a need for deep and precise reference frames for astrometric calibrations. The Deep Astrometric Standards (DAS) program is designed to establish such a frame, by providing absolute astrometry at the 5–10 mas level in four 10 deg2 Galactic fields, to a depth of V=25. The source of our basic reference frame is the UCAC2 catalog, significantly improved by additional observations, and new VLBI positions of radio-loud and optically visible QSOs. We describe all the major steps in the construction of the DAS fields and provide the current status of this project.
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9

Zhang, Yigong, Jiancheng Wang, Jie Su, Xiangming Cheng, and Zhenjun Zhang. "Astrometric Observations of a Near-Earth Object Using the Image Fusion Technique." Astronomical Journal 162, no. 6 (November 19, 2021): 250. http://dx.doi.org/10.3847/1538-3881/ac2c6f.

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Abstract The precise astrometric observation of small near-Earth objects (NEOs) is an important observational research topic in the astrometric discipline, which greatly promotes multidisciplinary research, such as the origin and evolution of the solar system, the detection and early warning of small NEOs, and deep-space navigation. The characteristics of small NEOs, such as faintness and fast moving speed, restrict the accuracy and precision of their astrometric observations. In the paper, we present a method to improve the accurate and precise astrometric positions of NEOs based on image fusion technique. The noise analysis and astrometric test from the observed images of the open cluster M23 are given. Using the image fusion technique, we obtain the sets of superimposed images and original images containing reference stars and moving targets, respectively. The final fused image set includes background stars with high signal-to-noise ratios and ideal NEO images simultaneously and avoids the saturation of background stars. Using the fused images, we can reduce the influence of telescope tracking and NEO ephemeris errors on astrometric observations, and our results indicate that the accuracy and precision of NEO Eros astrometry are improved obviously after we choose suitable image fuse mode.
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10

Neuhäuser, R., A. Seifahrt, T. Röll, A. Bedalov, and M. Mugrauer. "Detectability of Planets in Wide Binaries by Ground-Based Relative Astrometry with AO." Proceedings of the International Astronomical Union 2, S240 (August 2006): 261–63. http://dx.doi.org/10.1017/s1743921307004139.

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AbstractMany planet candidates have been detected by radial-velocity variations of the primary star; they are planet candidates, because of the unknown orbit inclination. Detection of the wobble in the two other dimensions, to be measured by astrometry, would yield the inclination and, hence, true mass of the companions. We aim to show that planets can be confirmed or discovered in a close visual stellar binary system by measuring the astrometric wobble of the exoplanet host star as a periodic variation of the separation, even from the ground. We test the feasibility with HD 19994, a visual binary with one radial velocity planet candidate. We use the adaptive optics camera NACO at the VLT with its smallest pixel scale (∼ 13 mas) for high-precision astrometric measurements. The separations measured in 120 single images taken within one night are shown to follow white noise, so that the standard deviation can be divided by the square root of the number of images to obtain the precision. In this paper we present the first results and investigate the achievable precision in relative astrometry with adaptive optics. With careful data reduction it is possible to achieve a relative astrometric precision as low as 50 μ as for a 0″.6 binary with VLT/NACO observations in one hour, the best relative astrometric precision ever achieved with a single telescope from the ground. The relative astrometric precision demonstrated here with AO at an 8-m mirror is sufficient to detect the astrometric signal of the planet HD 19994 Ab as periodic variation of the separation between HD 19994 A and B.
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11

Everall, Andrew, Douglas Boubert, Sergey E. Koposov, Leigh Smith, and Berry Holl. "Completeness of the Gaia-verse – IV. The astrometry spread function of Gaia DR2." Monthly Notices of the Royal Astronomical Society 502, no. 2 (February 1, 2021): 1908–24. http://dx.doi.org/10.1093/mnras/stab041.

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ABSTRACT Gaia Data Release 2 (DR2) published positions, parallaxes, and proper motions for an unprecedented 1331 909 727 sources, revolutionizing the field of Galactic dynamics. We complement this data with the astrometry spread function (ASF), the expected uncertainty in the measured positions, proper motions, and parallax for a non-accelerating point source. The ASF is a Gaussian function for which we construct the 5D astrometric covariance matrix as a function of position on the sky and apparent magnitude using the Gaia DR2 scanning law and demonstrate excellent agreement with the observed data. This can be used to answer the question ‘What astrometric covariance would Gaia have published if my star was a non-accelerating point source?’. The ASF will enable characterization of binary systems, exoplanet orbits, astrometric microlensing events, and extended sources that add an excess astrometric noise to the expected astrometry uncertainty. By using the ASF to estimate the unit weight error of Gaia DR2 sources, we demonstrate that the ASF indeed provides a direct probe of the excess source noise. We use the ASF to estimate the contribution to the selection function of the Gaia astrometric sample from a cut on astrometric_sigma5d_max showing high completeness for G < 20 dropping to ${\lt} 1{{\ \rm per\ cent}}$ in underscanned regions of the sky for G = 21. We have added an ASF module to the python package scanninglaw (https://github.com/gaiaverse/scanninglaw) through which users can access the ASF.
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12

Sahlmann, J., A. J. Burgasser, D. C. Bardalez Gagliuffi, P. F. Lazorenko, D. Ségransan, M. R. Zapatero Osorio, C. H. Blake, C. R. Gelino, E. L. Martín, and H. Bouy. "Astrometric orbits of spectral binary brown dwarfs – I. Massive T dwarf companions to 2M1059−21 and 2M0805+48." Monthly Notices of the Royal Astronomical Society 495, no. 1 (May 6, 2020): 1136–47. http://dx.doi.org/10.1093/mnras/staa1235.

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ABSTRACT Near-infrared spectroscopic surveys have uncovered a population of short-period, blended-light spectral binaries composed of low-mass stars and brown dwarfs. These systems are amenable to orbit determination and individual mass measurements via astrometric monitoring. Here, we present first results of a multiyear campaign to obtain high-precision absolute astrometry for spectral binaries using the Gemini-South and Gemini-North GMOS imagers. We measure the complete astrometric orbits for two systems: 2M0805+48 and 2M1059−21. Our astrometric orbit of 2M0805+48 is consistent with its 2-yr radial velocity orbit determined previously and we find a mass of $66^{+5}_{-14} M_\mathrm{Jup}$ for its T5.5 companion. For 2M1059−21, we find a 1.9-yr orbital period and a mass of $67^{+4}_{-5} M_\mathrm{Jup}$ for its T3.5 companion. We demonstrate that sub-milliarcsecond absolute astrometry can be obtained with both GMOS imagers and that this is an efficient avenue for confirming and characterizing ultracool binary systems.
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13

Sowmya, K., N. E. Nèmec, A. I. Shapiro, E. Işık, N. A. Krivova, and S. K. Solanki. "Predictions of Astrometric Jitter for Sun-like Stars. III. Fast Rotators." Astrophysical Journal 934, no. 2 (August 1, 2022): 146. http://dx.doi.org/10.3847/1538-4357/ac79b3.

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Abstract A breakthrough in exoplanet detections is foreseen with the unprecedented astrometric measurement capabilities offered by instrumentation aboard the Gaia space observatory. Besides, astrometric discoveries of exoplanets are expected from the planned space mission, Small-JASMINE. In this setting, the present series of papers focuses on estimating the effect of the magnetic activity of G2V-type host stars on the astrometric signal. This effect interferes with the astrometric detections of Earth-mass planets. While the first two papers considered stars rotating at the solar rotation rate, this paper focuses on stars having solar effective temperature and metallicity but rotating faster than the Sun, and consequently more active. By simulating the distribution of active regions on such stars using the Flux Emergence And Transport model, we show that the contribution of magnetic activity to the astrometric measurements becomes increasingly significant with increasing rotation rates. We further show that the jitter for the most variable periodic Kepler stars is high enough to be detected by Gaia. Furthermore, due to a decrease in the facula-to-spot area ratio for more active stars, the magnetic jitter is found to be spot dominated for rapid rotators. Our simulations of the astrometric jitter have the potential to aid the interpretation of data from Gaia and upcoming space astrometry missions.
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14

Brown, A. G. A. "Getting ready for the micro-arcsecond era." Proceedings of the International Astronomical Union 3, S248 (October 2007): 567–76. http://dx.doi.org/10.1017/s1743921308020176.

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AbstractAs the title of this symposium implies, one of the aims is to examine the future of astrometry as we move from an era in which thanks to the Hipparcos Catalogue everyone has become familiar with milliarcsecond astrometry to an era in which microarcsecond astrometry will become the norm. I will take this look into the future by first providing an overview of present astrometric programmes and how they fit together and then I will attempt to identify the most promising future directions. In addition I discuss the important conditions for the maximization of the scientific return of future large and highly accurate astrometric catalogues; catalogue access and analysis tools, the availability of sufficient auxiliary data and theoretical knowledge, and the education of the future generation of astrometrists.
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15

Bernstein, H. H. "Derivation of orbital parameters of very low mass companions in double stars from radial velocities and observations of space astrometry missions like HIPPARCOS, DIVA and GAIA." International Astronomical Union Colloquium 170 (1999): 410–15. http://dx.doi.org/10.1017/s0252921100048843.

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AbstractRadial velocity measurements are a well known high-precision method to obtain the orbits of extrasolar planets or brown dwarfs. However, this method is not able to determine the inclination which could be derived from astrometry. The astrometric effects of those objects are very minute, wherefore the interest of astronomers in astrometric techniques was very poor. This situation changes fundamentally since space astrometry observations are available. HIPPARCOS demonstrated the power of space astrometry and the extremely high accuracy of the DIVA, and especially the GAIA observations allows one to detect Jupiter- and Earth- like objects. The optimal estimation of the parameters of the orbit of extrasolar planets or brown dwarfs is a combination of radial velocity measurements and space astrometry observations. Here it is possible to overcome problems which are inherent in both observation methods, so space astrometry complements radial velocity observations and vice versa. This paper gives a method for the parameter estimation using both types of measurements.
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16

Yano, T., N. Gouda, H. Ueda, H. Koyama, Y. Kan-ya, and A. Taruya. "Elucidation of kinematical and dynamical structure of the Galactic bulge." Proceedings of the International Astronomical Union 3, S245 (July 2007): 39–44. http://dx.doi.org/10.1017/s1743921308017249.

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AbstractFuture space mission of astrometric satellite, GAIA and JASMINE (Japan Astrometry Satellite Mission for Infrared Exploration), will produce astrometric parameter, such as positions, parallaxes, and proper motions of stars in the Galactic bulge. Then kinematical information will be obtained in the future. Accordingly it is expected that our understanding of the dynamical structure will be greatly improved. Therefore it is important to make a method to construct a kinematical and dynamical structure of the Galactic bulge immediately.
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17

Fresneau, Alain. "Space Telescope Motion Limitations for Fine Guidance Sensor Astrometry." Symposium - International Astronomical Union 109 (1986): 643–47. http://dx.doi.org/10.1017/s0074180900077147.

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Basic Fine Guidance Sensor data have been simulated to correlate the Power Spectrum Densities (PSD) of the Space Telescope motion disturbances with the “seeing” of an astrometric target. The goal of this study is to describe whether a complete identification of the jitter of the line of sight during an astrometric observation is required to improve the precision of this observation. The present preliminary results indicate that the performance in Astrometry are close to the predictions.
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18

Gai, Mario, Alberto Vecchiato, Alessandro Sozzetti, Sebastiano Ligori, and Mario G. Lattanzi. "Gravitation Astrometric Measurement Experiment (GAME)." Proceedings of the International Astronomical Union 6, S276 (October 2010): 535–36. http://dx.doi.org/10.1017/s1743921311021119.

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AbstractGAME (Gravitation Astrometric Measurement Experiment) is a mission concept based on astronomical techniques for high precision measurements of interest to Fundamental Physics and cosmology, in particular the γ and β parameters of the Parameterized Post-Newtonian formulation of gravitation theories extending the General Relativity.High precision astrometry also provides the light deflection induced by the quadrupole moment of Jupiter and Saturn, and, by high precision determination of the orbits of Mercury and high elongation asteroids, the PPN parameter β.The astrometric and photometric capabilities of GAME may also provide crucial complementary information on a selected set of known exo-planets.
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19

White, Richard L., Stephen H. Lubow, and Bernie Shiao. "Improvements to Pan-STARRS1 Astrometry. II. Corrections for Differential Chromatic Refraction." Astronomical Journal 164, no. 2 (August 1, 2022): 73. http://dx.doi.org/10.3847/1538-3881/ac7ab6.

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Abstract In a previous paper, we applied the Gaia DR2 catalog to improve the astrometric accuracy of about 1.7 billion objects in Pan-STARRS1 Data Release 2 (PS1 DR2). We report here on further improvements made by utilizing Gaia EDR3 and correcting for the effects of differential chromatic refraction (DCR) in decl. We extend the correction algorithm in Paper I by iteratively subtracting color- and decl.-dependent PS1/Gaia EDR3 decl. residuals. We determine the astrometric improvement for ∼440 million reference objects that are point-like and cross-match to Gaia EDR3. For this set of objects, Gaia EDR3 provides a ∼3% improvement in PS1 astrometry over Gaia DR2, and DCR corrections provide an additional ∼5% improvement. DCR corrections increase substantially for objects observed away from the zenith. DCR corrections lead to an astrometric improvement of ∼30% for blue objects (0 < g − i < 1) that are 50° away from the zenith. The amplitude of systematic astrometric errors from these effects is substantially reduced to less than 1 mas for objects with PS1 colors in the range 0 < g − i < 4.5, which makes this a useful astrometric reference catalog in fields where there are few Gaia stars. The improved astrometric data will be available through the Mikulski Archive for Space Telescopes PS1 catalog interfaces.
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Kaplan, G. H. "Astrometric Interferometry — Can It Establish a Fundamental System?" Symposium - International Astronomical Union 141 (1990): 241–50. http://dx.doi.org/10.1017/s0074180900086897.

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The astrometric optical interferometer on Mt. Wilson is providing a new source of astrophysical and astrometric data on bright stars. The instrument, with 12-meter baselines, has been in operation since late 1986. The interferometer is capable of wide-angle astrometry, that is, the determination of very precise stellar positions within a reference frame defined by bright stars spread across a large area (of order one steradian) of the sky. This paper addresses the question of whether such an instrument can be used to establish a fundamental system — that is, one tied to the Earth in some well-defined way. Some astrometric data from this instrument are presented to illustrate the difficulties involved. Proposed means of addressing these problems in future instruments are discussed.
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21

Crosta, M. T., B. Bucciarelli, F. de Felice, M. G. Lattanzi, and A. Vecchiato. "Astrometric observable and relativistic astrometric catalogues." EAS Publications Series 30 (2008): 345–48. http://dx.doi.org/10.1051/eas:0830058.

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22

Lindegren, L., J. Hernández, A. Bombrun, S. Klioner, U. Bastian, M. Ramos-Lerate, A. de Torres, et al. "Gaia Data Release 2." Astronomy & Astrophysics 616 (August 2018): A2. http://dx.doi.org/10.1051/0004-6361/201832727.

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Context. Gaia Data Release 2 (Gaia DR2) contains results for 1693 million sources in the magnitude range 3 to 21 based on observations collected by the European Space Agency Gaia satellite during the first 22 months of its operational phase. Aims. We describe the input data, models, and processing used for the astrometric content of Gaia DR2, and the validation of these resultsperformed within the astrometry task. Methods. Some 320 billion centroid positions from the pre-processed astrometric CCD observations were used to estimate the five astrometric parameters (positions, parallaxes, and proper motions) for 1332 million sources, and approximate positions at the reference epoch J2015.5 for an additional 361 million mostly faint sources. These data were calculated in two steps. First, the satellite attitude and the astrometric calibration parameters of the CCDs were obtained in an astrometric global iterative solution for 16 million selected sources, using about 1% of the input data. This primary solution was tied to the extragalactic International Celestial Reference System (ICRS) by means of quasars. The resulting attitude and calibration were then used to calculate the astrometric parameters of all the sources. Special validation solutions were used to characterise the random and systematic errors in parallax and proper motion. Results. For the sources with five-parameter astrometric solutions, the median uncertainty in parallax and position at the reference epoch J2015.5 is about 0.04 mas for bright (G < 14 mag) sources, 0.1 mas at G = 17 mag, and 0.7 masat G = 20 mag. In the proper motion components the corresponding uncertainties are 0.05, 0.2, and 1.2 mas yr−1, respectively.The optical reference frame defined by Gaia DR2 is aligned with ICRS and is non-rotating with respect to the quasars to within 0.15 mas yr−1. From the quasars and validation solutions we estimate that systematics in the parallaxes depending on position, magnitude, and colour are generally below 0.1 mas, but the parallaxes are on the whole too small by about 0.03 mas. Significant spatial correlations of up to 0.04 mas in parallax and 0.07 mas yr−1 in proper motion are seen on small (< 1 deg) and intermediate (20 deg) angular scales. Important statistics and information for the users of the Gaia DR2 astrometry are given in the appendices.
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Benedict, G. F., J. T. McGraw, and T. R. Hess. "Relative Astrometry with Imaging Transit Telescopes." Symposium - International Astronomical Union 166 (1995): 9–12. http://dx.doi.org/10.1017/s0074180900227757.

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A CCD/Transit Instrument (CTI) has produced relative astrometry with standard errors less than 2.6% of a 1.55 arcsecond pixel for stars with V ≤ 17. Additional astrometric studies with existing data are required to better understand the ultimate contribution these devices can make to our science.The CTI is presently dismantled, awaiting a move to a new site. We briefly discuss the potential astrometric scientific returns from the exisiting data set, from a refurbished CTI, and from a similar device emplaced on the Moon.
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Zacharias, Norbert, Valeri V. Makarov, Charles T. Finch, Hugh C. Harris, Jeffrey A. Munn, and John P. Subasavage. "USNO Bright Star Catalog, Version 1." Astronomical Journal 164, no. 2 (July 5, 2022): 36. http://dx.doi.org/10.3847/1538-3881/ac686d.

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Abstract The USNO Bright Star Catalog (UBSC) is a new astrometric catalog of the 1423 brightest stars covering the entire sky, which is published online. It is nearly complete to V = 3 mag except for three stellar systems. A combined astrometric solution of the Hipparcos Intermediate Astrometry Data and two dedicated ground-based campaigns in 2013–2020 is the basis for this catalog. The astrometric parameters for each star include position coordinates, parallax, and proper motion components, and their covariances on the Hipparcos mean epoch 1991.25. 64% of the catalog are flagged as known or suspected double or binary stars. UBSC lists 68 stars missing in Gaia EDR3 and another 114 stars without Gaia parallaxes or proper motions. The formal precision achieved for proper motions is comparable to that of Gaia.
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Brandt, G. Mirek, Daniel Michalik, Timothy D. Brandt, Yiting Li, Trent J. Dupuy, and Yunlin Zeng. "htof: A New Open-source Tool for Analyzing Hipparcos, Gaia, and Future Astrometric Missions." Astronomical Journal 162, no. 6 (November 5, 2021): 230. http://dx.doi.org/10.3847/1538-3881/ac12d0.

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Abstract We present htof, an open-source tool for interpreting and fitting the intermediate astrometric data (IAD) from both the 1997 and 2007 reductions of Hipparcos, the scanning law of Gaia, and future missions such as the Nancy Grace Roman Space Telescope (NGRST). htof solves the astrometric parameters of any system for any arbitrary combination of absolute astrometric missions. In preparation for later Gaia data releases, htof supports arbitrarily high-order astrometric solutions (e.g., five-, seven-, and nine-parameter fits). Using htof, we find that the IAD of 6617 sources in Hipparcos 2007 might have been affected by a data corruption issue. htof integrates an ad hoc correction that reconciles the IAD of these sources with their published catalog solutions. We developed htof to study masses and orbital parameters of substellar companions, and we outline its implementation in one orbit fitting code (orvara). We use htof to predict a range of hypothetical additional planets in the β Pic system, which could be detected by coupling NGRST astrometry with Gaia and Hipparcos. htof is pip installable and available at https://github.com/gmbrandt/htof.
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Rioja, M., R. Dodson, G. Orosz, and H. Imai. "MultiView High Precision VLBI Astrometry at Low Frequencies." Proceedings of the International Astronomical Union 13, S336 (September 2017): 439–42. http://dx.doi.org/10.1017/s1743921317010560.

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AbstractObservations at low frequencies (<8GHz) are dominated by distinct direction dependent ionospheric propagation errors, which place a very tight limit on the angular separation of a suitable phase referencing calibrator and astrometry. To increase the capability for high precision astrometric measurements an effective calibration strategy of the systematic ionospheric propagation effects that is widely applicable is required. The MultiView technique holds the key to the compensation of atmospheric spatial-structure errors, by using observations of multiple calibrators and two dimensional interpolation. In this paper we present the first demonstration of the power of MultiView using three calibrators, several degrees from the target, along with a comparative study of the astrometric accuracy between MultiView and phase-referencing techniques. MultiView calibration provides an order of magnitude improvement in astrometry with respect to conventional phase referencing, achieving ~100micro-arcseconds astrometry errors in a single epoch of observations, effectively reaching the thermal noise limit.
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Shao, M., M. Colavita, D. Staelin, R. Simon, and K. Johnston. "Present Status and Future Plans for the Two Color Astrometric Interferometer Project." Symposium - International Astronomical Union 109 (1986): 331–40. http://dx.doi.org/10.1017/s0074180900076750.

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A 3-meter two telescope stellar interferometer has been used to observe fringes for the past year in order to study the feasibility of using long baseline interferometers for astrometry. We have demonstrated that two color fringe measurements are capable of significantly reducing the astrometric error due to atmospheric turbulence. Currently, we are investigating the thermal and mechanical sources of error in the instrument. The results of our study will be incorporated into the design of the 20 meter astrometric interferometer which will be built in the next 1–2 years. The key to the 20-meter interferometer is the laser system which we expect to monitor all the mechanical and thermal imperfections relevant to astrometry at the 10−3 to 10−4 arc sec level. A slight modification of this system could be used in a space based interferometer for 1 to 10 microarcsecond astrometry of faint objects.
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Meunier, N., A. M. Lagrange, and S. Borgniet. "Activity time series of old stars from late F to early K." Astronomy & Astrophysics 644 (December 2020): A77. http://dx.doi.org/10.1051/0004-6361/202038710.

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Context. Stellar activity strongly affects and may prevent the detection of Earth-mass planets in the habitable zone of solar-type stars with radial velocity technics. Astrometry is in principle less sensitive to stellar activity because the situation is more favourable: the stellar astrometric signal is expected to be fainter than the planetary astrometric signal compared to radial velocities. Aims. We quantify the effect of stellar activity on high-precision astrometry when Earth-mass planets are searched for in the habitable zone around old main-sequence solar-type stars. Methods. We used a very large set of magnetic activity synthetic time series to characterise the properties of the stellar astrometric signal. We then studied the detectability of exoplanets based on different approaches: first based on the theoretical level of false positives derived from the synthetic time series, and then with blind tests for old main-sequence F6-K4 stars. Results. The amplitude of the signal can be up to a few times the solar value depending on the assumptions made for activity level, spectral type, and spot contrast. The detection rates for 1 MEarth planets are very good, however, with extremely low false-positive rates in the habitable zone for stars in the F6-K4 range at 10 pc. The standard false-alarm probability using classical bootstrapping on the time series strongly overestimates the false-positive level. This affects the detection rates. Conclusions. We conclude that if technological challenges can be overcome and very high precision is reached, astrometry is much more suitable for detecting Earth-mass planets in the habitable zone around nearby solar-type stars than radial velocity, and detection rates are much higher for this range of planetary masses and periods when astrometric techniques are used than with radial velocity techniques.
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Hyland, L. J., M. J. Reid, S. P. Ellingsen, M. J. Rioja, R. Dodson, G. Orosz, C. R. Masson, and J. M. McCallum. "Inverse Multiview. I. Multicalibrator Inverse Phase Referencing for Microarcsecond Very Long Baseline Interferometry Astrometry." Astrophysical Journal 932, no. 1 (June 1, 2022): 52. http://dx.doi.org/10.3847/1538-4357/ac6d5b.

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Abstract Very Long Baseline Interferometry (VLBI) astrometry is a well established technique for achieving ±10 μas parallax accuracies at frequencies well above 10 GHz. At lower frequencies, uncompensated interferometer delays associated with the ionosphere play the dominant role in limiting the astrometric accuracy. Multiview is a novel VLBI calibration method, which uses observations of multiple quasars to accurately model and remove time-variable, directional-dependent changes to the interferometer delay. Here we extend the Multiview technique by phase-referencing data to the target source (“inverse Multiview”) and test its performance. Multiple observations with a four-antenna VLBI array operating at 8.3 GHz show single-epoch astrometric accuracies near 20 μas for target–reference quasar separations up to about 7°. This represents an improvement in astrometric accuracy by up to an order of magnitude compared to standard phase-referencing.
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Mishra-Sharma, Siddharth. "Inferring dark matter substructure with astrometric lensing beyond the power spectrum." Machine Learning: Science and Technology 3, no. 1 (January 19, 2022): 01LT03. http://dx.doi.org/10.1088/2632-2153/ac494a.

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Abstract Astrometry—the precise measurement of positions and motions of celestial objects—has emerged as a promising avenue for characterizing the dark matter population in our Galaxy. By leveraging recent advances in simulation-based inference and neural network architectures, we introduce a novel method to search for global dark matter-induced gravitational lensing signatures in astrometric datasets. Our method based on neural likelihood-ratio estimation shows significantly enhanced sensitivity to a cold dark matter population and more favorable scaling with measurement noise compared to existing approaches based on two-point correlation statistics. We demonstrate the real-world viability of our method by showing it to be robust to non-trivial modeled as well as unmodeled noise features expected in astrometric measurements. This establishes machine learning as a powerful tool for characterizing dark matter using astrometric data.
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Honma, Mareki, Takumi Nagayama, Tomoya Hirota, Naoko Matsumoto, Nobuyuki Sakai, and Noriyuki Kawaguchi. "Maser astrometry with VERA and Galactic structure." Proceedings of the International Astronomical Union 8, S287 (January 2012): 386–90. http://dx.doi.org/10.1017/s174392131200734x.

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AbstractSince 2007 VERA (VLBI Exploration of Radio Astrometry) has been producing astrometric results (distances and/or proper motions) for Galactic maser sources. Nearly 30 parallaxes have been obtained for star-forming regions and late-type stars. By using VERA's astrometric results for star-forming regions, combined with those obtained with VLBA and EVN, fundamental Galactic parameters and Galactic structure may be derived. Our results show that R0 = 8.4 ± 0.4 kpc and Ω⊙ ≡ Ω0 + V⊙/R0=30.7±0.8 km s−1 kpc−1, and also show that the rotation curve of the Galaxy is nearly flat. The determinations of Galactic parameters and structures demonstrate that the maser astrometry can not only contribute significantly to research of individual maser sources, but also to studies of the structure of the Galaxy.
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Sergienko, V. I., A. G. Radchuk, B. A. Pavlov, and V. S. Kudeeva. "A Fully-Automated System of Astrometric Data Collection and Processing." Symposium - International Astronomical Union 109 (1986): 795–98. http://dx.doi.org/10.1017/s0074180900077342.

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The automated astronomical system “Astrometrist” for the determination of the parameters of terrestrial rotation (DTRP) is intended for astrometric and meteorological data collection and on-line processing and feedback to the user by a computer.
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Høg, Erik. "GIER: A Danish computer from 1961 with a role in the modern revolution of astronomy - II." Proceedings of the International Astronomical Union 12, S330 (April 2017): 92–93. http://dx.doi.org/10.1017/s1743921317005336.

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AbstractA Danish computer, GIER, from 1961 played a vital role in the development of a new method for astrometric measurement. This method, photon counting astrometry, ultimately led to two satellites with a significant role in the modern revolution of astronomy. A GIER was installed at the Hamburg Observatory in 1964 where it was used to implement the entirely new method for the measurement of stellar positions by means of a meridian circle, at that time the fundamental instrument of astrometry. An expedition to Perth in Western Australia with the instrument and the computer was a success. This method was also implemented in space in the first ever astrometric satellite Hipparcos launched by ESA in 1989. The Hipparcos results published in 1997 revolutionized astrometry with an impact in all branches of astronomy from the solar system and stellar structure to cosmic distances and the dynamics of the Milky Way. In turn, the results paved the way for a successor, the one million times more powerful Gaia astrometry satellite launched by ESA in 2013. Preparations for a Gaia successor in twenty years are making progress.
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Lloyd, James P., Ben R. Oppenheimer, and James R. Graham. "The Potential of Differential Astrometric Interferometry from the High Antarctic Plateau." Publications of the Astronomical Society of Australia 19, no. 3 (2002): 318–22. http://dx.doi.org/10.1071/as01072.

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AbstractThe low infrared background and high atmospheric transparency are the principal advantages of Antarctic Plateau sites for astronomy. However, the poor seeing (between 1 and 3 as) negates much of the sensitivity improvements that the Antarctic atmosphere offers, compared to mid-latitude sites such as Mauna Kea or Cerro Paranal. The seeing at mid-latitude sites, though smaller in amplitude, is dominated by turbulence at altitudes of 10–20 km. Over the Antarctic Plateau, virtually no high altitude turbulence is present in the winter. The mean square error for an astrometric measurement with a dual-beam, differential astrometric interferometer in the very narrow angle regime is proportional to the integral of h2C2N(h). Therefore, sites at which the turbulence occurs only at low altitudes offer large gains in astrometric precision. We show that a modest interferometer at the South Pole can achieve 10 μ as differential astrometry 300 times faster than a comparable interferometer at a good mid-latitude site, in median conditions. Science programs that would benefit from such an instrument include planet detection and orbit determination, and astrometric observation of stars microlensed by dark matter candidates.
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Dravins, D., L. Lindegren, S. Madsen, and J. Holmberg. "Astrometric Radial Velocities From Hipparcos." Highlights of Astronomy 11, no. 1 (1998): 564. http://dx.doi.org/10.1017/s153929960002219x.

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AbstractSpace astrometry now permits accurate determinations of stellar radial motion, without using spectroscopy. Although the feasibility of deducing astrometric radial velocities from geometric projection effects was realized already by Schlesinger (1917), only with Hipparcos has it become practical. Such a program has now been carried out for the moving clusters of Ursa Major, Hyades, and Coma Berenices. Realized inaccuracies reach about 300 m/s (Dravins et al. 1997).Discrepancies between astrometric and spectroscopic radial velocities reveal effects (other than stellar motion) that affect wavelength positions of spectral lines. Such are caused by stellar surface convection, and by gravitational redshifts. A parallel program (Gullberg & Dravins 1997) is analyzing high-precision spectroscopic radial velocities for different spectral lines in these stars, using the ELODIE radial-velocity instrument atHaute-Provence.
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36

Kaplan-Lipkin, Avi, Bruce Macintosh, Alexander Madurowicz, Krishnamurthy Sowmya, Alexander Shapiro, Natalie Krivova, and Sami K. Solanki. "Multiwavelength Mitigation of Stellar Activity in Astrometric Planet Detection." Astronomical Journal 163, no. 5 (April 11, 2022): 205. http://dx.doi.org/10.3847/1538-3881/ac56e0.

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Abstract Astrometry has long been a promising technique for exoplanet detection. At the theoretical limits, astrometry would allow for the detection of smaller planets than previously seen by current exoplanet search methods, but stellar activity may make these theoretical limits unreachable. Astrometric jitter of a Sun-like star due to magnetic activity in its photosphere induces apparent variability in the photocenter of order 0.5 mR ⊙. This jitter creates a fundamental astrophysical noise floor preventing detection of lower-mass planets in a single spectral band. By injecting planet orbits into simulated solar data at five different passbands, we investigate mitigation of this fundamental astrometric noise using correlations across passbands. For a true solar analog and a planet at 1 au semimajor axis, the 6σ detection limit set by stellar activity for an ideal telescope at the best single passband is 0.01 Earth masses. We found that pairs of passbands with highly correlated astrometric jitter due to stellar activity, but with less motion in the redder band, enable higher-precision measurements of the common signal from the planet. Using this method improves detectable planet masses at 1 au by up to a factor of 10, corresponding to at best 0.005 Earth masses for a Sun-like star with a perfect telescope. Given these results, we recommend that future astrometry missions consider proceeding with two or more passbands to reduce noise due to stellar activity.
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37

Taheri, Mojtaba, Alan W. McConnachie, Paolo Turri, Davide Massari, David Andersen, Giuseppe Bono, Giuliana Fiorentino, Kim Venn, Jean-Pierre Véran, and Peter B. Stetson. "Optimal Differential Astrometry for Multiconjugate Adaptive Optics. I. Astrometric Distortion Mapping using On-sky GeMS Observations of NGC 6723." Astronomical Journal 163, no. 4 (March 23, 2022): 187. http://dx.doi.org/10.3847/1538-3881/ac5747.

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Abstract The Extremely Large Telescope and the Thirty Meter Telescope will use state of the art multiconjugate adaptive optics (MCAO) systems to obtain the full D 4 advantage that their apertures can provide. However, to reach the full astrometric potential of these facilities for on-sky science requires understanding any residual astrometric distortions from these systems and find ways to measure and eliminate them. In this work, we use Gemini multiconjugate adaptive optic system (GeMS) observations of the core of NGC 6723 to better understand the on-sky astrometric performance of MCAO. We develop new methods to measure the astrometric distortion fields of the observing system, which probe the distortion at the highest possible spatial resolution. We also describe methods for examining the time-variable and static components of the astrometric distortion. When applied to the GeMS Gemini South Adaptive Optics Imager (GSAOI) data, we are able to see the effect of the field rotator at the subpixel level, and we are able to empirically derive the distortion due to the optical design of GeMS/GSAOI. We argue that the resulting distortion maps are a valuable tool to measure and monitor the on-sky astrometric performance of future instrumentation. Our overall astrometry pipeline produces high-quality proper motions with an uncertainty floor of ∼45 μas yr−1. We measure the proper motion dispersion profile of NGC 6723 from a radius of ∼10″ out to ∼1′ based on ∼12,000 stars. We also produce a high-quality optical–near infrared color–magnitude diagram, which clearly shows the extreme horizontal branch and main-sequence knee of this cluster.
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38

Ducourant, C., O. Wertz, A. Krone-Martins, R. Teixeira, J. F. Le Campion, L. Galluccio, J. Klüter, et al. "Gaia GraL: Gaia DR2 gravitational lens systems." Astronomy & Astrophysics 618 (October 2018): A56. http://dx.doi.org/10.1051/0004-6361/201833480.

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Context. Thanks to its spatial resolution, the ESA/Gaia space mission offers a unique opportunity to discover new multiply imaged quasars and to study the already known lensed systems at sub-milliarcsecond astrometric precisions. Aims. In this paper, we address the detection of the known multiply imaged quasars from the Gaia Data Release 2 (DR2) and determine the astrometric and photometric properties of the individually detected images found in the Gaia DR2 catalogue. Methods. We have compiled an exhaustive list of quasar gravitational lenses from the literature to search for counterparts in the Gaia DR2. We then analysed the astrometric and photometric properties of these Gaia’s detections. To highlight the tremendous potential of Gaia at the sub-milliarcsecond level we finally performed a simple Bayesian modelling of the well-known gravitational lens system HE0435-1223, using Gaia DR2 and HST astrometry. Results. From 481 known multiply imaged quasars, 206 have at least one image found in the Gaia DR2. Among the 44 known quadruply imaged quasars of the list, 29 have at least one image in the Gaia DR2, 12 of which are fully detected (2MASX J01471020+4630433, HE 0435-1223, SDSS1004+4112, PG1115+080, RXJ1131-1231, 2MASS J11344050-2103230, 2MASS J13102005-1714579, B1422+231, J1606-2333, J1721+8842, WFI2033-4723, WGD2038-4008), eight have three counterparts, eight have two and one has only one. As expected, the modelling of HE0435-1223 shows that the model parameters are significantly better constrained when using Gaia astrometry compared to HST astrometry, in particular the relative positions of the background quasar source and the centroid of the deflector. The Gaia sub-milliarcsecond astrometry also significantly reduces the parameter correlations. Conclusions. Besides providing an up-to-date list of multiply imaged quasars and their detection in the Gaia DR2, this paper shows that more complex modelling scenarios will certainly benefit from Gaia sub-milliarcsecond astrometry.
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Gai, M., A. Vecchiato, A. Riva, D. Busonero, M. Lattanzi, B. Bucciarelli, M. Crosta, and Z. Qi. "Astrometric Precision Tests on TESS Data." Publications of the Astronomical Society of the Pacific 134, no. 1033 (March 1, 2022): 035004. http://dx.doi.org/10.1088/1538-3873/ac584a.

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Abstract Astrometry at or below the microarcsec level with an imaging telescope assumes that the uncertainty on the location of an unresolved source can be an arbitrarily small fraction of the detector pixel, given a sufficient photon budget. This paper investigates the geometric limiting precision, in terms of CCD pixel fraction, achieved by a large set of star field images, selected among the publicly available science data of the Transiting Exoplanet Survey Satellite (TESS) mission. The statistics of the distance between selected bright stars (G ≃ 5 mag), in pixel units, is evaluated, using the position estimate provided in the TESS light curve files. The dispersion of coordinate differences appears to be affected by long term variation and noisy periods, at the level of 0.01 pixel. The residuals with respect to low-pass filtered data (tracing the secular evolution), which are interpreted as the experimental astrometric noise, reach the level of a few milli-pixel or below, down to 1/5900 pixel. Saturated images are present, evidencing that the astrometric precision is mostly preserved across the CCD columns, whereas it features a graceful degradation in the along column direction. The cumulative performance of the image set is a few micropixel across columns, or a few 10 μpx along columns. The idea of astrometric precision down to a small fraction of a CCD pixel, given sufficient signal to noise ratio, is confirmed by real data from an in-flight science instrument to the 10−6 pixel level. Implications for future high precision astrometry missions are briefly discussed.
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Tichý, Miloš, Jana Tichá, and Michal Kočer. "KLENOT PROJECT - NEAR EARTH OBJECTS FOLLOW-UP PROGRAM." Proceedings of the International Astronomical Union 10, S318 (August 2015): 319–20. http://dx.doi.org/10.1017/s1743921315007176.

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AbstractNEO research is a great challenge just now - for science, for exploration and for planetary defence. Therefore NEO discoveries, astrometric follow-up, orbit computations as well as physical studies are of high interest both to science community and humankind.The KLENOT Project of the Klet Observatory, South Bohemia, Czech Republic pursued the confirmation, early follow-up, long-arc follow-up and recovery of Near Earth Objects since 2002. Tens of thousands astrometric measurements helped to make inventory of NEOs as well as to understand the NEO population. It ranked among the world most prolific professional NEO follow-up programmes during its first phase from 2002 to 2008. The fundamental improvement of the 1.06-m KLENOT Telescope was started in autumn 2008. The new computer controlled paralactic mount was built to substantially increase telescope-time efficiency, the number of observations, their accuracy and limiting magnitude.The testing observations of the KLENOT Telescope Next Generation (NG) were started in October 2011. The new more efficient CCD camera FLI ProLine 230 was installed in summer 2013. The original Klet Software Package has been continually upgraded over the past two decades of operation. Along with huge hardware changes we have decided for essential changes in software and the whole KLENOT work-flow. Using the current higher computing power available, enhancing and updating our databases and astrometry program, the core of our software package, will prove highly beneficial. Moreover, the UCAC4 as the more precise astrometric star catalog was implemented. The modernized KLENOT System was put into full operation in September 2013. This step opens new possibilities for the KLENOT Project, the long-term European Contribution to Monitoring and Cataloging Near Earth Objects.KLENOT Project Goals are confirmatory observations of newly discovered fainter NEO candidates, early follow-up of newly discovered NEOs, long-arc follow-up astrometry of NEOs in need of further data. The higher priority is given to Potentially Hazardous Asteroids (PHAs) and Virtual Impactors (VIs), recoveries of NEOs in the second opposition and also follow-up astrometry of radar or mission targets, special follow-up requests and follow-up astrometry of other unusual objects (comets, bright TNOs) including analysis of cometary features of suspected bodies, and also search for new asteroids, especially NEOs as well as other objects showing unusual motion.The KLENOT Telescope is located at the Klet Observatory, South Bohemia, Czech Republic (Central Europe), at geographical position: latitude 14° 17' 17” E, longitude 48° 51' 48”N, elevation 1068 meters above sea level, in a rather dark site in the middle of the Protected Landscape Area Blanský les. Average number of clear nights per year about 120. Our IAU/MPC code is 246KLENOT Project Advantages: •full observing time is dedicated to the KLENOT team•quick changes in an observing plan possible, even during an observing night•long-term NEO activities at Klet (since 1992)•experienced observers/measurers visually validate each moving object candidate•real-time processing of targeted objectsKLENOT Next Generation Telescope technical data (since 2013): •new computer controlled paralactic mount•1.06-m f/3 main mirror (Zeiss)•four lenses primary focus corrector•1.06-m f/2.7 optical system•CCD camera FLI ProLine PL230•chip e2v 2048 × 2048 pixels, pixel size 15 microns, Peltier cooling•FOV 37 × 37 arcminutes, image scale 1.1 arcseconds per pixel•limiting magnitude mV=21.5 mag. for 120-sec exposure timeKLENOT Project First Phase Results(2002-2008)total of 52,658 astrometric measurements of 5,867 bodies, it contains: •13,342 astrometric measurements of 1,369 NEAs (MPC,NEODys)•confirmation and astrometry of 623 NEAs from NEOCP (MPECs)•recoveries of 4 comets and 16 NEAs (including 196P/Tichý)•astrometry of 157 Virtual Impactors (CLOMON, SENTRY)•detection of cometary features of 34 bodies (IAUCs)•discovery of splitting of comet C/2004 S1 (Van Ness)•independent discovery of 4 fragments of comet 73P/S-W 3•asteroid discoveries - 750 bodies•3 NEOs - Apollo 2002 LK, Aten 2003 UT55, Apollo 2006 XR4, 1 JFA 2004 RT109The first KLENOT Project Next Generation Results (since 2011)total of 10,054 astrometric measurements of 1,298 bodies, it contains: •2,211 astrometric measurements of 263 NEAs(MPC,NEODys)•confirmation and astrometry of 143 NEAs from NEOCP (MPECs)•astrometry of 18 Virtual Impactors (CLOMON, SENTRY)•detection of cometary features of 5 bodies (IAUCs)
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41

Sahlmann, J., E. G. Nelan, P. Chayer, B. McLean, and M. Lallo. "Optimisation of JWST operations with the help of Gaia." Proceedings of the International Astronomical Union 12, S330 (April 2017): 98–99. http://dx.doi.org/10.1017/s1743921317006275.

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AbstractThe James Webb Space Telescope (JWST) is scheduled for launch in 2018. To operate and observe efficiently, JWST will rely on various external astrometric and photometric catalogues, in particular the HST Guide Star Catalog (GSC), for instance to locate sources accurately on the sky. The incorporation of the Gaia astrometric catalog will improve the absolute astrometry of the GSC and is therefore relevant for JWST operations. We outline how the JWST Science and Operations Center hosted at the Space Telescope Science Institute (STScI) intends to use the Gaia survey results to improve upon operational aspects such as the guiding and the geometric focal plane characterisation of JWST.
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42

Dravins, Dainis. "Absolute Wavelength Shifts — A New Diagnostic for Rapidly Rotating Stars." Symposium - International Astronomical Union 215 (2004): 27–32. http://dx.doi.org/10.1017/s0074180900195130.

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Accuracies reached in space astrometry now permit the accurate determination of astrometric radial velocities, without any use of spectroscopy. Knowing this true stellar motion, spectral shifts intrinsic to stellar atmospheres can be identified, for instance gravitational redshifts and those caused by velocity fields on stellar surfaces. The astrometric accuracy is independent of any spectral complexity, such as the smeared-out line profiles of rapidly rotating stars. Besides a better determination of stellar velocities, this permits more precise studies of atmospheric dynamics, such as possible modifications of stellar surface convection (granulation) by rotation-induced forces, as well as a potential for observing meridional flows across stellar surfaces.
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43

Unwin, Stephen C. "Precision Astrometry with the Space Interferometry Mission." Symposium - International Astronomical Union 202 (2004): 474–76. http://dx.doi.org/10.1017/s0074180900218548.

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SIM - the Space Interferometry Mission - will be the first of a new generation of space instruments using interferometry. Designed for precision astrometry, it will be a 10-meter optical interferometer providing 4 microarcsecond (μas) absolute position measurements over the whole sky, using a grid of stars (and quasars) for a reference frame. Maintaining its astrometric precision on stars as faint as 20 magnitude, it will far surpass the capability of ground-based astrometry, opening up a new era of space-based astrometry. A wide range of astrophysics problems will be addressed by SIM, including formation and dynamics of our Galaxy, calibration of the cosmic distance scale, and fundamental stellar astrophysics. SIM will search for planetary companions to nearby stars, by detecting the well-known astrometric ‘wobble’ signature with a single-measurement precision of 1 μas, enabling the detection of planets down to about an Earth mass. SIM will serve as a technology pathfinder for NA8A's Terrestrial Planet Finder. Launch is currently planned for 2008, with a mission duration of 5 years.
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McNally, Derek. "Astrometric angles." Nature 319, no. 6056 (February 1986): 824. http://dx.doi.org/10.1038/319824a0.

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45

Nucita, A. A., F. De Paolis, G. Ingrosso, M. Giordano, and L. Manni. "Astrometric microlensing." International Journal of Modern Physics D 26, no. 05 (April 2017): 1741015. http://dx.doi.org/10.1142/s0218271817410152.

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In the next future, astrometric microlensing will offer a new channel for investigating the nature of both lenses and sources involved in a gravitational microlensing event. The effect, corresponding to the shift of the position of the multiple image centroid with respect to the source star location, is expected to occur on scales from micro-arcoseconds to milli-arcoseconds depending on the characteristics of the lens-source system. Here, we consider different classes of events (single/binary lens acting on a single/binary source) also accounting for additional effects including the finite source size, the blending and orbital motion. This is particularly important in the era of Gaia observations which are making possible astrometric measurements with unprecedented quality.
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Seidelmann, P. K. "Prospects for Future Astrometric Missions." Symposium - International Astronomical Union 179 (1998): 79–85. http://dx.doi.org/10.1017/s0074180900128232.

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Hipparcos and the Hubble Space Telescope have demonstrated the astrometric capabilities in space. SIM and GAIA are being studied for future missions. There have been many proposals for new astrometry missions from many different countries, but most of them have not been funded.The best possibility for a mission within the next five years would be through a collaborative effort for a small, fast, cheap spacecraft which could be a precursor for future, larger, more accurate missions, which are under study.
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Colomer, F., D. Graham, T. Krichbaum, B. O. Rönnäng, P. de Vicente, A. Barcia, R. S. Booth, A. Witzel, J. Gómez-González, and A. Baudry. "Astrometry of SiO Masers: First detections of circumstellar SiO maser emission at λ = 7 mm using three European VLBI stations." International Astronomical Union Colloquium 131 (1991): 338–41. http://dx.doi.org/10.1017/s0252921100013592.

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The positions of circumstellar molecular masers relative to each other, to the central star, and to the compact extragalactic sources are of astrophysical and astrometrical interest. The SiO masers at λ = 7 mm are especially interesting as the maser spots are located very close to the photosphere of the star. For astrometry we would like to see a proper distribution of the maser spots and several epochs of observations in order to estimate the position of the central star and eventual motions of the spots.The optical position of this star can be determined with ground-based astrometric facilities, and many of the stars showing SiO emission are in theHipparcoscatalogue. A link between the Optical and Radio Reference Frames is achieved in a second step relating the maser spot positions to the Extragalactic Reference Frame (EGRF) established with VLBI (Baudryet al., 1984).
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48

Rodeghiero, G., M. Sawczuck, J. U. Pott, M. Glück, E. Biancalani, M. Häberle, H. Riechert, et al. "Development of the Warm Astrometric Mask for MICADO Astrometry Calibration." Publications of the Astronomical Society of the Pacific 131, no. 999 (April 12, 2019): 054503. http://dx.doi.org/10.1088/1538-3873/ab0c40.

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49

Lindegren, Lennart, and Dainis Dravins. "Astrometric radial velocities for nearby stars." Astronomy & Astrophysics 652 (August 2021): A45. http://dx.doi.org/10.1051/0004-6361/202141344.

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Context. Under certain conditions, stellar radial velocities can be determined from astrometry, without any use of spectroscopy. This enables us to identify phenomena, other than the Doppler effect, that are displacing spectral lines. Aims. The change of stellar proper motions over time (perspective acceleration) is used to determine radial velocities from accurate astrometric data, which are now available from the Gaia and HIPPARCOS missions. Methods. Positions and proper motions at the epoch of HIPPARCOS are compared with values propagated back from the epoch of the Gaia Early Data Release 3. This propagation depends on the radial velocity, which obtains its value from an optimal fit assuming uniform space motion relative to the solar system barycentre. Results. For 930 nearby stars we obtain astrometric radial velocities with formal uncertainties better than 100 km s−1; for 55 stars the uncertainty is below 10 km s−1, and for seven it is below 1 km s−1. Most stars that are not components of double or multiple systems show good agreement with available spectroscopic radial velocities. Conclusions. Astrometry offers geometric methods to determine stellar radial velocity, irrespective of complexities in stellar spectra. This enables us to segregate wavelength displacements caused by the radial motion of the stellar centre-of-mass from those induced by other effects, such as gravitational redshifts in white dwarfs.
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50

Geisler, R., J. Setiawan, Th Henning, D. Queloz, A. Quirrenbach, R. Launhardt, A. Müller, S. Reffert, and P. Weise. "Preparing the exoplanet search with PRIMA: searching for reference stars and target characterization." Proceedings of the International Astronomical Union 3, S249 (October 2007): 61–63. http://dx.doi.org/10.1017/s1743921308016384.

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AbstractThe PRIMA (Phase-Referenced Imaging and Micro-arcsecond Astrometry) facility at ESO VLTI (Paranal observatory) is expected to be commissioned in mid 2008. The ESPRI (Exoplanet Search with PRIMA) consortium is currently preparing an astrometric survey to search for extrasolar planets. To achieve the scientific goal of this survey, a careful selection of target and reference stars is necessary. Apart from catalog search and modelling, extensive and dedicated preparatory observations are indispensable. Here we present two aspects of the preparatory observation programs: A high dynamic range near infrared (NIR) imaging survey to search for astrometric reference stars around the preselected target stars and characterization of the target stars by using high-resolution spectroscopy.
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