To determine the precise positions of stars in CCD frames, various centering algorithms have been proposed for astrometry. The effective point spread function (ePSF) and the Gaussian centering algorithms are two representative centering algorithms. This paper compares in detail and investigates these two centering algorithms in performing data reduction. Specifically, synthetic star images in different conditions (i.e. profiles, fluxes, backgrounds and full width at half maximums) are generated and processed. We find that the difference in precision between the two algorithms is related to the profiles of the star images. Therefore, the precision comparison results using an ideal Gaussian-profile star image cannot be extended to other more specific experimental scenarios. Based on the simulation results, the most appropriate algorithm can be selected according to the image characteristics of observations, and the loss of precision of other algorithms can be estimated. The conclusions are verified using observations captured by the 1-m and 2.4-m telescopes at Yunnan Observatory.
Asteroid detections in astronomical images may appear as trails due to a combination of their apparent rate of motion and exposure duration. Nearby asteroids in particular typically have high apparent rates of motion and acceleration. Their recovery, especially on their discovery apparition, depends upon obtaining good astrometry from the trailed detections. We present an analytic function describing a trailed detection under the assumption of a Gaussian point spread function (PSF) and constant rate of motion. We have fit the function to both synthetic and real trailed asteroid detections from…
Context: Stellar occultations, greatly enhanced by the publication of the Gaia data releases, permit not only the determination of asteroid size and shape, but also the retrieval of additional, accurate astrometry, with a possible relevant impact on the study of dynamical properties. The use of Gaia as reference catalogue and the recent implementation of an improved error model for occultation astrometry offer the opportunity to test its global astrometric performance on the existing data set of observed events, dominated by minor planets belonging to the main belt. Aims: We aim to explore the…
The Infrared Array Camera (IRAC) on the Spitzer Space Telescope currently offers the greatest potential for high-precision astrometry of faint mid-IR sources across arcminute-scale fields, which would be especially valuable for measuring parallaxes of cold brown dwarfs in the solar neighborhood and proper motions of obscured members of nearby star-forming regions. To more fully realize IRAC's astrometric capabilities, we have sought to minimize the largest sources of uncertainty in astrometry with its 3.6 and 4.5 $μ$m bands. By comparing different routines that estimate stellar positions, we h…
The achievement of $μ$arcsec relative astrometry with ground-based, near infrared, extremely large telescopes requires a significant endeavour of calibration strategies. In this paper we address the removal of instrument optical distortions coming from the ELT first light instrument MICADO and its adaptive optics system MAORY by means of an astrometric calibration mask. The results of the test campaign on a prototype mask (scale 1:2) has probed the manufacturing precision down to $\sim$ 50nm/1mm scale, leading to a relative precision $δσ\sim 5e-5$. The assessed manufacturing precision indicate…
Precision astrometry is an integral component of successful pulsar timing campaigns. Astrometric parameters are commonly derived by fitting them as parameters of a timing model to a series of pulse times of arrival (TOAs). TOAs measured to microsecond precision over several-year spans can yield position measurements with sub-milliarcsecond precision. However, timing-based astrometry can become biased if a pulsar displays any red spin noise, which can be compared to the red noise signal produced by the stochastic gravitational wave background. We investigate how noise of different spectral type…
The VLTI instrument GRAVITY will provide very powerful astrometry by combining the light from four telescopes for two objects simultaneously. It will measure the angular separation between the two astronomical objects to a precision of 10 microarcseconds. This corresponds to a differential optical path difference (dOPD) between the targets of few nanometers and the paths within the interferometer have to be maintained stable to that level. For this purpose, the novel metrology system of GRAVITY will monitor the internal dOPDs by means of phase-shifting interferometry. We present the four-step …
Context. Thanks to more than 20 years of monitoring, the radial velocity (RV) method has detected long-period companions (P > 10yr) around several dozens of stars. Yet, the true nature of these companions remains unclear because of the uncertainty as to the inclination of the companion orbital plane. Aims. We wish to constrain the orbital inclination and the true mass of long-period single companions. Methods. We used a Markov Chain Monte Carlo (MCMC) fitting algorithm to combine RV measurements with absolute astrometry and, when available, relative astrometry data. Results. We have lifted the…
In this paper we explore the maximum precision attainable in the location of a point source imaged by a pixel array detector in the presence of a background, as a function of the detector properties. For this we use a well-known result from parametric estimation theory, the so-called Cramer-Rao lower bound. We develop the expressions in the 1-dimensional case of a linear array detector in which the only unknown parameter is the source position. If the object is oversampled by the detector, analytical expressions can be obtained for the Cramer-Rao limit that can be readily used to estimate the …
Abridged: Direct imaging has made significant progress over the past decade leading to the detection of several giant planets. Observing strategies and data rates vary from instrument to instrument and wavelength, and can result in tens of thousands of images to be combined. We here present a new approach, tailored for VLT/NaCo observations performed with the Annular Groove Phase Mask (AGPM) coronagraph at $L'$ filter. Our pipeline aims at improving the post-processing of the observations on two fronts: identifying the location of the star behind the AGPM to better align the science frames and…
Mass and radius measurements of stars are important inputs for models of stellar structure. Binary stars are of particular interest in this regard, because astrometry and spectroscopy of a binary together provide the masses of both stars as well as the distance to the system, while interferometry can both improve the astrometry and measure the radii of the stars. In this work we simulate parameter recovery from intensity interferometry, especially the challenge of disentangling the radii of two stars from their combined interferometric signal. Two approaches are considered: separation of the v…
Taking a large amount of images, the Cassini Imaging Science Subsystem (ISS) has been routinely used in astrometry. In ISS images, disk-resolved objects often lead to false detection of stars that disturb the camera pointing correction. The aim of this study was to develop an automated processing method to remove the false image stars in disk-resolved objects in ISS images. The method included the following steps: extracting edges, segmenting boundary arcs, fitting circles and excluding false image stars. The proposed method was tested using 200 ISS images. Preliminary experimental results sho…
Point spread function (PSF) reconstruction (PSF-R) is a well established technique to determine reliably and accurately the PSF from Adaptive Optics (AO) control loop data. We have successfully applied this technique to improve the precision on photometry and astrometry to observation of NGC6121 obtained with SPHERE/ZIMPOL as it will be presented in a forthcoming letter. Firstly, we present the methodology we followed to reconstruct the PSF combining pupil-plane and focal-plane measurements using using our PSF-R method PRIME (Beltramo-Martin et al. 2019), with upgrade of both the model and bes…
(Abridged) The detection of forming planets in disks around young stars remains elusive, and state-of-the-art observational techniques provide somewhat ambiguous results. It has been reported that the pre-transitional T Tauri star LkCa 15 could host three planets; candidate planet b is in the process of formation, as inferred from its H$α$ emission. However, a more recent work casts doubts on the planetary nature of the previous detections. We have observed LkCa 15 with ISIS/WHT. The spectrograph's slit was oriented towards the last reported position of LkCa 15 b (parallel direction) and 90deg…
A practical re-weighting scheme of data fitting: application to asteroids orbit determination with Gaia
LTE, Observatoire de Paris, Université PSL, Sorbonne Université, Université de Lille, LNE, CNRS（巴黎天文台，巴黎文理研究大学，索邦大学，里尔大学）；Institute of Applied Astronomy, Russian Academy of Sciences（俄罗斯科学院应用天文学研究所）；DiRAC Institute and the Department of Astronomy, University of Washington（华盛顿大学天文学系及DIRAC研究所）；Institut Polytechnique des Sciences Avancées (IPSA)（高等科学理工学院）
