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The release of Gaia catalogue is revolutionary to the astronomy of Solar system objects. After some effects such as atmospheric refraction and CCD geometric distortion have been taken into account, the astrometric precision for ground-based telescopes can reach the level of tens of milli-arcseconds (mas). If an object approaches a reference star in a small relative angular distance (less than 100 arcsec), which is called close approach event in this work, the relative positional precision between the object and reference star will be further improved since the systematic effects of atmospheric turbulence and local telescope optics can be reduced. To obtain the precise position of a main-belt asteroid in an close approach event, a second-order angular velocity model with time is supposed in the sky plane. By fitting the relationship between the relative angular distance and observed time, we can derive the time of maximum approximation and calculate the corresponding position of the asteroid. In practice, five nights' CCD observations including 15 close approach events of main-belt asteroid (702) Alauda are taken for testing by the 1 m telescope at Yunnan Observatory, China. Compared with conventional solutions, our results show that the positional precision significantly improves, which reaches better than 4 mas, and 1 mas in the best case when referenced for the Jet Propulsion Laboratory (JPL) ephemeris in both right ascension and declination. [ABSTRACT FROM AUTHOR]

Aims. The purpose of this work is to investigate an astrometric method to accurately measure the precise position of the center of mass of the dwarf planet Haumea and its satellites, based on the unresolved images taken by ground-based telescopes. Methods. A total of 29 nights of CCD observations are reported of the dwarf planet Haumea, taken during the period from February 7 to May 25, 2022 by three telescopes located in Yunnan Province, China. The pixel centroid of the unresolved image of Haumea and its satellites is measured with respect to its reference stars in the Gaia EDR3 star catalog. The new theoretical position of Haumea (the center of mass of Haumea and its satellites is assumed) is computed by the Jet Propulsion Laboratory (JPL) Horizon System. The observed minus computed (OC) positional residuals of Haumea in right ascension and in declination show a significant and periodic variation. Furthermore, when the ephemeris position of Hi'iaka, the larger and brighter satellite of Haumea, with respect to Haumea itself is taken from the Miriade ephemeris service of the Institut de mécanique céleste et de calcul des éphémérides (IMCCE) website, the observed positional measurements of the center of mass of Haumea and Hi'iaka are derived precisely after using a new method. Results. The (OC) positional residuals are confirmed to be caused mainly by the motion of Hi'iaka around its primary. The final mean (OC) residuals of Haumea and its satellites are found with respect to the most recent version of the JPL ephemeris. As a by-product, a parameter that connects the mass ratio to the light ratio of Haumea and Hi'iaka (by Eq. (2) in the text), is introduced to derive the final mean (OC) residuals. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]