Stellar occultation by the asteroid (853) Nansenia on 8 April 2021

Abstract

Observations of occultations of stars by asteroids provide useful information about the shape and size of asteroids. Ground-based multisite observations are especially valuable in that regard. To this end, on the joint initiative of the Main Astronomical Observatory of the National Academy of Sciences of Ukraine and Astronomical Observatory of Taras Shevchenko National University of Kyiv, the development of Ukrainian network of stellar occultation observing stations is currently underway. Asteroids vary greatly in size with small asteroids being dominating. Occultations by small asteroids are visible across a rather narrow path. In such a case, it will not be feasible to make use of the entire network. Moreover, it is not always possible to conduct observations using telescopes with the required primary mirror diameter and light-gathering power or those equipped with special high-speed cameras. First of all, it applies to asteroids small in diameter for which the shadow tracks may run far from stationary telescopes. If this is the case, amateur telescopes or movable systems are employed. The aperture of such telescopes does not enable to acquire observations of occultations of faint stars with short exposure times. However, any observations are useful as they may help to determine the limits of asteroid diameter directly using techniques typical for the photometry of stellar occultations. This paper presents the results of observations of the star UCAC4 516-047388 occultation by the asteroid (853) Nansenia on 8 April 2021 performed at the Kryzhanovka observation station of Odessa I.I. Mechnikov National University. This observing station will become a part of the network of stellar occultation observers which is currently being set up. A Schmidt telescope(D = 271.25 mm, F = 440 mm) and Videoscan-415-2001 CCD camera were used to carry out observations. The ICX415AL chip (with the number of effective pixels 782 (H) x 582 (V) and unit cell size of 8.3 μm x 8.3 μm) serves as an interline image sensor for the specified CCD camera. The spectral sensitivity of the CCD camera approximately corresponds to the visible range. The NTP based PC system clock was used for the frame time-stamping. A GPS receiver with a specialised application running in the background was used to set (synchronise) the imaging system computer clock. The exposure duration for the occultation recording was chosen to be 5 seconds in order to acquire an image of the star adequate for photometric measurements, on the one hand, but to minimise the exposure time thus improving temporal resolution, on the other hand. The star UCAC4 516-047388 has a low brightness of 14.2 m which provides another reason for a long exposure time of 5 seconds to be used. This exposure time exceeds the maximum duration of occultation of 2.8 sec. An approach that enables us to draw a conclusion that the occultation has taken place and also to estimate the duration of such a phenomenon is adopted to construe the photometric (light) curve. Our calculations yield the duration of occultation τ = 2.0 ± 1.2 sec, which corresponds to the predicted value within the error. This paper also provides a description of the custom software, designed to process observations of occultations in the form of a series of graphic (image) files, which was employed for the stellar occultation by the asteroid (853) Nansenia. This software application can also be used to process observations of amateurs who will become involved in observing stellar occultations.