Abstract

On Photoelectric Observations of Occultations of Stars by the Moon
and by Asteroids With a New Portable Photometer

Eugene M. Trunkovsky

Zeitauflösungen von 1 ms bei Sternbedeckungen durch den Mond erlauben eine Bestimmung der Winkelgröße eines Sterns mit einer Auflösung in der Größenordnung einer Millibogensekunde.

When we record with a high time resolution of the order of 1 ms a diffraction curve resulting from the star's occultation by the Moon, we have a possibility, first, to determine directly the angular size of a star with angular resolution of the order of milliarcsecond, i.e. to obtain valuable and even unique astrophysical information, and, second, to find the moment of geometric occultation of the stellar disk centre with an accuracy of the order of 1 msec.

It is known that high-accurate timing of lunar occultations of stars provide a valuable information for many applications such as study of the lunar motion and relief of the lunar surface, refinement of coordinates of stars, etc. With the purpose of determination of occultation time with very high accuracy one should take into account that the moment of occultation of a point light source corresponds to that point on the diffraction curve where light flux is equal to {L2+0.25*(L1-L2)}, L1 and L2 being flux levels before and after occultation, respectively. However, if the angular size of a star is non-negligible then the moment of geometric occultation of the stellar disk centre will be shifted towards larger flux values. This moment can be found as one of the parameters when an occultation curve is processed by fitting an optimum diffraction model. A good accuracy can be reached in the determining of the occultation moment if the stop time of data recording in the computer is determined with an accuracy not worse than 1 ms, with use of shortwave radio signals giving accurate time.

A number of photoelectric diffraction curves of the lunar occultations of stars has been recorded at the observatories of the Sternberg Astronomical Institute (SAI). Angular sizes of some single and close double stars have been found, and in some cases the occultation times have been determined with an accuracy of 1-3 ms.

Last years some efforts were undertaken also in order to obtain photoelectric light curves of occultations of stars by asteroids. Photometric recording of such events with a high temporal resolution permits to determine asteroidal sizes and shape, to refine asteroidal orbits and to obtain valuable information about the occulted star itself. In particular in some cases it is possible to study angular structure of the star occulted, especially if the star is close double or multiple one. Predictions of asteroidal occultations provided by IOTA give in most cases enough information so as to try to record the event expected from most suitable observational sites. Though there were not in the meanwhile any successful photoelectric observations of asteroidal occultations of stars in Russia (as far as I know) one may hope to obtain them in the near future.

Recently a new portable photoelectric photometer has been build and tested in our Institute. This instrument is mainly intended for occultation observations and allows one to bring it relatively easily to different observational sites in order to record occultation light curves. The photometer operates in photon-counting mode under control of the built-in micro-computer and an external portable PC, so it is automatized, and the data is transfered into computer memory by means of a controlling computer program. A set of different light filters in visual spectral range and a set of diaphragms with different diameters are available; one can previously choose the diaphragm of suitable size and select the most suitable spectral band, and besides both elements can be changed very quickly. The system photometer plus PC ensures a continuous recording of the light flux from objects under study with a high time resolution down to 1 ms.

A considerable number of photoelectric light curves for various occultation events is planned to be recorded with this new instrument.

Dr. Eugene M. Trunkovsky, Sternberg Astronomical Institute, Moscow State University
Address: Universitetskij prospect 13, Moscow, 119899, Russia E-Mail: tem@sai.msu.ru Internet: http://uranus.sai.msu.ru/~tem/
Telephone: +7 095 9391672 Fax: +7 095 9328841

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