Optical variability of some quasars important to ICRF-Gaia CRF link

The Gaia optical observations started a few years ago. As a result, the Gaia Celestial Reference Frame (Gaia CRF) should replace the International CRF (ICRF). This could be done via extragalactic radio sources (mostly quasars - QSOs) visible in optical domain. During about 2.5 years (for the period July 2016 { April 2019) of our original observations of some QSOs outside ICRF list we collected observations in the V and R bands for five objects and their 30 comparison stars. Photometry stability of these objects is of importance for astrometry and the mentioned link. Because of it we did investigation of brightness variability of objects and their suitable comparison stars, and the F{test was performed. As a result, only the brightness of one object (1556+335) does not show variability. Other four objects were examined to determine the quasiperiods of their light curves using the method of Least Squares: 1535+231 (3.1 years in V, and 1.7 and 5.2 years in R filter), 1607+604 (2.7 years in V, and 1.3 and 2.3 years in R), 1722+119 (1.3 and 2.7 years in V, and 1.3 and 5.3 years in R), and 1741+597 (6.5 years in V, and 1.3 and 4.0 years in R). After a similar analysis of variability of comparison stars, the conclusion is that all of them are useful for differential photometry. Also, we provide our finding charts for these objects with suitable comparison stars.

The Tycho Catalogue: Extension of Optical Reference Frame

The Tycho Catalogue contains astrometric and photometric data for the one million brightest stars on the sky. About 900000 stars are high quality astrometric reference stars. Photometry in a blue and visual band is provided. Two extensions of the catalogue are under way. One is the extension to one magnitude fainter stars by a second reduction of the original Tycho observations, resulting in a catalogue with a total of about three million stars. The other extension is the derivation of more accurate proper motions by means of the Astrographic Catalogue positions as first epoch. This latter work will be carried out first for the one million stars, and later for all the three million, resulting in the Tycho Reference Catalogue (TRC).