High-precision Astrometric Observations of the SRG Space Observatory with RTT-150 for Orbit Correction Control

The Russian–Turkish 1.5-m telescope (hereafter RTT-150) takes an active part in the work on the ground optical support of observations by the Russian Spectrum–Röntgen–Gamma (SRG) observatory (with the participation of Germany) aimed at the identification and classification of new X-ray sources detected by the SRG telescopes. The instrumental capabilities of RTT-150 (occasionally, without any significant usage of telescope time and detriment for astrophysical programs) are used in astrometric observations to control the correction of the orbit of the SRG space observatory itself. In these cases, RTT-150 provides the Ballistic Center of the Keldysh Institute of Applied Mathematics of the Russian Academy of Sciences with high-precision astrometric measurements of the SRG position, which have been used to successfully keep it in its operational orbit for more than three years of its exploitation. Beginning from the stage of flight to the zone of the Lagrange point 
 in July 2019 and at the main phase of the all-sky survey mission until March 2023, 96 sets (with a duration of about 30 min each) of high-precision positional observations with more than 5000 astrometric and photometric measurements were carried out at the RTT-150 observing facility. In most cases, the positional accuracy of the measurements dependent on seeing and apparent SRG brightness agrees with the accuracy of the provided ephemerides, 
 and 
 in declination and right ascension, respectively. In the summer months in projection onto the central region of the Galaxy in fields extremely rich in sources, the astrometric accuracy is 
, sufficient for the SRG detection after the corrections and subsequent refinements of its trajectory. Geographically, RTT-150 is the southernmost instrument among the Russian telescopes for the ground astrophysical support of SRG. This fact is also extremely important in positional SRG observations at the lowest positions in declination 
. We present the results of our astrometric and photometric observations with RTT-150 and our astrometric data reduction technique in various fields up to those extremely rich in sources.