In the night of October 14, 2021 in T.B. Omarov Assy-Turgen observatory by means of AZT-20 telescope with back-illuminated CMOS-camera “Kepler” excellent values of background upper-limit (UL ~ 24.8 mag) was registered with astronomical seeing ~ 1.7″ during an optical afterglow observation of gamma-ray burst GRB 210912A (r’-filter, 92 snapshots with 60s. exposure). It demonstrates high stability and transparency of atmosphere in the Observatory.