Minimum number of observations required to detect the curved motion of the secondary black hole as a function of the observed orbital period and semi-major axis in angular units. Observations are performed annually. We assume the nominal angular resolution of 6 µas for BHEX (solid lines), and 15 µas for the ngEHT (dashed lines). The red line depicts the assumption used in synthetic data simulations: the best case for the number of BHEX observations (three) of a binary across a nominal 2 year mission.