Procedure for estimating absolute proper motions. Panels a1 and a2 show the displacements along the X and Y directions in four epochs of a probable field star (blue triangles) relative to the mean motion of NGC 1978. Similarly, panels a3 and a4 show the displacements of a candidate cluster member (red dots). The m_F814W vs. m_F555W − m_F814W CMD of stars in the field of view of NGC 1978 is plotted in panel b, while panel c shows relative stellar proper motions against m_F814W. Aqua crosses are probable field stars selected on the basis of their proper motions. The stars used as references to calculate relative proper motions are colored black, while the remaining stars with cluster-like proper motions are gray. Panels d1 and d2 show the histogram of the difference between our relative proper motions and the absolute proper motions from Gaia eDR3.