Photon Ring Astrometry I: A Simple Spin Measurement Technique for High-Resolution Images of M87*
This work proposes using relative astrometry—measuring shifts between M87's direct image and its first lensed photon-ring sub-image—to constrain black hole spin. By separating these shifts into parallel and transverse components, the study finds that the transverse shift is strongly correlated with spin, offering a method to measure it without needing complex emission models.