Three instances where beam walk could occur, causing stars at slightly different sky angles to illuminate different parts of optical elements. Top: shear introduced at the telescope by focusing and recollimating the beam. "FSM" stands for the "Fast Steering Mirror," which provided tip–tilt corrections (first-order adaptive optics) and recollimated the light after the telescope. Second from top: shear within a collimated beam over large optical paths. Second from bottom: shear at focus of delay line optics (DL; the movable mirrors that provide optical delays). Bottom: the shear of two beams by amount Δ causing only partial overlap.