Wonders of active optics: rotating crosses

In the video, cross-like stellar images with a four-fold symmetry are generated by introducing a deformation of the primary mirror with four "high" and four "low" points at the mirror periphery (i.e., 45° apart, cf. ESO Press Photo eso9940e) and defocussing by moving the secondary mirror towards or away from the primary mirror. Then, maintaining the position of the secondary mirror, the deformation of the primary mirror is "rotated" by changing the support forces so that the "high" and "low" points shift along the mirror periphery in steps of approximately 20°. As a consequence, the cross-like stellar images with their four-fold symmetry now rotate around their centres by 20° with each step. This spectacular effect, where the images all rotate around their individual centres, cannot be achieved by rotating the light detector in the focal plane, since all stellar images would then rotate around one common centre in the field.