Wonders of Active Optics: Images with six-fold symmetry
The video clip shows the effects of six-fold symmetry deformation of the primary mirror.
During the sequence shown in ESO Press Video eso9940, the secondary mirror is first moved in one direction along the optical axis and then back to the "best focus" position, then to the other direction and finally again back to the "best focus" position; this sequence is repeated twice for clarity. As in the case of astigmatism, the lines rotate when one goes from one side of the best focus to the other side. However, the rotation angle is now only 30°, not 90° as in the case of astigmatism.