Wonders of active optics: letters
The ultimate application of this kind of mirror deformations and shifts is the generation of arbitrary patterns . The type of patterns that can be made is restricted by the fact that even a comparatively thin mirror like the VLT main mirror can only be deformed inside certain limits. Within the given range of forces available, deformations can only be generated for geometrical shapes that are comparatively smooth.
The name of the VLT may be written in the sky with images that ressemble the shapes of the letters "V", "L" and T", as generated from a force pattern with no more than eight extreme points at the edge or three lines ("ridges" and/or "valleys") across the mirror surface.
The video 07e/99 shows the real, distorted images, obtained at ANTU and affected by quickly changing atmospheric turbulence.Kredit: