VLT Active Optics
Situated in the Atacama Desert, astronomers at the Very Large Telescope in Chile experience great seeing conditions. In order to make the most of them, the telescopes have to perform the best they can, working around various sources of error. Observations are extremely sensitive to the shape of the mirrors- the VLT sees in infrared and visible light, which has a wavelength of a few hundred nanometers, and as a rule the mirror would have to be precise to 1/20th of this. The issues with the Hubble Space Telescope when it was first launched were due to the mirror being too flat at the edges by about 2.2 microns- that’s 4 times the wavelength of visible light!
The solution is to use active optics. The 8.2m primary mirror has 150 separate supports, which can all be moved individually to subtly change the shape of the mirror, and the secondary mirror can be tilted. A reference star is observed within the field of view of the telescope, and adjustments are made accordingly, allowing objects to become sharper (see top gif). Smaller but faster corrections can be made using adaptive optics to allow for atmospheric disturbance.
With a few calculations to make a specific shape of mirror, the VLT can even spell out its name using distorted light from a single star!
Information source & further reading here. Gifs made by me from this video, credit ESO.