Active Phasing Experiment (APE) for the ELT

The ELT will have a segmented mirror. For the adaptive optics to work properly and correct the blurring introduced by the atmosphere turbulence, the segments have to be aligned as if their were part of a single large mirror, with a nanometer accuracy (1 millionth of a mm). In order to experiment with the challenges of this alignment, the APE system will be installed on the VLT, mimicking a segmented mirror and its issues. On the image, the 61 segment APE mirror is being tested in Garching.