Figure 4: Principle of 27-beam phase analyzer for adaptive phasing with the triplet algorithm.

1. Converging beams are received from the segmented recombining mirror RM, equipped with phasing actuators PA. Beam splitters BS1 and BS2 extract groups of sub-pupils to generate partial images, needed for the phase analysis algorithm. There are three beam-splitting elements such as BS2, having a flat and partially reflective upper face, and an anti-reflective lower face with three facets which direct the beams towards three separate images. An additional layer of faceted beam splitters, not shown, can be added to provide 27 single images, serving for corrections of tip-tilt and focus.
   
2. The full recombined image is surrounded by 9 triple images and 3 ninetuple images. In each case, image sizes are optimized for pixel matching.
   
3. For a simple phasing response, the arrangement of actuators PA carrying the elements of recombining mirror RM follows the groups' hierachy. The full set of images, dispersed by spectro-imaging attachment SI, is received on a single photon-counting camera PC.