APE segment pattern recognition in new phasing techniques

The aperture of future Extremely Large Telescopes will be composed of hundreds of individual segments which require the development of new robust phasing techniques based on the concept of pupil plane detection. The misalignments of the segments produce amplitude variations at the location of the segment edges recorded on the phasing camera. To analyze the signals which contain the information about the segmentation error, the position of the segment borders on a CCD image must be determined with a sub-pixel accuracy. In the framework of the Active Phasing Experiment (APE) carried out at ESO, we have developed two methods to retrieve the segmented pattern. One is based on the Hough transform and the other one on the correlation of the images with a hexagonal pattern. After a description of both methods, we shall present the results achieved so far with simulations. Finally, the performances of the two methods will be compared. This project forms part of the ELT Design Study and is supported by the European Commission, within Framework Programme 6, contract No 011863.