The Giant Magellan Telescope (GMT) consists of seven 8.365 m segments with a separation of 0.345 m. A uniquechallenge for GMT lies in phasing these segments and, in particular, how to measure segment piston optically.In this paper, we present a results of a phasing strategy using a non-redundant pupil mask. We show how thismethod can be extended to measure segment piston differences of more than half a wave by using broadbandlight.
