Piston-tip-tilt mirror array in the wide field phasing testbed for the Giant Magellan Telescope

The Wide Field Phasing Testbed (WFPT) will be used to test phasing and active optics systems planned for the doubly segmented Giant Magellan Telescope (GMT). The testbed consists of a set of optical relays including segmented and deformable mirrors (DMs) that represent the GMT primary and secondary. Displacements, tilts, and clocking of the GMT M1 and M2 segments generate discontinuous wavefront errors that cannot be accurately reproduced by only continuous-surface deformable mirrors. Therefore, two segmented Piston-Tip-Tilt (PTT) mirror arrays placed at the M1 and M2 conjugate positions augment the DMs to reproduce these wavefronts in the WFPT. They must have large stroke (≥10 µm piston) with high temporal stability (⪅20 nm) to avoid drifts from corrupting the sequential AGWS measurements. They must also have very narrow gaps between the mirror clear apertures to mimic the GMT pupil geometry. The pupil size at the PTT array was scaled to fit hexagonal 17mm mirror segments, producing a pupil of approximately 50.25 mm in diameter. Each of the seven segment assemblies consists of a custom segment base component, to which a set of three piezo actuators are epoxied. We selected lead zirconate titanate (PZT) discrete stack actuators which incorporate strain gauges to allow for closed-loop operation, thus eliminating the hysteresis and creep effects of the actuator. A two-axis flexure is bonded to each piezo, opposite the bonded base. The hexagonal mirror is bonded to the three flexures. The assembly, testing and integration challenges of two arrays completed in June 2022 is discussed.