The Giant Magellan Telescope (GMT) is a gregorian 25.5-meter diameter primary mirror (M1) made of 7 8.4-meter diameter circular aspheric segments. The secondary mirror (M2) is a 1/8th down-scale model of M1 madeof 7 1- meter diameter segments. Each segments has positionners to adjust its 6 rigid body motions. The gureof M1 segment is controlled with 44 bending modes and M2 segments are deformed using 672 actuators. Inthe active and adaptive operation modes of the GMT, around a dozen wavefront sensors (WFS) are selectivelyused to monitor the optical aberrations building-up into the telescope. A dedicated control system feeds backthe WFS measurements to the mirrors actuators to deliver image quality optimized for the eld of view of eachscientic instrument. This paper describes the GMT Dynamic Optical Simulation (DOS) tool. DOS integratesthe optical and mechanical model of the telescope together with the control system. DOS is a cloud-basedoptical propagation software package with build-in models for both geometric and Fourier optics and a fullcontrol algorithm development environment.

We present in this paper preliminary simulation results aimed at validating the GMT piston control strategy. Wewill in particular consider an observing mode in which an Adaptive Optics (AO) system is providing fast on-axisWF correction with the Adaptive Secondary Mirror (ASM), while the phasing system using multiple SegmentPiston Sensors (SPS) makes sure that the seven GMT segments remain phased. Simulations have been performedwith the Dynamic Optical Simulation (DOS) tool developed at the GMT Project Oce, which integrates theoptical and mechanical models of GMT. DOS fast ray-tracing capabilities allows us to properly simulate theeect of eld-dependent aberrations, and in particular, the so-called Field Dependent Segment Piston (FDSP)mode arising when a segment tilt on M1 is compensated on-axis by a segment tilt on M2. We will show thatwhen using an asterism of SPS, our scheme can properly control both segment piston and the FDSP mode.