The Infrared Imaging Spectrograph (IRIS) for TMT: closed-loop adaptive optics while dithering

The InfraRed Imaging Spectrograph (IRIS) is the first-lightclient instrument for the Narrow Field Infrared Adaptive OpticsSystem (NFIRAOS) on the Thirty Meter Telescope(TMT). IRIS includes three natural guide star(NGS) On-Instrument Wavefront Sensors(OIWFS) to measure tip/tilt and focus errors in the instrument focal plane. NFIRAOS also has an internal natural guide star wavefront sensor, and IRIS and NFIRAOS must precisely coordinate the motions of their wavefront sensorpositioners to track the locations of NGSs while the telescopeis dithering(offsetting the telescopeto cover more area), to avoid a costly re-acquisition time penalty. First, we present an overview of the sequencing strategy for all of the involved subsystems. We then predict the motion of the telescopeduring dithersbased on finite-element models provided by TMT, and finally analyze latency and jitter issues affecting the propagation of position demands from the telescopecontrol system to individual motor controllers.