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

TL;DR

This paper discusses the coordination of wavefront sensors in the IRIS instrument for TMT, focusing on maintaining precise alignment during telescope dithering through sequencing strategies and motion prediction.

Contribution

It introduces a sequencing strategy and analyzes latency and jitter issues for wavefront sensor coordination during dithering in the IRIS instrument.

Findings

Predicted telescope motion during dithers using finite-element models.

Identified latency and jitter challenges in sensor positioner coordination.

Proposed strategies to mitigate re-acquisition time penalties.

Abstract

The InfraRed Imaging Spectrograph (IRIS) is the first-light client instrument for the Narrow Field Infrared Adaptive Optics System (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 sensor positioners to track the locations of NGSs while the telescope is dithering (offsetting the telescope to 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 telescope during dithers based on finite-element models provided by TMT, and finally analyze latency and jitter issues affecting the propagation of position demands from the telescope control system to individual motor controllers.