The LSST real-time active optics system

Abstract

The Large Synoptic Survey Telescope is an 8.4m telescope now under construction on Cerro Pachon in Chile. This ground-based telescope is designed to conduct a decade-long time domain survey of the optical sky. Achieving the fundamental science goals of the LSST requires high quality imaging that is consistent over its full 3.5 degree Field of view. LSST will use an Active Optics System (AOS) to achieve this goal. Unlike most other systems, the LSST Active Optics correction will combine an open-loop model with a slow (30 s) closed loop feedback. Although it is not fast enough to correct for atmospheric distortions, the AOS will correct in real time the system aberrations mostly introduced by gravity, temperature gradients and other system distortions. The LSST AOS uses a combination of 4 curvature wavefront sensors (CWS) located on the edge of the LSST Field of view. The information coming from the 4 CWSs are combined to calculate the appropriate corrections to be sent to LSST's three mirrors, all equipped with actuators. The AOS software is composed of a Wavefront Estimation Pipeline (WEP) to estimate the error at each of the wavefront sensors and an Active Optics Controller (AOC) to reconstruct the state of the 3-mirrors system and find the optimal correction. In this paper, we describe the design and implementation of the AOS as well as its calibration.