LSST Active Optics System Software Architecture Sandrine J. Thomas a , Paul Lotz a , Srinivasan Chandrasekharan a , Bo Xin a , Charles Claver a , George Angeli a , Jacques Sebag a , and Gregory P. Dubois-Felsmann b a LSST, 950 N. Cherry Av, Tucson, AZ, USA b Caltech, 1200 E California Blvd, Pasadena, CA 91125, USA ABSTRACT The Large Synoptic Survey Telescope (LSST) is an 8-meter class wide-field telescope now under construction on Cerro Pach´ on, near La Serena, Chile. This ground-based telescope is designed to conduct a decade-long time domain survey of the optical sky. In order to achieve the LSST scientific goals, the telescope requires delivering seeing limited image quality over the 3.5 degree field of view. Like many telescopes, LSST will use an Active Optics System (AOS) to correct in near real-time the system aberrations primarily introduced by gravity and temperature gradients. The LSST AOS uses a combination of 4 curvature wavefront sensors (CWS) located on the outside of the LSST field-of-view. The information coming from the 4 CWS is combined to calculate the appropriate corrections to be sent to the 3 different mirrors composing LSST. The AOS software incorporates a wavefront sensor estimation pipeline (WEP) and an active optics control system (AOCS). The WEP estimates the wavefront residual error from the CWS images. The AOCS determines the correction to be sent to the different degrees of freedom every 30 seconds. In this paper, we describe the design and implementation of the AOS. More particularly, we will focus on the software architecture as well as the AOS interactions with the various subsystems within LSST. Keywords: Large Synoptic Survey Telescope, Active Optics, Wide Field of View, Curvature Sensing 1. INTRODUCTION The Large Synoptic Survey Telescope (LSST) is an 8.4 meter diameter telescope now under construction on Cerro Pachon in Chile. 1, 2 The telescope is composed of three aspheric mirrors: the 8.4m primary mirror (M1), a 3.4m secondary mirror (M2) and a 5m tertiary mirror (M3). The primary and tertiary mirrors form a single monolithic mirror called the M1M3 3, 4 . LSST has one instrument, a three-lens camera that directs the light path onto a 3.2 gigapixel camera, with a field-of-view (FOV) of 3.5 degrees. 5 The camera has 6 different filters, u,g,r,i,z,y. The overall system image quality budget for the LSST is 0.4 arcsec FWHM with 0.25 arcsec allocated to the telescope and 0.3 arcsec associated to the camera. LSST is a seeing-limited telescope and thus these performance do not include the errors coming from the atmosphere. In order to optimize the image quality across the 3.5 degree FOV of the camera, LSST relies on an Active Optics System (AOS). Both M1M3 and M2 are equipped with actuators that allow for the AOS to control the shape of the mirror surface and their position. 6 In addition, M2 and the camera are on hexapods allowing further positioning adjustment. 7 The wavefront control is performed by determining a base set of control parameters based on both finite element analysis (FEA) and on-sky open-loop measurements as well as by adding low temporal frequency closed loop corrections determined from real-time wavefront measurements. The open-loop model may take the form of equations or tables. In this paper, we also refer to this as Look-up Table (LUT). The LUT provides near optimum values for all actuator forces and hexapods positions and depends primarily on elevation angle and temperature. Although constructed using FEA models, it will be verified extensively during the commissioning phase. Further author information: (Send correspondence to S.J.T.) S.J.T.: E-mail: sthomas@lsst.org, Telephone: 1 520 318 8227 Ground-based and Airborne Telescopes VI, edited by Helen J. Hall, Roberto Gilmozzi, Heather K. Marshall, Proc. of SPIE Vol. 9906, 99063B · © 2016 SPIE · CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2231798 Proc. of SPIE Vol. 9906 99063B-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 01/05/2017 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx