Bo Xin

Active optics in Large Synoptic Survey Telescope

The Large Synoptic Survey Telescope (LSST) has a 3.5º field of view and F/1.2 focus that makes the performance quite sensitive to the perturbations of misalignments and mirror surface deformations. In order to maintain the image quality, LSST has an active optics system (AOS) to measure and correct those perturbations in a closed loop. The perturbed wavefront errors are measured by the wavefront sensors (WFS) located at the four corners of the focal plane. The perturbations are solved by the non-linear least square algorithm by minimizing the rms variation of the measured and baseline designed wavefront errors. Then the correction is realized by applying the inverse of the perturbations to the optical system. In this paper, we will describe the correction processing in the LSST AOS. We also will discuss the application of the algorithm, the properties of the sensitivity matrix and the stabilities of the correction. A simulation model, using ZEMAX as a ray tracing engine and MATLAB as an analysis platform, is set up to simulate the testing and correction loop of the LSST AOS. Several simulation examples and results are presented.

Overview of the LSST active optics system

The LSST will utilize an Active Optics System to optimize the image quality by controlling the surface figures of the mirrors (M1M3 and M2) and maintain the relative position of the three optical systems (M1M3 mirror, M2 mirror and the camera). The mirror surfaces are adjusted by means of figure control actuators that support the mirrors. The relative rigid body positions of M1M3, M2 and the camera are controlled through hexapods that support the M2 mirror cell assembly and the camera. The Active Optics System (AOS) is principally operated off of a Look-Up Table (LUT) with corrections provided by wave front sensors.

Manufacture and final tests of the LSST monolithic primary/tertiary mirror

The LSST M1/M3 combines an 8.4 m primary mirror and a 5.1 m tertiary mirror on one glass substrate. The combined mirror was completed at the Richard F. Caris Mirror Lab at the University of Arizona in October 2014. Interferometric measurements show that both mirrors have surface accuracy better than 20 nm rms over their clear apertures, in nearsimultaneous tests, and that both mirrors meet their stringent structure function specifications. Acceptance tests showed that the radii of curvature, conic constants, and alignment of the 2 optical axes are within the specified tolerances. The mirror figures are obtained by combining the lab measurements with a model of the telescope’s active optics system that uses the 156 support actuators to bend the glass substrate. This correction affects both mirror surfaces simultaneously. We showed that both mirrors have excellent figures and meet their specifications with a single bending of the substrate and correction forces that are well within the allowed magnitude. The interferometers do not resolve some small surface features with high slope errors. We used a new instrument based on deflectometry to measure many of these features with sub-millimeter spatial resolution, and nanometer accuracy for small features, over 12.5 cm apertures. Mirror Lab and LSST staff created synthetic models of both mirrors by combining the interferometric maps and the small highresolution maps, and used these to show the impact of the small features on images is acceptably small.

An integrated modeling framework for the Large Synoptic Survey Telescope (LSST)

All of the components of the LSST subsystems (Telescope and Site, Camera, and Data Management) are in production. The major systems engineering challenges in this early construction phase are establishing the final technical details of the observatory, and properly evaluating potential deviations from requirements due to financial or technical constraints emerging from the detailed design and manufacturing process. To meet these challenges, the LSST Project Systems Engineering team established an Integrated Modeling (IM) framework including (i) a high fidelity optical model of the observatory, (ii) an atmospheric aberration model, and (ii) perturbation interfaces capable of accounting for quasi static and dynamic variations of the optical train. The model supports the evaluation of three key LSST Measures of Performance: image quality, ellipticity, and their impact on image depth. The various feedback loops improving image quality are also included. The paper shows application examples, as an update to the estimated performance of the Active Optics System, the determination of deployment parameters for the wavefront sensors, the optical evaluation of the final M1M3 surface quality, and the feasibility of satisfying the settling time requirement for the telescope structure.

Real time wavefront control system for the Large Synoptic Survey Telescope (LSST)

The LSST is an integrated, ground based survey system designed to conduct a decade-long time domain survey of the optical sky. It consists of an 8-meter class wide-field telescope, a 3.2 Gpixel camera, and an automated data processing system. In order to realize the scientific potential of the LSST, its optical system has to provide excellent and consistent image quality across the entire 3.5 degree Field of View. The purpose of the Active Optics System (AOS) is to optimize the image quality by controlling the surface figures of the telescope mirrors and maintaining the relative positions of the optical elements. The basic challenge of the wavefront sensor feedback loop for an LSST type 3-mirror telescope is the near degeneracy of the influence function linking optical degrees of freedom to the measured wavefront errors. Our approach to mitigate this problem is modal control, where a limited number of modes (combinations of optical degrees of freedom) are operated at the sampling rate of the wavefront sensing, while the control bandwidth for the barely observable modes is significantly lower. The paper presents a control strategy based on linear approximations to the system, and the verification of this strategy against system requirements by simulations using more complete, non-linear models for LSST optics and the curvature wavefront sensors.

Prototype pipeline for LSST wavefront sensing and reconstruction

The Large Synoptic Survey Telescope (LSST) uses an Active Optics System (AOS) to maintain system alignment and surface figure on its three large mirrors. Corrective actions fed to the LSST AOS are determined from 4 curvature based wavefront sensors located on the corners of the inscribed square within the 3.5 degree field of view. Each wavefront sensor is a split detector such that the halves are 1mm on either side of focus. In this paper we describe the development of the Active Optics Pipeline prototype that simulates processing the raw image data from the wavefront sensors through to wavefront estimation on to the active optics corrective actions. We also describe various wavefront estimation algorithms under development for the LSST active optics system. The algorithms proposed are comprised of the Zernike compensation routine which improve the accuracy of the wavefront estimate. Algorithm development has been aided by a bench top optical simulator which we also describe. The current software prototype combines MATLAB modules for image processing, tomographic reconstruction, atmospheric turbulence and Zemax for optical ray-tracing to simulate the closed loop behavior of the LSST AOS. We describe the overall simulation model and results for image processing using simulated images and initial results of the wavefront estimation algorithms.

LSST primary/tertiary monolithic mirror

At the core of the Large Synoptic Survey Telescope (LSST) three-mirror optical design is the primary/tertiary (M1M3) mirror that combines these two large mirrors onto one monolithic substrate. The M1M3 mirror was spin cast and polished at the Steward Observatory Mirror Lab at The University of Arizona (formerly SOML, now the Richard F. Caris Mirror Lab at the University of Arizona (RFCML)). Final acceptance of the mirror occurred during the year 2015 and the mirror is now in storage while the mirror cell assembly is being fabricated. The M1M3 mirror will be tested at RFCML after integration with its mirror cell before being shipped to Chile.

The LSST camera corner raft conceptual design: a front-end for guiding and wavefront sensing

The Large Synoptic Survey Telescope (LSST) is a proposed ground based telescope that will perform a comprehensive astronomical survey by imaging the entire visible sky in a continuous series of short exposures. Four special purpose rafts, mounted at the corners of the LSST science camera, contain wavefront sensors and guide sensors. Wavefront measurements are accomplished using curvature sensing, in which the spatial intensity distribution of stars is measured at equal distances on either side of focus by CCD detectors. The four Corner Rafts also each hold two guide sensors. The guide sensors monitor the locations of bright stars to provide feedback that controls and maintains the tracking of the telescope during an exposure. The baseline sensor for the guider is a Hybrid Visible Silicon hybrid-CMOS detector. We present here a conceptual mechanical and electrical design for the LSST Corner Rafts that meets the requirements imposed by the camera structure, and the precision of both the wavefront reconstruction and the tracking. We find that a single design can accommodate two guide sensors and one split-plane wavefront sensor integrated into the four corner locations in the camera.

An overview of the LSST system integration and commission plan (Conference Presentation)

The Commissioning Phase of the LSST Project is the final stage in the combined NSF and DOE funded LSST construction project. The LSST commission phase is planned to start early in 2020 and be completed near the end of 2022, ending with the LSST Observatory system ready to start survey operations. Commissioning includes the assembly of the three principal subsystems (Telescope, Camera and Data Management) into the LSST Observatory System and the integration and test (AIT Phase-1) Early System AIT Phase-2) Full System AIT and Phase-3) Science Validation where a series of mini-surveys are used to characterize the system with respect to the survey performance specifications in the SRD/LSR and functionality of the, leading to operations readiness. The Science Validation Phase concludes with an Operations Readiness Review (ORR). The LSST System Assembly, Integration and Test and Commissioning effort has been planned out over several phases The first phase of commissioning under Early AIT the scheduler will be exercised and all safety checks verified for autonomous operation; and early DM algorithm testing will be performed with on-sky data from ComCam using a commissioning computing cluster at the Base Facility. The second phase of activities under Full System AI&T is designed to complete the technical integration of the three principal subsystems and EPO, show full compliance with system level requirements as detailed in the Observatory System Specifications and system level interface control documents, and provide full scale data for further DM/EPO software and algorithmic testing and development. System level requirements that flow directly to subsystems without any further derivation will be tested for compliance, at the subsystem level and below, under the supervision of Project Systems Engineering. This document includes the general approach and goals for these tests. It is expected that roughly four (4) months into the Full System AI&T phase the telescope and camera will be fully integrated and routinely producing science grade images over the full field of view (FOV), at which point “System First Light” will be declared. Following System First Light will be an intensive data acquisition period design to test the image processing pipelines and validate the derived science products that are to be delivered by the LSST survey. The third and final phase of activities under Science Validation is designed to fully characterize the system performance specifications detailed in LSST System Requirements Document and the range of demonstrated performance per the LSST Science Requirements. These activities are based on the measured “On-Sky” performance and informed simulations of the LSST system. In this paper we describe the inputs and assumptions to the commissioning plan, a summary of the activities in each phase, management strategies and expected outcomes.

Dome seeing sensitivity analysis for LSST

High spatial resolution thermal unsteady CFD simulations of LSST are performed and processed to provide image degradation due to dome seeing in FWHM. An analysis of the sensitivity of the image quality to certain important geometric features and aerothermal properties is presented. More specifically, the influence of the LSST vent light baffles and windscreen, the wind speed and the surface temperature of components such as the primary and secondary mirrors, the camera, the telescope structure and dome exterior is assessed and conclusions are drawn. The secondary mirror and camera surface temperatures are found to be among the most critical in minimizing LSST dome seeing.