A convenient telescope performance metric for imaging through turbulence

This paper provides an overview of the various image quality metrics used in astronomical imaging and explains in details a new metric, the Normalized Point Source Sensitivity. It is based on the Equivalent Noise Area concept, an extension of the EE80% metric and is intuitively linked to the required science integration time. As it was proved in recent studies, the PSSN metric properly accounts for image degradation due to the spatial frequency content of a given telescope aberration and the effects of various errors can be multiplicatively combined, like those expressed in Central Intensity Ratio. Extensions of the metric for off-axis imaging and throughput degradation are presented. Wavelength and spatial frequency dependence of PSSN are discussed. While the proper calculation of the PSSN metric requires the precise knowledge of the PSF of both the optics and atmosphere, there is a straightforward approximation linking PSSN to the Zernike decomposition of the OPD. Besides the summary of various aspects of the Point Source Sensitivity, the paper provides many numerical examples derived for the Thirty Meter Telescope

Statistical approach to systems engineering for the Thirty Meter Telescope

Core components of systems engineering are the proper understanding of the top level system requirements, their allocation to the subsystems, and then the verification of the system built against these requirements. System performance, ultimately relevant to all three of these components, is inherently a statistical variable, depending on random processes influencing even the otherwise deterministic components of performance, through their input conditions. The paper outlines the Stochastic Framework facilitating both the definition and estimate of system performance in a consistent way. The environmental constraints at the site of the observatory are significant design drivers and can be derived from the Stochastic Framework, as well. The paper explains the control architecture capable of achieving the overall system performance as well as its allocation to subsystems. An accounting for the error and disturbance sources, as well as their dependence on environmental and operational parameters is included. The most current simulations results validating the architecture and providing early verification of the preliminary TMT design are also summarized

Wind loads on ground-based telescopes

One of the factors that can influence the performance of large optical telescopes is the vibration of the telescope structure due to unsteady wind inside the telescope enclosure. Estimating the resulting degradation in image quality has been difficult because of the relatively poor understanding of the flow characteristics. Significant progress has recently been made, informed by measurements in existing observatories, wind-tunnel tests, and computational fluid dynamic analyses. We combine the information from these sources to summarize the relevant wind characteristics and enable a model of the dynamic wind loads on a telescope structure within an enclosure. The amplitude, temporal spectrum, and spatial distribution of wind disturbances are defined as a function of relevant design parameters, providing a significant improvement in our understanding of an important design issue

Curvature Wavefront Sensing for the Large Synoptic Survey Telescope

The Large Synoptic Survey Telescope (LSST) will use an active optics system (AOS) to maintain alignment and surface figure on its three large mirrors. Corrective actions fed to the LSST AOS are determined from information derived from 4 curvature wavefront sensors located at the corners of the focal plane. Each wavefront sensor is a split detector such that the halves are 1mm on either side of focus. In this paper we describe the extensions to published curvature wavefront sensing algorithms needed to address challenges presented by the LSST, namely the large central obscuration, the fast f/1.23 beam, off-axis pupil distortions, and vignetting at the sensor locations. We also describe corrections needed for the split sensors and the effects from the angular separation of different stars providing the intra- and extra-focal images. Lastly, we present simulations that demonstrate convergence, linearity, and negligible noise when compared to atmospheric effects when the algorithm extensions are applied to the LSST optical system. The algorithm extensions reported here are generic and can easily be adapted to other wide-field optical systems including similar telescopes with large central obscuration and off-axis curvature sensing.Comment: 26 pages, 10 figure

Thermal performance prediction of the TMT optics

Thermal analysis for the Thirty Meter Telescope (TMT) optics (the primary mirror segment, the secondary mirror, and the tertiary mirror) was performed using finite element analysis in ANSYS and I-DEAS. In the thermal analysis, each of the optical assemblies (mirror, mirror supports, cell) was modeled for various thermal conditions including air convections, conductions, heat flux loadings, and radiations. The thermal time constant of each mirror was estimated and the temperature distributions of the mirror assemblies were calculated under the various thermal loading conditions. The thermo-elastic analysis was made to obtain the thermal deformation based on the resulting temperature distributions. The optical performance of the TMT optics was evaluated from the thermally induced mirror deformations. The goal of this thermal analysis is to establish thermal models by the FEA programs to simulate for an adequate thermal environment. These thermal models can be utilized for estimating the thermal responses of the TMT optics. In order to demonstrate the thermal responses, various sample time-dependent thermal loadings were modeled to synthesize the operational environment. Thermal responses of the optics were discussed and the optical consequences were evaluated

Thermal modeling of the TMT Telescope

Thermal modeling of the Thirty Meter Telescope (TMT) was conducted for evaluations of thermal performances by finite element (FE) and optical analysis tools. The thermal FE models consist of the telescope optical assembly systems, instruments, laser facility, control and electronic equipments, and telescope structural members. A three-consecutive-day thermal environment data was implemented for the thermal boundary created by Computational Fluid Dynamics (CFD) based on the environment conditions of the TMT site. Temporal and spatial temperature distributions of the optical assembly systems and the telescope structure were calculated under the environmental thermal conditions including air convections, conductions, heat flux loadings, and radiations. With the calculated temperature distributions, the thermo-elastic analysis was performed to predict thermal deformations of the telescope structure and the optical systems. The line of sight calculation was made using the thermally induced deformations of the optics and structures. Merit function routines (MFR) were utilized to calculate the Optical Path Difference (OPD) maps after repositioning the optics based on a best fit of M1 segment deformations. The goal of this thermal modeling is to integrate the mechanical and optical deformations in order to simulate the thermal effects with the TMT site environment data from CFD

Dynamic analysis of TMT

Dynamic disturbance sources affecting the optical performance of the Thirty Meter Telescope (TMT) include unsteady wind forces inside the observatory enclosure acting directly on the telescope structure, unsteady wind forces acting on the enclosure itself and transmitted through the soil and pier to the telescope, equipment vibration either on the telescope itself (e.g. cooling of instruments) or transmitted through the soil and pier, and potentially acoustic forces. We estimate the characteristics of these disturbance sources using modeling anchored through data from existing observatories. Propagation of forces on the enclosure or in support buildings through the soil and pier to the telescope base are modeled separately, resulting in force estimates at the telescope pier. We analyze the resulting optical consequences using integrated modeling that includes the telescope structural dynamics, control systems, and a linear optical model. The dynamic performance is given as a probability distribution that includes the variation of the external wind speed and observing orientation with respect to the wind, which can then be combined with dome seeing and other time- or orientation-dependent components of the overall error budget. The modeling predicts acceptable dynamic performance of TMT

Active optics challenges of a thirty-meter segmented mirror telescopy

Ground-based telescopes operate in a turbulent atmosphere that affects the optical path across the aperture by changing both the mirror positions (wind seeing) and the air refraction index in the light path (atmospheric seeing). In wide field observations, when adaptive optics is not feasible, active optics are the only means of minimizing the effects of wind buffeting. An integrated, dynamic model of wind buffeting, telescope structure, and optical performance was devleoped to investigate wind energy propagation into primary mirror modes and secondary mirror rigid body motion.Although the rsults showed that the current level of wind modeling was not appropriate to decisively settle the need for optical feedback loops in active optics, the simulations strongly indicated the capability of a limited bandwidth edge sensor loop to maintain the continuity of the primary mirror inside the preliminary error budget. It was also found that the largest contributor to the wind seeing is image jitter, i.e. OPD tip/tilt

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