Lianqi Wang

The Real-Time controller (RTC) for the Narrow Field Infrared Adaptive Optics System (NFIRAOS) for TMT final design

The Real-Time Controller (RTC) for the Thirty Meter Telescope (TMT) Narrow Field Infrared Adaptive Optics System (NFIRAOS) is the software and server hardware that converts wavefront error measurements into wavefront corrector demands, at the heart of the laser guide star multi-conjugate adaptive optics (MCAO) or natural guide star adaptive optics (NGS AO). The RTC takes input from up to six Shack-Hartmann Laser Guide Star wavefront sensors (LGS WFS), one high-order Natural Guide Star Pyramid Wavefront Sensor (PWFS), up to three Shack-Hartmann On-Instrument wavefront sensors (OIWFS) that are located in the client science instruments, and up to 4 on-detector guide windows (ODGW) also in the client instruments. The RTC controls two deformable mirrors conjugated to 0km (DM0) and 11.8km (DM11). DM0 is mounted on a tip/tilt stage (TTS). During the final design phase we performed prototyping to verify that off-the-shelf servers using general purpose CPUs are able to support the maximum 800 Hz frequency at which the RTC is required to operate. We also considered methods to provide live data streams to a graphical user interface without impacting the AO system performance. This paper will discusses the outcome of the impact of jitter and latency on loop speed in our prototype and an overview of the RTC pipeline, including the many “knobs” that can be turned to fine-tune the behavior of NFIRAOS in different observing modes, and under different observing conditions.

Point spread function reconstruction simulations for laser guide star multi-conjugate adaptive optics on extremely large telescopes

This paper discusses point spread function reconstruction (PSFR) simulations for laser guide star (LGS) multi-conjugate adaptive optics (MCAO) on extremely large telescopes (ELTs). The Multithreaded Adaptive Optics Simulator (MAOS), configured to simulate the Thirty Meter Telescope (TMT) Narrow Field InfraRed Adaptive Optics System (NFIRAOS), provided simulated telemetry. For median Mauna Kea turbulence conditions (0.55arcsec seeing) and for the expected NFRIAOS laser guide star (LGS) wavefront sensor (WFS) measurement noise, the actuator error based PSFR algorithm leads to Strehl Ratio (SR), enclosed energy (EE) and PSF profile errors below 8% in Z-band, 5% in J-band and 2% in Kband assuming perfect knowledge of the turbulence and wind profiles and of measurement noise. Further algorithm optimization could in principle reduce this residual error level.

NFIRAOS adaptive optics for the Thirty Meter Telescope

NFIRAOS (Narrow-Field InfraRed Adaptive Optics System) will be the first-light multi-conjugate adaptive optics system for the Thirty Meter Telescope (TMT). NFIRAOS houses all of its opto-mechanical sub-systems within an optics enclosure cooled to precisely -30°C in order to improve sensitivity in the near-infrared. It supports up to three client science instruments, including the first-light InfraRed Imaging Spectrograph (IRIS). Powering NFIRAOS is a Real Time Controller that will process the signals from six laser wavefront sensors, one natural guide star pyramid WFS, up to three low-order on-instrument WFS and up to four guide windows on the client instrument’s science detector in order to correct for atmospheric turbulence, windshake, optical errors and plate-scale distortion. NFIRAOS is currently preparing for its final design review in late June 2018 at NRC Herzberg in Victoria, British Columbia in partnership with Canadian industry and TMT.

Optimizing multi-LGS WFS AO performance in the context of sodium profile evolution and non-common path aberration

For Extremely Large Telescope (ELT) adaptive optics (AO) systems, multiple Sodium Laser Guide Star (LGS) wavefront sensors (WFSs) are required to achieve high sky coverage and diffraction limited performance. However, temporal and spatial variation of the sodium profile causes measurement biases that appear at all time scales and vary between LGS WFSs. To make things worse, optical design residuals, polishing and alignment errors also create non-common-path aberrations (NCPA) that vary between sub-apertures and different WFS, causing LGS WFS to work significantly off null with a nonlinear response. The induced aberrations are consequently non-radially symmetric, even for center launch laser beams with polar coordinate detectors. Natural guide star (NGS) based truth wavefront sensors are often suggested as a method of sensing these LGS WFS aberrations, but a single sensor will suffer strong anisoplanatism that may introduce additional errors. In this paper, we present mitigation strategies and performance estimations based on simulations for the Thirty Meter Telescope (TMT) Narrow Field Infrared AO system (NFIRAOS).

Roof- and Pyramid Wavefront Sensing with Extended Sources

We generalize the standard Fourier optics measurement equations for the roofand pyramid wavefront sensor (RWFS and PWFS) concepts to the case of extended (non-point) sources. Samples of such sources include (i) extended astronomical objects such as moons, (ii) point sources with modulation (i.e., tip/tilt nutation), and (iii) elongated laser guide stars. For simple source profiles, for example top-hat or Gaussian functions, the effect of the extended source on the measurement can be represented analytically as a filter function for each of these sample cases. For the RWFS, we use this representation to evaluate the impact of extended source on the wavefront reconstruction error due to noise, as a function of the number of Zernike modes estimated. A limited set of sample results are also presented for more computationally intensive case of the PWFS. The trends are qualitatively similar to the RWFS, but with a modestly larger sensitivity to noise when reconstructing 50 or more Zernike modes. Finally, the measurement equations developed for extended sources indicated that the RWFS and PWFS wavefront measurements may be biased (i.e., nonzero measurements for a null wavefront) when the source is asymmetrical, but the magnitude of this effect has not been studied yet either analytically or via simulation.

Thirty Meter Telescope Adaptive Optics System Error Budgets and Requirements Traceability

The Thirty Meter Telescope (TMT) uses error budgets to understand and track the expected science parameters of the Observatory. In this paper, we demonstrate how the top-level requirements for wavefront error in both Multi-Conjugate Adaptive Optics (MCAO) and Natural Guide Star Adaptive Optics (NGSAO) modes have been decomposed and allocated between various sources that may cause performance degradation. We also show how those values have been integrated into the requirements for each individual subsystem. By integrating these error budgets into our requirements management process, we are able to maintain traceability between science and design, and understand how changes at a low-level could affect the overall AO performance of the Thirty Meter Telescope.

High fidelity optical modeling for the TMT

The Thirty Meter Telescope (TMT) is a Ritchey-Chritien optical telescope with a 30-meter diameter primary mirror made up of 492 hexagonal segments. Such a large and complex optical system requires detailed modeling of the optical performance during the design phase. An optical modeling computational framework has been developed to support activities related to wavefront & image performance prediction. The model includes effects related to mirror shape sensing & control, mirror alignment & phasing, M1 segment control, low order wavefront correction, adaptive optics simulation for high order wavefront correction, and high contrast imaging. Here we give an overview of this optical simulation framework, the modeling tools and algorithms that are used, and a set of sample analyses. These tools have been used in many aspects of the system design process from mirror specification to instrument & sensor design to algorithm development and beyond.

NFIRAOS —TMT Early Light Adaptive Optics System

NFIRAOS is the early-light facility Adaptive Optics System for the Thirty Meter Telescope. We present the specifications, novel architecture and design of NFIRAOS

Advanced NGS-mode control in NFIRAOS using split-tomography

Controllers based on simple and double integrators are compared to Linear-Quadratic-Gaussian controllers for the Natural-Guide Star loop of NFIRAOS, the 1st light multi-conjugate Adaptive Optics facility for the Thirty Meters Telescope.