Olivier Lardière

Testing the pyramid truth wavefront sensor for NFIRAOS in the lab

For today and future adaptive optics observations, sodium laser guide stars (LGSs) are crucial; however, the LGS elongation problem due to the sodium layer has to be compensated, in particular for extremely large telescopes. In this paper, we describe the concept of truth wavefront sensing as a solution and present its design using a pyramid wavefront sensor (PWFS) to improve NFIRAOS (Narrow Field InfraRed Adaptive Optics System), the first light adaptive optics system for Thirty Meter Telescope. We simulate and test the truth wavefront sensor function under a controlled environment using the HeNOS (Herzberg NFIRAOS Optical Simulator) bench, a scaled-down NFIRAOS bench at NRC-Herzberg. We also touch on alternative pyramid component options because despite recent high demands for PWFSs, we suffer from the lack of pyramid supplies due to engineering difficulties.

Fast modulation and dithering on a pyramid wavefront sensor bench

A pyramid wavefront sensor (PWFS) bench has been setup at NRC-Herzberg (Victoria, Canada) to investigate, first, the feasibility of a double roof prism PWFS, and second, test the proposed pyramid wavefront sensing methodology to be used in NFIRAOS for the Thirty Meter Telescope. Traditional PWFS require shallow angles and strict apex tolerances, making them difficult to manufacture. Roof prisms, on the other hand, are common optical components and can easily be made to the desired specifications. Understanding the differences between a double roof prism PWFS and traditional PWFS will allow for the double roof prism PWFS to become more widely used as an alternative to the standard pyramid, especially in a laboratory setting. In this work, the response of the double roof prism PWFS as the amount of modulation is changed, is compared to an ideal PWFS modelled using the adaptive optics toolbox, OOMAO in MATLAB. The object oriented toolbox uses physical optics to model complete AO systems. Fast modulation and dithering using a PI mirror has been implemented using a micro-controller to drive the mirror and trigger the camera. The various trade offs of this scheme, in a controlled laboratory environment, are studied and reported.

Upgrading the MMT AO system with a near-infrared Pyramid wavefront sensor

There are long existing limitations of the sky coverage of astronomical Adaptive Optics (AO) systems that use natural guide stars (NGSs) as reference sources. In this work, we present numerical simulations and lab test results of an optical NGS pyramid wavefront sensor (PWFS) for the MMT AO system. The potential increase of sky coverage benefits from the gain in sensitivity of the PWFS in a closed-loop NIR AO system compared with the optical Shack-Hartmann wavefront sensor (SHWFS). The upgraded MMT AO WFS system will use IR avalanche photodiode (APD) array with extremely low readout noise (at sub-electron level), run at a high frame rate (over 1kHz), and cover the wavelength range from 0.85-1.8 μm. This upgraded system will access a larger portion of the sky by looking at fainter, redder reference stars. We use ”yao” simulation to show the expected limiting magnitude gain of NIR PWFS compared with the existing optical SHWFS. The sky coverage will increase by 11 times at the Galactic plane and by 6 times at the North Galactic Pole when compared to traditional optical WFSs. This novel WFS will also enable observations of the dust obscured plane of the Galaxy, where the optical light of most stars is more extincted. We demonstrate the basic lab test with a set of double roof prisms. We evaluate the overall performance of the PWFS on our lab AO bench, present captured micro-pupil images and do wavefront reconstruction. We will upgrade to SAPHIRA and pyramid prism for later lab test. We plan to implement this system at MMT and carry out on-sky tests in Spring 2019.

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.

Calibration and test procedures for the NFIRAOS deformable mirror prototypes

A test setup and detailed plan for safe characterization of prototype deformable mirrors (DMs) for the Thirty Meter Telescope’ s Narrow Field Infrared Adaptive Optics System (NFIRAOS) are presented. The DM size and performance requirements for NFIRAOS are such that prototypes must be built and tested before commissioning the final deliverables in order to mitigate risk. There are two prototypes under test; the actuators have been constructed with the pitch, size and stroke range specified for the full scale DMs, and on the order of 15% of the total number of actuators required by DM0, the ground-conjugated DM. The diameters of the active areas of the prototypes are approximately 35% of the full DM0 diameter. The performance in terms of stroke, linearity, hysteresis and overall controllability must meet requirements at room temperature and at -30 degrees Celsius. NRC HAA has implemented a test setup to characterize the performance of the DM prototypes in this thermal environment. A testing procedure has also been developed to verify the technology up to its limits, while protecting from damage. A primary risk of damage comes from excessive inter-actuator stroke which must be carefully controlled, particularly in the case of non-linear and hysteretic actuators. A detailed calibration procedure and actuator protection scheme has been developed.

Double-Pyramid Wavefront Sensors: Tolerance Relaxation and Cheaper Alternatives using Achromatic Double-Roof Prisms

We present the current design of the double-pyramid planned for the TMT-NFIRAOS Visible Natural Guide Star WFS and its manufacture challenges to meet all the requirements in terms of tip size, pupil image quality and mapping on the detector pixels. We show that the angular tolerance of the pyramid can be relaxed from ±6 arcsec. to ±2 arcmin. without significant impact on the AO performance, which is within the capabilities of most optical suppliers and reduces the fabrication cost by a factor 10 or more. Then we compare the pyramid design to a new concept using two double-roof prisms, optically equivalent to an achromatic double-pyramid and offering very similar performance. Roof prisms are easier to manufacture to tolerance than pyramids since there is no tip. We also present an adjustable version of the achromatic double-roof prism allowing very fine adjustments of the positions of the four pupils onto the detector pixels, which can relax the manufacture tolerance of the prisms even more.

Current Status and Implementation of Pyramid Truth Wavefront Sensor on NFIRAOS Simulation Bench at NRC-Herzberg

We describe the components and functions and report the recent upgrades/modifications of the HeNOS (Herzberg NFIRAOS Optical Simulator) bench, a NFIRAOS (Narrow Field InfraRed Adaptive Optics System) simulator bench at NRC-Herzberg. The recent upgrade includes the new natural guide star simulator, the new science camera, and the implementation of a truth wavefront sensor using a pyramid. The truth wavefront sensing simulation involves the reproduction of Shack-Hartmann wavefront sensor elongation due to the sodium layer profile, which we simulate with a set of defocus on the ground layer deformable mirror. This is the first truth wavefront sensor made with a pyramid in use today.

Optomechanical design of TMT NFIRAOS Subsystems at INO

The adaptive optics system for the Thirty Meter Telescope (TMT) is the Narrow-Field InfraRed Adaptive Optics System (NFIRAOS). Recently, INO has been involved in the optomechanical design of several subsystems of NFIRAOS, including the Instrument Selection Mirror (ISM), the NFIRAOS Beamsplitters (NBS), and the NFIRAOS Source Simulator system (NSS) comprising the Focal Plane Mask (FPM), the Laser Guide Star (LGS) sources, and the Natural Guide Star (NGS) sources. This paper presents an overview of these subsystems and the optomechanical design approaches used to meet the optical performance requirements under environmental constraints.

Optical design of infrared pyramid wavefront sensor for the MMT

We report the optical design of an infrared (0.85-1.8 μm) pyramid wavefront sensor (IRPWFS) that is designed for the 6.5m MMT on telescope adaptive optics system using the latest developments in low-noise infrared avalanche photodiode arrays. The comparison between the pyramid and the double-roof prism based wavefront sensors and the evaluation of their micro pupils’ quality are presented. According to our analysis, the use of two double-roof prisms with achromatic materials produces the competitive performance when compared to the traditional pyramid prism, which is difficult to manufacture. The final micro pupils on the image plane have the residual errors of pupil position, chromatism, and distortion within 1/10 pixel over the 2×2 arcsecond field of view, which meet the original design goals.