The study of cold or obscured, red astrophysical sources can significantly benefit from adaptive optics (AO) systems employing infrared (IR) wavefront sensors. One particular area is the study of exoplanets around M-dwarf stars and planet formation within protoplanetary disks in star-forming regions. Such objects are faint at visible wavelengths but bright enough in the IR to be used as a natural guide star for the AO system. Doing the wavefront sensing at IR wavelengths enables high-resolution AO correction for such science cases, with the potential to reach the contrasts required for direct imaging of exoplanets. To this end, a new near-infrared pyramid wavefront sensor (PyWFS) has been added to the Keck II AO system, extending the performance of the facility AO system for the study of faint red objects. We present the Keck II PyWFS, which represents a number of firsts, including the first PyWFS installed on a segmented telescope and the first use of an IR PyWFS on a 10-m class telescope. We discuss the scientific and technological advantages offered by IR wavefront sensing and present the design and commissioning of the Keck PyWFS. In particular, we report on the performance of the Selex Avalanche Photodiode for HgCdTe InfraRed Array detector used for the PyWFS and highlight the novelty of this wavefront sensor in terms of the performance for faint red objects and the improvement in contrast. The system has been commissioned for science with the vortex coronagraph in the NIRC2 IR science instrument and is being commissioned alongside a new fiber injection unit for NIRSPEC. We present the first science verification of the system—to facilitate the study of exoplanets around M-type stars.
Electro-Multiplying CCDs offer a unique combination of speed, sub-electron noise and quantum efficiency. These features make them extremely attractive for astronomical adaptive optics. The SOUL project selected the Ocam2k from FLI as camera upgrade for the pyramid wavefront sensor of the LBT SCAO systems. Here we present results from the laboratory characterization of the 3 of the custom Ocam2k cameras for the SOUL project. The cameras showed very good noise (0.4e- and 0.4 - 0.7e- for binned modes) and dark current values (1.5e-). We measured the camera gain and identified the dependency on power cycle and frame rate. Finally, we estimated the impact of these gain variation in the SOUL adaptive optics system. The impact on the SOUL performance resulted to be negligible.
MAORY is one of the approved instruments for the European Extremely Large Telescope. It is an adaptive optics module, enabling high-angular resolution observations in the near infrared by real-time compensation of the wavefront distortions due to atmospheric turbulence and other disturbances such as wind action on the telescope. An overview of the instrument design is given in this paper.
The Natural Guide Star (NGS) Wavefront Sensor (WFS) sub-system of MAORY implements 3 Low-Order and Reference (LOR) WFS needed by the Multi-Conjugate Adaptive Optics (MCAO) system. Each LOR WFS has 2 main purposes: first, to sense the fast low-order modes that are affected by atmospheric anisoplanatism and second, to de-trend the LGS measurements from the slow spatial and temporal drifts of the Sodium layer. These features require to implement 2 different WFS sharing the same NGS and optical breadboard but being respectively a 2×2 Shack-Hartman Sensor (SHS) working at infrared wavelengths and a slow 10×10 SHS at visible bands. The NG WFS sub-system also provides a common support plate for the 3 WFS and their control electronics and cabling. The paper summarizes the status of the preliminary design of the LOR Module on the road to the MAORY Preliminary Design Review (PDR), focusing mainly on the description and analysis of the opto-mechanical arrangement foreseen for the NGS WFS sub-system. Performances and the design trade-offs of the NGS WFS sub-system are analyzed in a complementary paper. First, the requirement imposed by MAORY AO system are discussed. Then the paper gives an overview of the opto-mechanical arrangement for the main components of the sub-system: the support plate, the 3 WFS units and their interfaces to the instrument rotator. In the end the paper discusses the sub-system pointing and WFE budgets derived from different analyses. The design concept for the electronic devices of the sub-system, the cabinet arrangement and the cabling sheme are given in second complementary paper.
A future upgrade of the Keck II telescope’s adaptive optics system will include a near-infrared pyramid wavefront sensor. It will benefit from low-noise infrared detector technology, specifically the avalanche photodiode array SAPHIRA (Leonardo). The system will either operate with a natural guide star in a single conjugated adaptive optics system, or using a laser guide star (LGS), with the pyramid working as a low-order sensor. We present a study of the pyramid sensor’s performance via end-to-end simulations, including an analysis of calibration strategies. For LGS operation, we compare the pyramid to LIFT, a focal-plane sensor dedicated to low-order sensing.
MAORY is the Multi-Conjugate Adaptive Optics module for the European ELT. It will provide a wide-field correction for the first-light instrument MICADO. The Low-Order wavefront modes will be sensed on 3 Natural Guide Stars with Shack-Hartmann Wavefront Sensors, so-called the LO WFS. In the presented work, we focus on the numerical study of the main aspects that depend on the LO WFS design and operational use: low-order sensing performance and sky coverage.
Wavefront sensing in the infrared is highly desirable for the study of M-type stars and cool red objects, as they are sufficiently bright in the infrared to be used as the adaptive optics guide star. This aids in high contrast imaging, particularly for low mass stars where the star-to-planet brightness ratio is reduced. Here we discuss the combination of infrared detector technology with the highly sensitive Pyramid wavefront sensor (WFS) for a new generation of systems. Such sensors can extend the capabilities of current telescopes and meet the requirements for future instruments, such as those proposed for the giant segmented mirror telescopes. Here we introduce the infrared Pyramid WFS and discuss the advantages and challenges of this sensor. We present a new infrared Pyramid WFS for Keck, a key sub-system of the Keck Planet Imager and Characterizer (KPIC). The design, integration and testing is reported on, with a focus on the characterization of the SAPHIRA detector used to provide the H-band wavefront sensing. Initial results demonstrate a required effective read noise <1e– at high gain.
The development of new low-noise infrared detectors, such as RAPID (CEA LETI/Sofradir) or SAPHIRA (Selex), has given the possibility to consider infrared wavefront sensing at low ux. We propose here a comparative study of near infrared (J and H bands) wavefront sensing concepts for mid and high orders estimation on a 8m- class telescope, relying on three existing wavefront sensors: the Shack-Hartmann sensor, the pyramid sensor and the quadri-wave lateral shearing interferometer. We consider several conceptual designs using the RAPID camera, making a trade-off between background flux, optical thickness and compatibility with a compact cryostat integration. We then study their sensitivity to noise in order to compare them in different practical scenarios. The pyramid provides the best performance, with a gain up to ∼ 0.5 magnitude, and has an advantageous setup.
Wavefront sensing in the near infrared has become an attractive option with the advent of new low-noise infrared detectors, such as the SAPHIRA (Selex) and RAPID (CEA/Sofradir) APD arrays. The performance improvements obtained with the H2RG-based Keck I near-infrared tip-tilt sensor is motivating the implementation of a near-infrared low-order sensor for Keck II. The recently proposed focal plane sensor algorithm LIFT could fulfill this role. We show here an analysis of performance, demonstrating that LIFT would provide a significant gain (∼ 1 magnitude) over the current tip/tilt sensor at low flux, as well as the first experimental validation of LIFT on Keck with a calibration source.
We discuss the advantages of wavefront sensing at near-infrared (IR) wavelengths with low-noise detector technologies that have recently become available. In this paper, we consider low order sensing with laser guide star (LGS) adaptive optics (AO) and high order sensing with natural guide star (NGS) AO. We then turn to the application of near-IR sensing with the W. M. Keck Observatory (WMKO) AO systems for science and as a demonstrator for similar systems on extremely large telescopes (ELTs). These demonstrations are based upon an LGS AO near-IR tip-tilt-focus sensor and our collaboration to implement a near-IR pyramid wavefront sensor (PWFS) for a NGS AO L-band coronagraphic imaging survey to identify exoplanet candidates.
Laser assisted adaptive optics systems rely on Laser Guide Star (LGS) Wave-Front Sensors (WFS) for high order aberration measurements, and rely on Natural Guide Stars (NGS) WFS to complement the measurements on low orders such as tip-tilt and focus. The sky-coverage of the whole system is therefore related to the limiting magnitude of the NGS WFS. We have recently proposed LIFT, a novel phase retrieval WFS technique, that allows a 1 magnitude gain over the usually used 2×2 Shack-Hartmann WFS. After an in-lab validation, LIFT’s concept has been demonstrated on sky in open loop on GeMS (the Gemini Multiconjugate adaptive optics System at Gemini South). To complete its validation, LIFT now needs to be operated in closed loop in a laser assisted adaptive optics system. The present work gives a detailed analysis of LIFT’s behavior in presence of high order residuals and how to limit aliasing effects on the tip/tilt/focus estimation. Also, we study the high orders’ impact on noise propagation. For this purpose, we simulate a multiconjugate adaptive optics loop representative of a GeMS-like 5 LGS configuration. The residual high orders are derived from a Fourier based simulation. We demonstrate that LIFT keeps a high performance gain over the Shack-Hartmann 2×2 whatever the turbulence conditions. Finally, we show the first simulation of a closed loop with LIFT estimating turbulent tip/tilt and focus residuals that could be induced by sodium layer’s altitude variations.
Laser Tomographic systems, such as ATLAS, will rely on natural guide stars to sense low order aberrations.
LIFT is a novel focal plane wavefront sensor (WFS), performing a maximum likelihood phase retrieval on a
single image, with better sensitivity than a 2x2 Hartmann-Shack WFS. We first present a characterization of
LIFT’s noise propagation performance and working domain by means of simulations. We then show the results
of experiments on ONERA’s test bench for LIFT. These experiments validate the estimation of tip/tilt and
focus, in monochromatic light and in large bandwidth (for a spectral resolution of 3), as well as the expected
noise propagation. They also confirm the validity of the imaging models used for simulations. Finally, we focus
on the preparation of an on-sky validation based on Gemini Multi-conjugate adaptive optics System (GeMS)
calibration data at Gemini Southern Observatory.