Which range of magnitudes for layer oriented MCAO?

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.

Laser guide stars (LGSs) have been deployed for the last 20-30 years in ground-based astronomical telescopes to overcome the limited sky coverage of classical adaptive optics (AO) systems. Unfortunately, slow altitude drifts of the sodium layer compromise focus measurements, generating the so-called slow focus error, and, consequently, a natural guide star (NGS) is needed to compensate that error. The Keck I telescope AO system uses a 20x20 Shack-Hartmann (SH) wavefront sensor (WFS) for slow focus tracking (with a 5x5 mode used on fainter stars). This approach is far from optimal due to limited sky coverage, since the available NGSs are usually very faint. Our goal is to develop a different technique for slow focus tracking and make it fully operational using focal plane wavefront sensing (FPWFS), which can significantly increase sky coverage and allow slow focus tracking at higher frequencies, reducing the lag error. The Keck I near-infrared (NIR) tip-tilt sensor, known as TRICK, is used to obtain the focal plane images without any hardware modifications being necessary. We develop, characterize, and compare three different FPWFS algorithms, namely Gerchberg-Saxton (GS), linearized focal plane technique (LiFT), and Gaussian fit (Gf). These algorithms are studied for the specific purpose of slow focus sensing in the NIR (H and K bands) using numerical simulations and data collected at Keck in 2025 (bench and on-sky). The three algorithms were studied and characterized against different criteria such as linearity, computational costs, and resistance to low signal-to-noise ratio and/or residuals. From the results obtained, the main candidate for an on-sky deployment was GS, for which the main deciding factor was its higher stability and robustness under the presence of residuals. For that reason, on-sky tests in closed loop were made with GS. On-sky tests showed promising results, with GS successfully compensating for purposely introduced focus errors, even under the presence of high turbulence conditions. These tests represent an important step toward the full operationalization of this tool, expected in the coming months. This work can also be extrapolated to other existing 8-10 m class telescopes, or even future 30-40 m class telescopes, where the use of FPWFS can significantly improve sky coverage and reduce the lag error.

The effect of focus anisoplanatism upon the performance of an astronomical laser guide star (LGS) adaptive optics (AO) system can in principle be reduced if the lowest order wavefront aberrations are sensed and corrected using a natural guide star (NGS). For this approach to be useful, the noise performance of the wavefront sensor (WFS) used for the NGS measurements must be optimized to enable operation with the dimmest possible source. Two candidate sensors for this application are the Shack-Hartmann sensor and “phase-diverse phase retrieval,” a comparatively novel approach in which the phase distortion is estimated from two or more well-sampled, full-aperture images of the NGS measured with known adjustments applied to the phase profile. We present analysis and simulation results on the noise-limited performance of these two methods for a sample LGS AO observing scenario. The common parameters for this comparison are the NGS signal level, the sensing wavelength, the second-order statistics of the phase distortion, and the RMS detector read noise. Free parameters for the two approaches are the Shack-Hartmann subaperture geometry, the focus biases used for the phase-diversity measurements, and the algorithms used to estimate the wavefront. We find that phase-diverse phase retrieval provides consistently superior wavefront estimation accuracy when the NGS signal level is high. For lower NGS signal levels on the order of 103 photodetection events, the Shack-Hartmann (phase diversity) approach is preferred at a RMS detector read noise level of 5 (0) electrons/pixel.

The Gran Telescopio Canarias Adaptive Optics (GTCAO) will measure the wavefront with a Shack-Hartmann sensor. This wavefront sensor (WFS) is based on the CCD220, an electron-multiplying CCD (EMCCD) that achieves sub-electron readout noise, increasing the signal to noise ratio when weak natural guide stars (NGS) are used as reference. GTCAO will start its operation in telescope with NGS, using only one wavefront sensor, and later it will incorporate a Laser Guide Star (LGS) and consequently a second WFS, also based on an EMCCD. Both EMCCDs and a third one used as spare, have been characterized and compared including the system gain, electron- multiplication gain, readout noise vs gain, excess noise and linearity. The EM gain calibration is important to keep all EMCCD channels in the linear regime and the camera manufacturer carries it out, but it is reported that the multiplication gain may suffer ageing and degradation even if the camera is not in use. This suggests the need to monitor this ageing. In this paper it is proposed and tested a procedure for predictive maintenance that re-characterize the system gain, electron- multiplication gain and linearity periodically in order to predict the eventual ageing of the EMCCD multiplying registers. This procedure can be carried out quickly while the detector is installed in the WFS and in operational status. In order to provide the required illumination, the GTCAO calibration system is used.

We present the status and plans for the Keck All sky Precision Adaptive optics (KAPA) program. KAPA includes four key science programs, an upgrade to the Keck I laser guide star (LGS) adaptive optics (AO) facility to improve image quality and sky coverage, AO telemetry based point spread function (PSF) estimates for all science exposures, and an educational component focused on broadening the participation of women and underrepresented groups in instrumentation. For the purpose of this conference we will focus on the AO facility upgrade which includes implementation of a new laser, wavefront sensor and real-time controller to support laser tomography, the laser tomography system itself, and modifications to an existing near-infrared tip-tilt sensor to support multiple natural guide star (NGS) and focus measurements.

The Advanced Instrumentation and Technology Center at the Australian National University is building a Laser Tomography Adaptive Optics Test Bed for Extremely Large Telescopes. The optical test bench is using three Laser Guide Stars (LGS) propagating through three phase screens. The LGS wavefronts are sampled with a 16 × 16 Shack-Hartmann wavefront sensor (SH-WFS). Cone effect, spot elongation and Sodium layer density fluctuations are reproduced on the bench. Two Natural Guide Stars (NGS), on-axis and off-axis, are also added to the bench. The wavefront of the on-axis NGS is corrected with a DM located in the optical path of both the LGSs and the on-axis NGS. The DM commands are derived from the tomographic estimate of the on-axis NGS wavefront using the measurements of the 3 LGS WFSs. The off-axis NGS wavefront is sampled with a 6 × 6 SH-WFS and is emulating tip-tilt, focus and truth sensing. A DM located in front of the off-axis NGS WFS is correcting the off-axis NGS wavefront. The commands of this DM are also derived from the tomographic reconstructor. In the paper, the design of the LTAO test bed is presented.

The Subaru laser guide star adaptive optics (AO) system was installed at the Nasmyth focus of the Subaru Telescope, and had the first light with natural guide star on October 2006. The AO system has a 188-element curvature based wavefront sensor with photon-counting avalanche photodiode (APD) modules. It measures high-order terms of wavefront using either of a single laser (LGS) or natural guide star (NGS) within a 2' diameter field. The AO system has also a source simulator. It simulates LGS and NGS beams, simultaneously, with and without atmospheric turbulence by two turbulent layer at about 0 and 6 km altitudes, and reproduces the cone effect for the LGS beam. We describe the design, construction, and integration of the curvature wavefront sensor and calibration source unit.

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.

The adaptive optics (AO) system requires natural guide stars (NGS) to differentiate the focus error generated due to vertical shift in the sodium centroid altitude together with atmospheric turbulence. This dependency on NGS restricts achieving useable sky coverage and AO operation during dawn. One of the proposed approaches to distinguish these two focus errors is to monitor continuously the region of the sodium layer where the LGS is generated. The classical approaches used for this task, direct imaging, and LIDAR technology, have limitations. The technique of continuously amplitude modulating the guidestar laser (GSL) according to the pseudo-random binary sequence (PRBS) has the potential to overcome the limitations of classical approaches and reduce the dependency of the AO system on the NGS. The focus error is the major issue in large telescopes; therefore, we extended the PRBS technique to one of the large telescopes, the Gaint Magellan Telescope through numerical simulations. The study shows that modulation strength of 0.47 is required to fulfil the requirements during high sodium column density. Previously, it was intended to experimentally verify the technique with an LGS facility installed at the 1.8 m EOS telescope, Mount Stromlo, Canberra, Australia. Due to unavoidable circumstances, we now intend to perform the experiment with the LGS facility at the 2.3 m ANU telescope, Siding Spring Observatory (SSO), Coonabarabran, Australia. The paper presents the numerical simulations conducted with parameters of SSO, hardware selection for the experiment based on the system requirements, and preliminary test results from the laboratory.

International audience

Adaptive optics systems based on sodium Laser Guide Stars may exploit Natural Guide Stars to solve intrinsic limitations of artificial beacons, such as tilt indetermination and, in the case of Multi-Conjugate Adaptive Optics, tilt anisoplanatism. Natural Guide Stars are also required to mitigate the impact of the sodium layer structure and variability: on a 40-meter class telescope, as a consequence of the perspective elongation effect and of the finite Laser Guide Star Wavefront Sensor field of view, spurious wavefront aberrations are generated. The sodium layer may also have transverse structures leading to significant differential effects among Laser Guide Stars. All these issues show up in MAORY, a concept of a Multi-Conjugate Adaptive Optics module for the European Extremely Large Telescope. Starting from the analysis of the input perturbations to be measured, we derive preliminary requirements for the Natural Guide Star Wavefront Sensor and describe its conceptual design. The Wavefront Sensor uses three natural stars; each wavefront sensor probe consists of a fast tip-tilt and focus channel and a slow channel for monitoring the aberrations induced by the sodium layer. The dual-channel design allows the efficient exploitation of the few natural stars typically available in the field of view, while providing a way to monitor the potential anisoplanatism due to the differential sodium effects among the LGSs.

We present Canopus, the AO bench for Gemini's Multi Conjugate Adaptive Optics System (GEMS), a unique facility for the Gemini South telescope located at Cerro Pachon in Chile. The MCAO system uses five laser beacons in conjunction with different natural guide stars configurations. A deployable fold mirror located in the telescope Acquisition and Guiding Unit (A&G) sends the telescope beam to the entrance of the bench. The beam is split within Canopus into three main components: two sensing paths and the output corrected science beam. Light from the laser constellation (589nm) is directed to five Shack-Hartman wave front sensors (E2V-39 CCDs read at 800Hz). Visible light from natural guide stars is sent to three independent sensors arrays (SCPM AQ4C Avalanche Photodiodes modules in quad cell arrangement) via optical fibers mounted on independent stages and a slow focus sensor (E2V-57 back-illuminated CCD). The infrared corrected beam exits Canopus and goes to instrumentation for science. The Real Time Controller (RTC) analyses wavefront signals and correct distortions using a fast tip-tilt mirror and three deformable mirrors conjugated at different altitudes. The RTC also adjusts positioning of the laser beacon (Beam Transfer Optics fast steering array), and handles miscellaneous offloads (M1 figure, M2 tip/tilt, LGS zoom and magnification corrections, NGS probes adjustments etc.). Background optimizations run on a separate dedicated server to feed new parameters into the RTC.

The scientific productivity of laser guide star adaptive optics systems strongly depends on the sky coverage, which describes the probability of finding natural guide stars for the tip/tilt wavefront sensor(s) to achieve a certain performance. Knowledge of the sky coverage is also important for astronomers planning their observations. In this paper, we present an efficient method to compute the sky coverage for the laser guide star multiconjugate adaptive optics system, the Narrow Field Infrared Adaptive Optics System (NFIRAOS), being designed for the Thirty Meter Telescope project. We show that NFIRAOS can achieve more than 70% sky coverage over most of the accessible sky with the requirement of 191 nm total rms wavefront.

In order to achieve high sky coverage with natural guide star adaptive optics systems, the reference stars need to be chosen over a large field of view. But the size of the optical beam can become unmanageably large in current and planned future giant telescopes. This can render the optics unaffordable. To solve this issue, we have adopted two approaches - multiple fields of view and star-enlargers - for the LINC-NIRVANA layer-oriented, multiple-conjugated adaptive optics system. In this paper, we compare and contrast the advantages and disadvantages of various optical configurations for wide-field, natural guide star acquisition on current 8-meter and future 25-40 meter extremely large telescopes.

The Layer Oriented Wavefront Sensor for MAD has been used in the sky to achieve science. The preliminary results from a six night run and the perspectives in terms of achieved performances and projections of sky coverage for slightly more sophisticated system like the Multiple Field of View one are shown indicating that the sensor is keeping its promises.