Phasing the segmented Giant Magellan Telescope: progress in testbeds and prototypes

The Natural Guidestar Adaptive Optics (NGAO) system of the Giant Magellan Telescope (GMT) uses a visible-light modulated pyramid wavefront sensor (PWFS) to measure the wavefront aberrations, including residual segment piston errors. This paper presents an updated wavefront sensing and control architecture that increases the robustness of segment piston control. In the first place, the Natural Guide star Wavefront Sensor (NGWS) design has been updated to incorporate the recently proposed Holographic Dispersed Fringe Sensor (HDFS) as a second wavefront sensing channel measuring unambiguously differential segment piston errors as large as ~14 microns. In the second place, an optimized modal control strategy has been developed to compensate mode-by-mode for the optical gain of the PWFS. We report on the simulated NGAO performance results obtained when using a R=12 guide star, and with a turbulence strength of 𝑟₀ =12.8 cm.

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.

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 expected performance for the Giant Magellan Telescope (GMT) adaptive optics system. Using a custom adaptive optics modeling code developed at the University of Arizona, we are able to simulate the various aspects of atmospheric turbulence, wavefront sensing, and adaptive correction for the specific geometry of the GMT AO system. This allows us to explore the design space of the GMT and determine the performance of the system for various adaptive optics modes, including Ground-Layer AO. We present results showing the effect of the segmented aperture on AO performance, and discuss strategies for sensing and correction with the GMT aperture.

The near-Infrared Multi-Object Spectrograph (IRMOS) for TMT is one of the most powerful astronomical instruments ever envisioned. The combination of the collecting area of TMT, the unique image-sharpening capabilities of the Multi-Object Adaptive Optics (MOAO) system, and the multiplexing advantage of the multi-object integral-field spectra provided by the IRMOS back-end make it capable of addressing some of the leading scientific challenges of the coming decades. Here we present an overview of one potential IRMOS concept and then focus on the MOAO system. In particular we will describe our concept for the laser and natural guide star wavefront sensors, deformable mirrors and the calibration system of MOAO. For each of these design elements, we describe the key trade studies which help define each subsystem. From results of our studies, we assemble a MOAO ensquared energy budget. We find that 50% of the energy is ensquared within the 50 milli-arcsecond spatial pixel of the IRMOS integral field units for a wavelength of 1.65μm. Given the requirements placed on the MOAO system to achieve this performance, large ensquared energies can be achieved with even finer plate scales for wavelengths longer than 1.5μm.

<strong>11.1 Introduction</strong> Today, large-aperture modern telescope optical systems such as the ground-based Thirty Meter Telescope (TMT) and Giant Magellan Telescope (GMT) are divided into four parts: a large precision telescope radiation collector, a metrology and wavefront sensing and control (WFSC) system, the instrument that analyzes the complex wavefronts, and the detector that converts the complex wavefronts into intensity. Smaller-aperture space telescopes such as those used by the Wide-field Infrared Survey Explorer (WISE), Kepler, Stereo, and Ice, Cloud, and land Elevation Satellite (ICESat) do not require optical metrology and wavefront sensing and control because they are sufficiently small that their rigidity is obtained using mass, since the space system mass budget will accommodate it. In this chapter we expand the classical two-part astronomical optical system comprised of telescope and instrument into the much more capable (for large apertures) new three-part optical system of (1) the telescope, (2) the metrology and WFSC system, and (3) the instrument. At the heart of this new system is the two-stage optics system1 that collects, measures, and corrects the wavefront, and focuses it onto a field stop where the radiation passes into instruments that process the input complex wavefront for spectral, spatial (image), polarization, and radiometric information. In Chapters 8 and 9, we learned how scalar complex wavefronts propagate through an optical system to create an image, and we discussed the relationship between the wavefront and image quality. In Chapter 10, we learned that interferometry is a tool for analyzing the spectral (temporal) content and spatial structure of wavefronts. In this chapter, we show how wavefronts are manipulated to compensate for the following: <ul> &#x2022; telescope fabrication errors, &#x2022; the low mechanical stiffness characteristic of the new lightweight next-generation ground and space telescopes, and &#x2022; atmospheric turbulence. </ul>

From the cockpit of his United Airlines DC-8, en route from Chicago to Seattle at 37,000 feet, Capt. Homer Peterson first saw a broad vapor trail, much wider than that of a plane, just above the western horizon. Then he saw that the vapor was led by a glowing reddish-white object, followed by a flaming tail. Suddenly the object exploded like buckshot, each piece trailing fire. Spectacular though the explosion was, it did not take Capt. Peterson by surprise. About three hours before, he had received a TIP (Tracking and Impact Prediction) from the North American Air Defense Command, through his airline's operations office. The TIP had warned him to be on the lookout for a man-made object that nized reporting was done by United; then the field was opened up. Today 68 airlines make up the net, 50 of them outside the U.S. Only five major airlines in the world do not belong to the chain, one of which is Russia's Aeroflot, which wrote last year that it was waiting for the Soviet Academy of Sciences to decide whether such a project is as valuable as it is cracked up to be. The other holdouts are Trans World Airlines, Japan Air Lines, Scandinavian Air Service and Air France. Still under Roth's direction, VFON now represents almost 30,000 flight crew members who fly almost two million miles of air routes, most of them unduplicated. There have been 41 reports submitted on 14 reentering space objects (at least half of them Russian), wire and all radio reception-but, strangely, not transmission-cut out. The pilot who reported the sighting has been guaranteed anonymity by both VFON and Dr. Condon-a necessity if the network is going to receive any UFO reports at all. It's a matter of regaining the pilots' trust, Roth says. Besides the Smithsonian Astrophysical Observatory and the University of Colorado, VFON sends data of various kinds to the Environmental Science Services Administration, Northwestern University, the University of Arizona and a saucer-spotting organization called the National Investigations Committee for Aerial Phenomena. Even the National Aeronautics and Space Administration is interested; last month Roth briefed a space agency team

The Giant Magellan Telescope (GMT) is a 25-meter optical/infrared extremely large telescope that is being built by an international consortium of universities and research institutions. It will be located at the Las Campanas Observatory, Chile. The GMT primary mirror consists of seven 8.4-m borosilicate honeycomb mirror segments made at the Steward Observatory Mirror Lab (SOML). Six identical off-axis segments and one on-axis segment are arranged on a single nearly-paraboloidal parent surface having an overall focal ratio of f/0.7. The fabrication, testing and verification procedures required to produce the closely-matched off-axis mirror segments were developed during the production of the first mirror. Production of the second and third off-axis segments is underway. GMT incorporates a seven-segment Gregorian adaptive secondary to implement three modes of adaptive-optics operation: natural-guide star AO, laser-tomography AO, and ground-layer AO. A wide-field corrector/ADC is available for use in seeing-limited mode over a 20-arcmin diameter field of view. Up to seven instruments can be mounted simultaneously on the telescope in a large Gregorian Instrument Rotator. Conceptual design studies were completed for six AO and seeing-limited instruments, plus a multi-object fiber feed, and a roadmap for phased deployment of the GMT instrument suite is being developed. The partner institutions have made firm commitments for approximately 45% of the funds required to build the telescope. Project Office efforts are currently focused on advancing the telescope and enclosure design in preparation for subsystem- and system-level preliminary design reviews which are scheduled to be completed in the first half of 2013.

The Giant Magellan Telescope (GMT)¹ is a 25 m telescope composed of seven 8.4 m "unit telescopes", on a common mount. Each primary and conjugated secondary mirror segment will feed a common instrument interface, their focal planes co-aligned and co-phased. During telescope operation, the alignment of the optical components will deflect due to variations in thermal environment and gravity induced structural flexure of the mount. The ultimate co-alignment and co-phasing of the telescope is achieved by a combination of the Acquisition Guiding and Wavefront Sensing system (AGWS) and two segment-edge-sensing systems². An analysis of the capture range of the AGWS indicates that it is unlikely that that system will operate efficiently or reliably with initial mirror positions provided by open-loop corrections alone³. Since 2016 GMT have been developing a telescope metrology system, that is intended to close the gap between openloop modelling and AGWS operations. A prototyping campaign was initiated soon after receipt of laser metrology hardware in 2017. This campaign is being conducted in collaboration with the Large Binocular Telescope Observatory (LBTO), and hardware was first deployed on the LBT in August 2017. Since that time the system had been run and developed over some hundreds of hours on-sky. It has been shown to be capable of reliably measuring the relative positions of the main optics over ~ 10 m to a repeatability of ~ 1-2 microns RMS. This paper will describe the prototyping campaign to date, the basic design of the system, lessons learned and results achieved. It will conclude with a discussion of future prototyping efforts.

The MMT Adaptive optics exoPlanet characterization System (MAPS) is an exoplanet characterization program that encompasses instrument development, observational science, and education. The instrument we are developing for the 6.5m MMT observatory is multi-faceted, including a refurbished 336-actuator adaptive secondary mirror (ASM); two pyramid wavefront sensors (PyWFS's); a 1-kHz adaptive optics (AO) control loop; a high-resolution and long-wavelength upgrade to the Arizona infraRed Imager and Echelle Spectrograph (ARIES); and a new-AO-optimized upgrade to the MMT-sensitive polarimeter (MMT-Pol). With the completed MAPS instrument, we will execute a 60-night science program to characterize the atmospheric composition and dynamics of ~50-100 planets around other stars. The project is approaching first light, anticipated for Summer/Fall of 2022. With the electrical and optical tests complete and passing the review milestone for the ASM’s development, it is currently being tuned. The PyWFS's are being built and integrated in their respective labs: the visible-light PyWFS at the University of Arizona (UA), and the infrared PyWFS at the University of Toronto (UT). The top-level AO control software is being developed at UA, with an on-sky calibration algorithm being developed at UT. ARIES development continues at UA, and MMT-Pol development is at the University of Minnesota. The science and education programs are in planning and preparation. We will present the design and development of the entire MAPS instrument and project, including an overview of lab results and next steps.

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.

The University of Arizona and Smithsonian Astrophysical Observatory are jointly undertaking the development of the concept of a multiple-mirror telescope for astronoical uses. The system consists of six 1.8-m-aperture casse-grainian telescopes combined to produce a common focal surface in the center of the telescope. The early form of the telescope will provide for automatic alignment of the six independent systems. The telescope will be constructed to facilitate later extension of the alignment subsystem to include aperture synthesis (phasing) by methods proposed by Meinel and Shannon. The light-gathering power of the combination is equal to a standard telescope of 180-in. aperture, but the construction will cost very much less. The mounting will be of the alt-azimuth type. The telescope will be located at an elevation of 8,600 ft at the SAO's Mt. Hopkins Observing Station near Tucson.

Layer Oriented MCAO is a promising technique attempting to perform wide field of view correction with Natural Guide Stars. In the extended concept of Multiple Field of View Layer Oriented MCAO the wavefront sensor field of view is enlarged to collect more photons from more Natural Guide Stars and, in principle, significant sky coverages at any galactic latitude are achieved using only Natural Guide Stars. In this paper we address the problem of finding the best magnitude range for the Natural Guide Stars in order to achieve the best correction with the largest sky coverage in the Layer Oriented Multiple Field of View. For a given Field of View and sky direction we consider only the Natural Guide Stars within a given brightness range and we compute the equivalent integrated magnitude. Then we correlate the contribution in sky coverage of the previously considered Natural Guide Stars and we extrapolate which is the magnitude class giving the largest sky coverage. Once identified the more suitable Natural Guide Star magnitude class we discuss the possible implications in the design of a Multiple Field of View Layer Oriented wavefront sensor and we give the order of magnitude for the main parameters, i.e., maximum number of Natural Guide Stars and detector characteristics.

A 25 m four mirror live optics telescope is studied. M1 is spherical with 141 segments and f/0.96. M1 is re-imaged onto M4, also with 141 segments. Image FWHM is less than 0.10 arc sec over greater than 20 arc min. A horseshoe solution with a simple azimuth platform is applied. M1 segments are supported by a fine meniscus form truss structure, tied to the horseshoes by a coarser mesh. A FEM with 104 dof was developed and applied. Live optics control M1 and M4 segments (the latter with potentially high bandwidth), M1/M4 segment balancing and servos. Correction signals in tilt, coma and defocus are traced. A correlation tracker and a laser guide star system are included. Low and high wind speed regimes are studied. An end-to-end simulation model is developed, based on modal representation of our FEM. Image quality dependence on wind load is studied from segment piston and tilt deflections. Eigenmodes are recorded. Using wind time series, we study dynamic effects and image quality resulting from the 141 segment spots. Automatic segment control at a bandwidth of only 1 Hz gives excellent image quality. We foresee to reach a bandwidth greater than 50 Hz, securing a system partly adaptive, with effects of atmospheric wave front tilt removed through M4 segment tilting at high frequency. Further progress include optimization of mechanical design and end-to-end simulation model, wind tunnel testing and studies of wave front sensor, correlation tracker and instruments. A fully adaptive system is tentatively studied as is coherent operation at IR wavelengths.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Experiments have shown the reward adaptive-optics provides in improving the resolution of ground-based astronomical telescopes [1,2,3]. A critical contributor to adaptive-optics system performance is the control algorithm that converts wavefront sensor (WFS) measurements into the deformable mirror (DM) actuator commands. For the adaptive-optics systems in use today this control algorithm consists of a wavefront reconstruction step to estimate the instantaneous phase distortion to be compensated [4], followed by a servo control law to temporally filter this instantaneous estimate before it is applied to the deformable mirror [5]. So-called modal adaptive-optics systems can apply different temporal filters to separate spatial components, or modes, of the overall phase distortion [6]. Extensive analysis has been performed to evaluate and optimize the performance of these adaptive-optics control systems [7,8,9,10,11], but the results obtained depend on atmospheric parameters which are seldom known exactly and are constantly fluctuating. The uncertainty and variability of atmospheric conditions implies that an optimal degree of turbulence compensation cannot be achieved or maintained for long time intervals with a fixed control algorithm. A need exists for methods to update adaptive-optics control algorithms based upon actual system performance. Encouraging results have already been obtained demonstrating the value of emperically optimizing the control bandwidths for a modal adaptive-optics system [12]. In comparison, the subject of real-time adjustments to reconstruction matrices on the basis of measured system performance has received little attention.