HARMONI is the first light visible and near-IR integral field spectrograph for the ELT. It covers a large spectral range from 450nm to 2450nm with resolving powers from 3500 to 18000 and spatial sampling from 60mas to 4mas. It can operate in two Adaptive Optics modes - SCAO (including a High Contrast capability) and LTAO - or with NOAO. The project is preparing for Final Design Reviews. The AO wavefront sensor modules are assembled and tested in Marseille, requiring a large telescope simulator for realistic projection of various guide objects into the system. This is particularly challenging for the six laser guide stars, whose partially overlapping beams converge to a focus several meters behind the instrument entrance focus. To avoid building a well-corrected, meter-sized, optical system for this task, a single large lens is employed, associated with source modules individually equipped with spatial light modulators (SLM) capable both of simulating the atmosphere and the telescope's deformable mirror (M4) and of compensating the aberrations inherent to the simple simulator optics. In this paper, we present preliminary optical designs for the telescope simulator and the source units. We also describe prototype work, associated with theoretical analysis, to demonstrate the SLM's capabilities for generating sufficiently large aberrations.
HARMONI is the first light visible and near-IR integral field spectrograph for the ELT. It covers a large spectral range from 450 nm to 2450 nm with resolving powers from 3500 to 18000 and spatial sampling from 60 mas to 4 mas. It can operate in two Adaptive Optics modes - SCAO (including a High Contrast capability) and LTAO - or with no AO mode. To prepare the final design reviews, we have built an optical bench to emulate and characterize the performance of the laser guide star (LGS) wavefront sensor (WFS) to be used in HARMONI. The WFS is a classic Shack-Hartmann, nonetheless pushed to the extreme due to the size of the primary mirror of the ELT (39 m). The WFS is composed of a 80×80 double side microlens array (MLA), an optical relay made of 6 lenses in order to re-image the light coming from the MLA on the detector, and a CMOS camera using a Sony detector with 1608×1104 pixels, RON< 3e, and a frame rate of 500Hz. The sensor has a large number of pixels to provide a field-of-view wider than 15 arcsec per subaperture over the full pupil, which is required to image the elongated LGS spots. An innovative feature of our bench is the use of a spatial light modulator (SLM) which allows us to emulate the M4 deformable mirror (DM) and the real position of its actuators, together with the projected spiders in the pupil plane. We report on the design and performance of our bench, including the first interaction matrices using the ELT-M4 influence functions and a non-elongated source. We expect to implement a system to emulate an elongated source in order to grasp a better understanding of its effects on wavefront sensing.
HARMONI is the first light visible and near-IR integral field spectrograph for the ELT covering a large spectral range from 450nm to 2450nm with resolving powers from 3500 to 18000 and spatial sampling from 60mas to 4mas. It can operate in two Adaptive Optics modes - SCAO and LTAO - or with no AO. The project is preparing for Final Design Reviews. The laser Tomographic AO (LTAO) system provides AO correction with very high sky-coverage thanks to two systems: the Laser Guide Star Sensors (LGSS) and the Natural Guide Star Sensors (NGSS). LGSS is dedicated to the analysis of the wavefront coming from 6 laser guide stars created by the ELT. It is made of 6 independent wavefront sensor (WFS) modules mounted on a rotator of 600mm diameter to stabilise the pupil onto the microlens array in front of the detector. The optical design accepts elongated spots of up to 16 arcsec with no truncation using a CMOS detector from SONY. We will present the final optical and mechanical design of the LGSS based on freeform lenses to minimize the numbers of optical components and to accommodate for the diversity of sodium layer configurations. We will focus on rotator design, illustrating how we will move 1 tons with 90” accuracy in restrictive environment. Finally, we will present the strategy to verify the system in HARMONI context. The main challenge for the verification being how to test an AO system without access to the deformable mirror, part of the ELT.
HARMONI is the first light, adaptive optics assisted, integral field spectrograph for the European Southern Observatory’s Extremely Large Telescope (ELT). A work-horse instrument, it provides the ELT’s diffraction limited spectroscopic capability across the near-infrared wavelength range. HARMONI will exploit the ELT’s unique combination of exquisite spatial resolution and enormous collecting area, enabling transformational science. The design of the instrument is being finalized, and the plans for assembly, integration and testing are being detailed. We present an overview of the instrument’s capabilities from a user perspective, and provide a summary of the instrument’s design. We also include recent changes to the project, both technical and programmatic, that have resulted from red-flag actions. Finally, we outline some of the simulated HARMONI observations currently being analyzed.
Laser Guide Star wave-front sensing [LGSWFS] is a key element of tomographic Adaptive Optics system. For classical Shack-Hartmann Wave-Front Sensor, necessary trade-offs have to be made between the pupil spatial sampling, the sub-aperture field-of-view and the pixel sampling. For Extremely Large Telescope [ELT] scales, these trade-off are also driven by strong technical constraints, especially concerning the available detectors. We propose a sensitivity analysis, and we explore how these parameters impacts the final performance. We introduce the concept of super resolution, which allows to reduce the pupil sampling and allows proposing potential LGSWFS designs providing the best performance for ELT scales.
Laser guide star (LGS) wave-front sensing (LGSWFS) is a key element of tomographic adaptive optics system. However, when considering Extremely Large Telescope (ELT) scales, the LGS spot elongation becomes so large that it challenges the standard recipes to design LGSWFS. For classical Shack–Hartmann wave-front sensor (SHWFS), which is the current baseline for all ELT LGS-assisted instruments, a trade-off between the pupil spatial sampling [number of sub-apertures (SAs)], the SA field-of-view (FoV) and the pixel sampling within each SA is required. For ELT scales, this trade-off is also driven by strong technical constraints, especially concerning the available detectors and in particular their number of pixels. For SHWFS, a larger field of view per SA allows mitigating the LGS spot truncation, which represents a severe loss of performance due to measurement biases. For a given number of available detectors pixels, the SA FoV is competing with the proper sampling of the LGS spots, and/or the total number of SAs. We proposed a sensitivity analysis, and we explore how these parameters impacts the final performance. In particular, we introduce the concept of super resolution, which allows one to reduce the pupil sampling per WFS and opens an opportunity to propose potential LGSWFS designs providing the best performance for ELT scales.
The adaptive optics systems of future Extremely Large Telescopes (ELTs) will be assisted with laser guide stars (LGS) which will be created in the sodium layer at a height of ≈90 km above the telescopes. In a Shack–Hartmann wavefront sensor, the long elongation of LGS spots on the sub-pupils far apart from the laser beam axis constraints the design of the wavefront sensor (WFS) which must be able to fully sample the elongated spots without undersampling the non-elongated spots. To fulfill these requirements, a newly released large complementary metal oxide semiconductor sensor with 1100 × 1600 pixels and 9 μm pixel pitch could be employed. Here, we report on the characterization of such a sensor in terms of noise and linearity, and we evaluate its performance for wavefront sensing based on the spot centroid variations. We then illustrate how this new detector can be integrated into a full LGS WFS for both the European Southern Observatory’s ELT and the Thirty Meter Telescope.
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