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In this paper, we present the design and prototyping of the HARMONI Adaptive Optics Calibration Unit (AOCU). The AOCU consists of a set of on-axis sources (covering 0.5-2.4 μm) with a controllable wavefront shape. It will deploy into the instrument focal plane to inject calibration light into the rest of the system. The AOCU supports all-natural guide-star wavefront sensors for SCAO, HCAO, and LTAO.
The AOCU will be used to calibrate the WFSs, the internal interaction matrices of HARMONI, measure and compensate NCPAs between AO dichroics and the science detectors, and calibrate the pointing model zero position. The illumination assembly of the AOCU will consist of six diffraction-limited sources and a resolved source coupled into fibres. Because of the wide range of wavelengths and the spatial separations requirements, we use two endlessly single-mode fibres and a multimode fibre. In addition, several LED sources need to be coupled efficiently into the single-mode fibres. In this paper, we present the general AOCU design using off-the-shelf with a focus on the illumination and source module.
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 SCAO Sensors subsystem (SCAOS) is located within the Natural Guide Star Sensors (NGSS) system which includes several wavefront sensors (WFS) to cover the needs of the different HARMONI observing modes and operates in a cold, thermally stabilized (+2°C) and dry gas environment for thermal background limitation. To reach the required performance, the SCAOS will use different modules and mechanisms among which, two particularly critical devices have been prototyped and are tested: The SCAOS Pyramid Modulator Unit (SPMU) and the SCAOS Object Selection Mechanism (SOSM). Both devices are tip-tilt mirrors but have very different specifications (amplitude and speed). In this work, we will present and discuss the design, the assembly and the full test (performance, control) of the two systems, in both ambient and cold environments.
Characterizing the line spread function in integral field spectrographs from ground-based telescopes
HARMONI first light spectroscopy for the ELT: geometrical calibration in the data reduction software
In all configurations, the diffraction grating will lose a greater fraction of scientific light than any other single optic in the instrument. Additionally, manufacturers are often unable to measure the fraction of transmitted light at HARMONI's longest wavelengths. For these reasons, we have developed a setup to measure the efficiencies of transmission diffraction gratings across HARMONI's bandpass. The setup uses modulated signals, a single detector, and a lock-in amplifier to minimize sources of systematic errors. A modified version of this setup may be used to measure stray light. These setups and initial results are presented.
The focal plane mask wheel sits in the input focus of the cryostat. It provides 16 user-selectable positions for masks (28x40 mm) used in observation. The key driver for this mechanism is the high repeatability (±2.5 μm) required, equivalent to ~1mas in the input focal plane. The IAC has previously designed, manufactured, tested and put in operation cryogenic wheels with high repeatability; however, the challenge of obtaining a wheel with such repeatability requires testing new concepts of detent positioning systems.
The shutter allows for exposures shorter than the minimum read time of the near-IR detectors and is needed for any CCD observations with the visible cameras. A dual shutter design is needed to achieve the necessary open/close times (<20 ms), but this also provides some redundancy and a graceful failure mode for this critical device. To mitigate risks on the proper behaviour of a fast cryogenics shutter a prototype based on a simple concept has been manufactured. We present the design and results for the performed cryogenic tests of a mask wheel and a shutter prototypes that we have developed.
We show, in addition to the expected PSF degradation with the field direction, that the PSF retains a coherent core even at large off-axis distances. We demonstrated the large performance improvement of fine tuning the sampling frequency for dimer natural guide stars and an improvement of approx. 50% in SR can be reached above the nominal case. We show that using a smaller AO system with only 20x20 sub-apertures it is possible to further increase performance and maintain equivalent performance even for large off-axis angles.
We tour the processing of both the isoplanatic and anisoplanatic tilt modes using the spatio-angular approach whereby the wavefront is estimated directly in the pupil plane avoiding a cumbersome explicit layered estimation on the 35-layer profiles we're currently using.
Taking the case of Harmoni, we cover the choice of wave-front sensors, the number and field location of guide-stars, the optimised algorithms to beat down angular anisoplanatism and the performance obtained with different temporal controllers under split high-order/low-order tomography or joint tomography. We consider both atmospheric and far greater telescope wind buffeting disturbances. In addition we provide the sky-coverage estimates thus obtained.
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