In the last few years the concept of an active space telescope has been greatly developed, to meet demanding requirements with a substantial reduction of tolerances, risks and costs. This is the frame of the LATT project (an ESA TRP) and its follow-up SPLATT (an INAF funded R&D project). Within the SPLATT activities, we outline a novel approach and investigate, both via simulations and in the optical laboratory, two main elements: an active segmented primary with contactless actuators and a pyramid wavefront sensor (PWFS) to drive the correction chain. The key point is the synergy between them: the sensitivity of the PWFS and the intrinsic stability of a contactless-actuated mirror segment. Voice-coil, contactless actuators are in facts a natural decoupling layer between the payload and the optical surface and can suppress the high frequency vibration as we verified in the lab. We subjected a 40 cm diameter prototype with 19 actuators to an externally injected vibration spectrum; we then measured optically the reduction of vibrations when the optical surface is floating controlled by the actuators, thus validating the concept at the first stage of the design. The PWFS, which is largely adopted on ground-based telescope, is a pupil-conjugated sensor and offers a user-selectable sampling and capture range, in order to match different use cases; it is also more sensitive than Shack-Hartmann sensor especially at the low-mid spatial scales. We run a set of numerical simulations with the PWFS measuring the misalignment and phase steps of a JWST-like primary mirrors: we investigated the PWFS sensitivity in the sub-nanometer regime in presence of photon and detector noise, and with guide star magnitudes in the range 8 to 14. In the paper we discuss the outcomes of the project and present a possible roadmap for further developments.
All the aberrations not corrected by Adaptive Optics (AO) systems are important limitations for high contrast imaging at large telescopes. Among them, the most relevant ones are called Non-Common Path Aberrations (NCPA): these are present downstream of the separation of the optical paths to Wavefront Sensor and scientific focal plane. The typical approach to mitigate them is to set an offset on the AO system with the opposite sign of this NCPA. It can be obtained with a trial-and-error approach or with sophisticated focal plane Wavefront sensing. There is a need for a fast procedure to measure NCPA in order to limit the telescope downtime and to repeat, if needed, the correction procedure to cope with any temporal variation. Different methods exists to measure and compensate it introducing the correction as offset in the AO control loop. New approaches based on Neural Networks (NNs) have also been proposed. In this work, using simulated images, we test and describe the application of a supervised Multi-layer Perceptron (MLP) NN for the mitigation of NCPA in high contrast imaging at visible wavelengths. As shown in our previous work, we already tested the method on simulated images and showed that this method is robust even in the presence of turbulence-induced dynamic aberrations that are not labelled in the training phase of the NN corresponding to the typical AO residual of the daytime calibration. We tested the method on the GHOST optical test bench at ESO laboratories and preliminary results show the method is very promising, recovering almost completely the SR in an iterative correction process.
SHARK-VIS the Large Binocular Telescope (LBT) forthcoming optical high-contrast imager is completed and will soon see its first light. Thanks to the high performances of SOUL, the new LBT adaptive optics (AO) system, SHARK-VIS will extend the capabilities of one arm of the binocular telescope down to 400 nm, delivering spatial resolutions (≈ 15 mas) that in the infrared bands will be achieved only by future extremely large telescopes (ELTs). This document describes the instrument and its final laboratory test that assessed the expected performances. End to End test, done on a dedicated test-bench resembling on sky conditions, showed the capability to detect faint target with contrast lower than 10−4 at only 50mas away from a magR 12 point like source.
The Exoplanets at LBT with a Visible IFS for SHARK-VIS (ELVIS) is an add-on imaging spectrograph to be integrated in the new LBT high-contrast high-resolution AO-assisted imager SHARK-VIS. ELVIS is optimized for a medium/high spectral resolution of 10-20k with a limited bandwidth around the Hα, and it is planned fed by a small core (10-20 ⊘ µm) multi mode fiber bundle providing about 140 spaxels on a field of view around 300×300 sqmas. This instrument has a very compact design based on a VPH dispersing element to allow its installation within a standard 19” rack mount. As shown in the literature, young accreting sub-stellar and planetary companions are better detected and analyzed by these instruments allowing to reach contrast at least ten times fainter (in their Hα emission) with respect to standard imagers.
Large format deformable mirrors have been proposed in the last few years as key elements to implement active wave front correction for future space telescopes. Active optics is, in fact, an enabling technology for high stability, high contrast and high resolution systems. We present in this work a 40 cm diameter prototype, together with its laboratory characterization, based on voice-coil actuators. When the mirror is operated, such contact-less actuation allows the optical surface to float at a given distance from its support and the mirror is virtually decoupled from the mechanics; such condition offers an intrinsic isolation from external vibrations with no need for further damping devices. We demonstrated experimentally this concept in the laboratory on a dedicated interferometric setup, registering a substantial rejection of the vibrations injected. We will present in this work the test results and a roadmap for future developments.
In order to deliver high contrast, high resolution images, a keypoint in future space telescopes is to implement a correction chain, composed by a WaveFront Sensor (WFS) and an active/deformable mirror (DM) running in closed loop. While the current baseline for WFS is mostly focused on PSF sensing, no or little attention has been paid to a crucial element: after an initial startup phase, a space telescope will be operating in quasi diffraction-limited regime, so that a WFS shall be able to stably measure nanometer-level aberrations. In this scenario, the aberration signal in the PSF is overwhelmed by the bright core and to get rid of the photon noise a long time integration is required, which is converted into a more stringent stability requirement for the DM. We propose the implementation of the pyramid WFS (PWFS), which has been already demonstrated on-sky for high contrast at large ground based observatories. In this paper we present numerical simulations of a PWFS controlling an active primary in space. We will show the signal measured by the pyramid when a offset is applied on the mirror and the closed loop performances. The results indicate that the PWFS is able to detect nanometer-level low spatial scale aberrations and drive the active mirror with an optical stability consistent with typical requirements.
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