Seminars

Wavefront architecture of the ELT

The ELT Wavefront Control manages the 5 mirrors of the telescope in position and shape and delivers at the interface with the instruments a beam quality compatible with diffraction-limited performance after correction by post-focal AO systems. This is obtained if the spectral distribution of aberrations in the temporal and spatial domains lies within the dynamics of M4, the deformable mirror of the telescope. In this talk, I will present some properties of the ELT aperture, discuss the main components of telescope dynamic disturbances whose spectral distribution exceeds the envelope of the free atmosphere, and describe the elements of the telescope control strategy.

Lucie Leboulleux researcher at IPAG will present her work about  “High-contrast imagers robust to segmentation-due errors and low-wind effect”. Here is an abstract of a presentation :

Imaging and characterizing exoplanets down to Earth-like planets relies on instrument able to access objects with a contrast to their host star of the order of 10^(-10) to -10^(-8), at short angular separations (<0.1’’). However, such instruments are complex to design, extremely sensitive to optical aberrations and instabilities, and tend to be set on giant telescopes with a segmented mirror that increases the number of possible sources of instabilities: the Extremely Large Telescope (39m and 798 segments), the Thirty Meter Telescope (30m, 492 segments), the Giant Magellan Telescope (24m, 7 segments), possibly the Habitable World Observatory (ex-UVOIR) space telescope (6m)… Among other sources of errors, their coronagraphic instruments will be impacted by segment phasing errors: on LUVOIR, for instance, a 10^(-10) contrast implies phasing constraints down to 10pm RMS, which can currently not be accessed and maintained. During this seminar, I develop an innovative approach to this problematic, by telling the story of PASTIS (Pair-based Analytical model for Segmented Telescopes Imaging from Space), an analytical model first invented to characterize coronagraphic instruments and to set constraints on the telescope segment alignment, and then used to directly design instruments robust to segment phasing errors. This approach is also extended to two other types of aberrations expected on coming ground-based telescopes: post-adaptive optics system petaling effects and the low-wind effect, the latest already limiting VLT/SPHERE and Subaru telescope high-contrast performance.

Maxime Quesnel PhD student will present his work on A Simulator-based Autoencoder for Focal Plane Wavefront Sensing.

Laurie Paillier research engineer at ONERA will present her work about optical links from ground to space. Here is an abstract of her presentation :

Both the increasing imaging resolution of earth observation satellites and the advent of a space based globalized internet are currently urging for very high data rate transmissions between space and ground. With the promise to provide tens of Gbps per channel, optical links may become a major breakthrough technology, assuming that the
technological assets developed for the fibered networks can be exploited. Especially, phase modulation techniques have demonstrated their tremendous efficiency for fibered networks. The aim of this work was to investigate the feasibility of their transposition to the case of satellite-to-ground optical links, accounting for their specificities: laser phase noise, Doppler effect, and the impact of propagation through the turbulent atmosphere and its correction by adaptive optics. We investigated for different turbulence conditions two architectures of digital receiver: one based on a phase-locked loop, and the other one based on an open loop approach. As expected, the importance of the quality of the adaptive optics correction is highlighted. This work opens prospects for a strong increase in the achievable bit rate for coherent telemetry links when using higher-order constellations (QPSK and beyond).

Loïc Barbot PhD student will present his work on astronomical navigation. Here is the abstract of the presentation :

Astronomical navigation is still studied in merchant or naval academies as a method to fix the position of a ship. Considered as an emergency technic, it is implemented by an observer measuring with the sextant the angle between the horizon line and a celestial body. After calculations and plots, the position is obtained with an accuracy of several nautical miles (1 NM = 1,852 km).
Star tracking is used on satellites to measure their orientation in space in order to turn their solar panels towards the sun or their cameras and antennas towards the earth. This requires to identify several stars in the same field of view or in the fields of several star-trackers.
To be reliable, a stellar positioning system must have an accuracy comparable to satellite positioning, should operate day and night, even at low altitude where the atmosphere is dense in aerosols. If the position of the stars and the algorithms are well mastered, the detection of stars by daylight remains a challenge.
The MARIS STELLA project explores the potential of infrared cameras. The stars thus detectable are numerous but this technology has not yet reached its full maturity: the performances are improving over time. After a presentation of the current results, future tests will be described.

MUSE and the search for massive black in globular clusters