arXiv subjects

C. Boyer

Publications and source records attributed to C. Boyer.

7 recordsLinked to original sources

arXiv2026

PROVIDENCE: a ground station for space observation

PROVIDENCE is an ONERA-led project to install a new-generation optical ground station at the Haute-Provence Observatory (OHP) in South of France. Built around a 2.5-meter aperture, the station addresses five core science cases: space domain awareness, astronomy, laser activities and communications, atmospheric characterization, and instrumental prototyping. The project is organized into three parallel subprojects covering the telescope (Providence T), the building (Providence B), and the instruments (Providence I), the latter including the facility's first-light adaptive optics instrument, INTERSTELLAR. This paper presents the scientific motivation, the project architecture, the telescope technical specifications, the civil-engineering challenges associated with replacing the existing T152 equatorial telescope, and the station layout designed to host multiple co-active instrument teams. The project roadmap targets First Light in 2029, timed to track the close-approach flyby of asteroid Apophis.

astro-ph.IM↗

arXiv2025

Statistical Advantage of Softmax Attention: Insights from Single-Location Regression

Large language models rely on attention mechanisms with a softmax activation. Yet the dominance of softmax over alternatives (e.g., component-wise or linear) remains poorly understood, and many theoretical works have focused on the easier-to-analyze linearized attention. In this work, we address this gap through a principled study of the single-location regression task, where the output depends on a linear transformation of a single input token at a random location. Building on ideas from statistical physics, we develop an analysis of attention-based predictors in the high-dimensional limit, where generalization performance is captured by a small set of order parameters. At the population level, we show that softmax achieves the Bayes risk, whereas linear attention fundamentally falls short. We then examine other activation functions to identify which properties are necessary for optimal performance. Finally, we analyze the finite-sample regime: we provide an asymptotic characterization of the test error and show that, while softmax is no longer Bayes-optimal, it consistently outperforms linear attention. We discuss the connection with optimization by gradient-based algorithms.

cs.LG↗

arXiv2018

Magnetic and structural properties of Co$_2$MnSi based Heusler compound

The influence of antisite disorder occupancies on the magnetic properties of the half-metallic Co$_2$MnSi compound was studied by experimental techniques and first-principles calculations. The neutron diffraction studies show almost identical amount of Mn and Co disorders of 6.5\% and 7.6\%, which was found to be in good agreement with density functional theory (DFT) calculations of the stable Co$_2$MnSi system with the corresponding disorders. DFT studies reveal that antiferromagnetic interactions introduced by Mn disorder lead to a reduction of the net magnetic moment. The results are discussed in conjunction with neutron diffraction and magnetization measurements. Transport property measurement under magnetic field up to 9 Tesla revealed a positive magnetoresistance for bulk Co$_2$MnSi that persists up to room temperature. A Curie temperature of $\sim$1014 K was determined for the compound by high temperature electrical resistivity and dilatometry measurements.

cond-mat.mtrl-sci↗

arXiv2018

Magnetic and structural studies of G-phase compound Mn$_6$Ni$_{16}$Si$_7$

Transition metal compounds with complex crystal structures tend to demonstrate interesting magnetic coupling resulting in unusual magnetic properties. In this work, the structural and magnetic characterization of a single crystal of the Ni-Mn-Si based G-phase compound, Mn$_6$Ni$_{16}$Si$_7$, grown by the Czochralski method, is reported. In this structure isolated octahedral Mn$_6$ clusters form a f.c.c. lattice. As each octahedron consists of eight edge-sharing equilateral triangles, the possibility for geometric frustration exists. Magnetization and specific heat measurements showed two magnetic phase transitions at 197 K and 50 K, respectively. At 100 K neutron diffraction on powder samples shows a magnetic structure with k = (001) in which only four of the six Mn spins per cluster order along $<100>$ directions giving a two dimensional magnetic structure consistent with intra-cluster frustration. Below the 50 K phase transition the Mn spins cant away from $<100>$ directions and a weak moment develops on the two remaining Mn octahedral sites.

cond-mat.mtrl-sci↗

arXiv2012

Modeling of pulsed laser guide stars for the Thirty Meter Telescope project

The Thirty Meter Telescope (TMT) has been designed to include an adaptive optics system and associated laser guide star (LGS) facility to correct for the image distortion due to Earth's atmospheric turbulence and achieve diffraction-limited imaging. We have calculated the response of mesospheric sodium atoms to a pulsed laser that has been proposed for use in the LGS facility, including modeling of the atomic physics, the light-atom interactions, and the effect of the geomagnetic field and atomic collisions. This particular pulsed laser format is shown to provide comparable photon return to a continuous-wave (cw) laser of the same average power; both the cw and pulsed lasers have the potential to satisfy the TMT design requirements for photon return flux.

physics.atom-ph↗

arXiv2001

Gemini MCAO Control System

The Gemini Observatory is planning to implement a Multi Conjugate Adaptive Optics (MCAO) System as a facility instrument for the Gemini-South telescope. The system will include 5 Laser Guide Stars, 3 Natural Guide Stars, and 3 Deformable mirrors optically conjugated at different altitudes to achieve near-uniform atmospheric compensation over a 1 arc minute square field of view. The control of such a system will be split into 3 main functions: the control of the opto-mechanical assemblies of the whole system (including the Laser, the Beam Transfer Optics and the Adaptive Optics bench), the control of the Adaptive Optics System itself at a rate of 800FPS and the control of the safety system. The control of the Adaptive Optics System is the most critical in terms of real time performances. The control system will be an EPICS based system. In this paper, we will describe the requirements for the whole MCAO control system, preliminary designs for the control of the opto-mechanical devices and architecture options for the control of the Adaptive Optics system and the safety system.

cs.OH↗

arXiv1997

Performance of the CFHT Adaptive Optics Bonnette

Extensive results from the commissioning phase of PUEO, the adaptive optics instrument adaptor for the Canada-France-Hawaii telescope, are presented and discussed. Analyses of more than 750 images recorded with a CCD and a near-IR camera on 16 nights in wavelengths from B to H are used to derive the properties of the compensated wavefront and images in a variety of conditions. The performance characteristics of the system are analyzed and presented in several ways, in terms of delivered Strehl ratios, full-width-half-maxima, and quantities describing the improvements of both. A qualitative description is given of how the properties of the corrected images result from the structure function of the compensated phase. Under median seeing conditions, PUEO delivers essentially diffraction-limited images at H and K, images with FWHM ~0.1" at J and I, and provides significant gains down to B, with guide stars as faint as R = 14. During good conditions, substantial gains were realized with guide stars as faint as R = 17. A simple user-interface and software which automatically and continuously optimizes the mode gains during observations makes the operational efficiency extremely high. A few astronomical examples are briefly discussed.

astro-ph↗