arXiv subjects

Emmanuel Hugot

Publications and source records attributed to Emmanuel Hugot.

At least 19 recordsLinked to original sources

arXiv2025

Experimental study of the first telescope with a toroidal curved detector

We describe a practical implementation of the anamorphically curved detector concept. In order to demonstrate its advantages, a telescope lab prototype was developed, built, and tested. It is based on a 4-mirror all-spherical unobscured design, similar to that proposed by D. Shafer. The telescope is open at F/#=5.5 and its extended field of view is 10.6x8 degrees. We explore the design parameter space to demonstrate the change in gain introduced by the curved detector and to substantiate the chosen parameters. If the image surface is curved to the shape of a toroid, the image quality reaches the diffraction limit over the entire field by design. The design was optimized to use standard concave spherical mirrors. The detector was curved down toroidally with an exquisite surface precision of approximately 11 micrometers Peak-to-Valley. The experimental tests of the prototype have shown that use of the toroidally curved detector allows to increase the contrast in the field corner by up to 0.3 units, which is in good agreement with the modeling results. We also propose a few prospective applications of the demonstrated concept.

astro-ph.IM↗

arXiv2021

BlueMUSE: Project Overview and Science Cases

We present the concept of BlueMUSE, a blue-optimised, medium spectral resolution, panoramic integral field spectrograph based on the MUSE concept and proposed for the Very Large Telescope. With an optimised transmission down to 350 nm, a larger FoV (1.4 x 1.4 arcmin$^2$) and a higher spectral resolution compared to MUSE, BlueMUSE will open up a new range of galactic and extragalactic science cases allowed by its specific capabilities, beyond those possible with MUSE. For example a survey of massive stars in our galaxy and the Local Group will increase the known population of massive stars by a factor $>$100, to answer key questions about their evolution. Deep field observations with BlueMUSE will also significantly increase samples of Lyman-alpha emitters, spanning the era of Cosmic Noon. This will revolutionise the study of the distant Universe: allowing the intergalactic medium to be detected unambiguously in emission, enabling the study of the exchange of baryons between galaxies and their surroundings. By 2030, at a time when the focus of most of the new large facilities (ELT, JWST) will be on the infra-red, BlueMUSE will be a unique facility, outperforming any ELT instrument in the Blue/UV. It will have a strong synergy with ELT, JWST as well as ALMA, SKA, Euclid and Athena.

astro-ph.IM↗

arXiv2021

Curved detector-based optical design for the VLT/BlueMUSE instrument

BlueMUSE (Blue Multi Unit Spectroscopic Explorer) is a blue-optimised, medium spectral resolution, panoramic integral field spectrograph proposed for the Very Large Telescope (VLT) and based on the MUSE concept. BlueMUSE will open up a new range of galactic and extragalactic science cases allowed by its specific capabilities in the 350 - 580 nm range: an optimised end-to-end transmission down to 350 nm, a larger FoV (up to $1.4 \times 1.4$ arcmin$^2$) sampled at 0.3 arcsec, and a higher spectral resolution ($λ/Δλ\sim 3500$) compared to MUSE. To our knowledge, achieving such capabilities with a comparable mechanical footprint and an identical detector format ($4\text{k} \times 4\text{k}$, 15 $\mathrm{μm}$ CCD) would not be possible with a conventional spectrograph design. In this paper, we present the optomechanical architecture and design of BlueMUSE at pre-phase A level, with a particular attention to some original aspects such as the use of curved detectors.

astro-ph.IM↗

arXiv2020

3D printing for astronomical mirrors

3D printing, also called additive manufacturing, offers a new vision for optical fabrication in term of achievable optical quality and reduction of weight and cost. In this paper we describe two different ways to use this technique in the fabrication process. The first method makes use of 3D printing in the fabrication of warping harnesses for stress polishing, and we apply that to the fabrication of the WFIRST coronagraph off axis parabolas. The second method considers a proof of concept for 3D printing of lightweight X-Ray mirrors, targeting the next generation of X-rays telescopes. Stress polishing is well suited for the fabrication of the high quality off axis parabolas required by the coronagraph to image exoplanets.. Here we describe a new design of warping harness which can generate astigmatism and coma with only one actuator. The idea is to incorporate 3D printing in the manufacturing of the warping harness. The method depicted in this paper demonstrates that we reach the tight precision required at the mirrors surface. Moreover the error introduced by the warping harness fabricated by 3D printing does not impact the final error budget. Concerning the proof of concept project, we investigate 3D printing towards lightweight X-ray mirrors. We present the surface metrology of test samples fabricated by stereo lithography (SLA) and Selective Laser Sintering (SLS) with different materials. The lightweighting of the samples is composed of a series of arches. By complementing 3D printing with finite element analysis topology optimization we can simulate a specific optimum shape for the given input parameters and external boundary conditions. The next set of prototypes is designed taking to account the calculation of topology optimisation.

astro-ph.IM↗

arXiv2020

Superpolished OAPs for WFIRST CGI

Exoplanet imaging requires super polished off-axis parabolas (OAP) with the utmost surface quality. In this paper we describe an innovative manufacturing process combining 3D printing and stress polishing, to create a warping harness capable of producing any off axis parabola profile with a single actuator. The warping harness is manufactured by 3D printing. This method will be applied to the production of the WFIRST coronagraph's off axis parabolas. The evolution of the warping harness design is presented, starting from a ring warping harness generating astigmatism, to an innovative thickness distribution harness optimised to generate an off axis parabola shape. Several design options are available for the prototyping phase, with their advantages and disadvantages which will be discussed.

astro-ph.IM↗

arXiv2020

Use of 3D printing in astronomical mirror fabrication

In this paper we are exploring the possibilities of 3D printing in the fabrication of mirrors for astronomy. Taking the advantages of 3D printing to solve the existing problems caused by traditional manufacturing, two proof-of-concept mirror fabrication strategies are investigated in this paper. The First concept is a deformable mirror with embedded actuator supports system to minimise errors caused by the bonding interfaces during mirror assembly. The second concept is the adaption of the Stress Mirror Polishing (SMP) technique to a variety of mirror shapes by implemented a printed thickness distribution on the back side of the mirror. Design investigations and prototypes plans are presented for both studies.

astro-ph.IM↗

arXiv2020

Stress mirror polishing for future large lightweight mirrors: design using shape optimization

This study proposes a new way to manufacture large lightweight aspherics for space telescopes using Stress Mirror Polishing (SMP). This technique is well known to allow reaching high quality optical surfaces in a minimum time period, thanks to a spherical full-size polishing tool. To obtain the correct surface' aspheric shape, it is necessary to define precisely the thickness distribution of the mirror to be deformed, according to the manufacturing parameters. We first introduce active optics and Stress Mirror Polishing techniques, and then, we describe the process to obtain the appropriate thickness mirror distribution allowing to generate the required aspheric shape during polishing phase. Shape optimization procedure using PYTHON programing and NASTRAN optimization solver using Finite Element Model (FEM) is developed and discussed in order to assist this process. The main result of this paper is the ability of the shape optimization process to support SMP technique to generate a peculiar aspherical shape from a spherical optical surface thanks to a thickness distribution reshaping. This paper is primarily focused on a theoretical framework with numerical simulations as the first step before the manufacturing of a demonstrator. This two-steps approach was successfully used for previous projects.

astro-ph.IM↗

arXiv2020

Measuring the cophasing state of a segmented mirror with a wavelength sweep and a Zernike phase contrast sensor

The demand for higher resolution telescopes leads to segmented primary mirrors which need to be phased for operation. A phasing sensor applying a wavelength sweep technique provides a large capture range without modulating the position of individual mirror segments. This technique offers the potential to monitor the phasing state of a segmented telescope in parallel to the science observations. We evaluate the performance of the wavelength sweep technique using a Zernike phase contrast sensor for coarse phasing. Tests results on a dedicated bench show 112 nm rms precision. With the help of a simulation, we explain a known error of the method and we suggest ways for improvements.

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arXiv2020

Variable Curvature Displays: Optical Designs and Applications for VR/AR/MR Headsets

In the present paper, we discuss the design of a projection system with curved display and its enhancement by variably adjusting the curvature. We demonstrate that the focal surface curvature varies significantly with a change of the object position and that it can easily be computed with the Seidel aberration theory. Using this analytically derived curvature value as the starting point, we optimise a refocusable projection system with 90 degrees field of view and F/#=6.2 . It is demonstrated that such a system can provide stable image quality and illumination when refocusing from infinity to 1.5 m. The gain in spatial resolution is as high as 1.54 times with respect to a flat focal surface. Furthermore, we prove that a silicon die can be curved to the required shape with a safety factor of 4.3 in terms of the mechanical stress. Finally, it is shown that the developed system can be used in a virtual reality headset providing high resolution, low distortion and a flexible focusing mode.

physics.optics↗

arXiv2019

Next-generation telescopes with curved focal surface for ultra-low surface brightness surveys

In spite of major advances in both ground- and space-based instrumentation, the ultra-low-surface brightness universe (ULSB) still remains a largely unexplored volume in observational parameter space. ULSB observations provide unique constraints on a wide variety of objects, from the Zodiacal light all the way to the optical cosmological background radiation, through dust cirri, mass loss shells in giant stars, LSB galaxies and the intracluster light. These surface brightness levels (>28-29 mag arcsec^-2) are observed by maximising the efficiency of the surveys and minimising or removing the systematics arising in the measurement of surface brightness. Based on full-system photon Monte Carlo simulations, we present here the performance of a ground-based telescope aimed at carrying out ULSB observations, with a curved focal surface design. Its off-axis optical design maximises the field of view while minimising the focal ratio. No lenses are used, as their multiple internal scatterings increase the wings of the point spread function (PSF), and the usual requirement of a flat focal plane is relaxed through the use of curved CCD detectors. The telescope has only one unavoidable single refractive surface, the cryostat window, and yet it delivers a PSF with ultra-compact wings, which allows the detection, for a given exposure time, of surface brightness levels nearly three orders of magnitude fainter than any other current telescope.

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arXiv2018

Design and modelling of spectrographs with holographic gratings on freeform surfaces

In the present paper we demonstrate the approach of using a holographic grating on a freeform surface for advanced spectrographs design. We discuss the surface and groove pattern description used for ray-tracing. Moreover, we present a general procedure of diffraction efficiency calculation, which accounts for the change of hologram recording and operation conditions across the surface. The primary application of this approach is the optical design of the POLLUX spectropolarimeter for the LUVOR mission project where a freeform holographic grating operates simultaneously as a cross-disperser and a camera with high resolution and high dispersion. The medium ultraviolet channel design of POLLUX is considered in detail as an example. Its resolving power reaches [126,000-133,000] in the region of 118.5-195 nm. Also, we show a possibility to use a similar element working in transmission to build an unobscured double-Schmidt spectrograph. The spectral resolving power reaches 4000 in the region 350-550 nm and remains stable along the slit.

astro-ph.IM↗

arXiv2018

SPHERE on-sky performance compared with budget predictions

The SPHERE (spectro-photometric exoplanet research) extreme-AO planet hunter saw first light at the VLT observatory on Mount Paranal in May 2014 after ten years of development. Great efforts were put into modelling its performance, particularly in terms of achievable contrast, and to budgeting instrumental features such as wave front errors and optical transmission to each of the instrument's three focal planes, the near infrared dual imaging camera IRDIS, the near infrared integral field spectrograph IFS and the visible polarimetric camera ZIMPOL. In this paper we aim at comparing predicted performance with measured performance. In addition to comparing on-sky contrast curves and calibrated transmission measurements, we also compare the PSD-based wave front error budget with in-situ wave front maps obtained thanks to a Zernike phase mask, ZELDA, implemented in the infrared coronagraph wheel. One of the most critical elements of the SPHERE system is its high-order deformable mirror, a prototype 40x40 actuator piezo stack design developed in parallel with the instrument itself. The development was a success, as witnessed by the instrument performance, in spite of some bad surprises discovered on the way. The devastating effects of operating without taking properly into account the loss of several actuators and the thermally and temporally induced variations in the DM shape will be analysed, and the actions taken to mitigate these defects through the introduction of specially designed Lyot stops and activation of one of the mirrors in the optical train will be described.

astro-ph.IM↗

arXiv2018

Curved detectors developments and characterization: application to astronomical instruments

Many astronomical optical systems have the disadvantage of generating curved focal planes requiring flattening optical elements to project the corrected image on flat detectors. The use of these designs in combination with a classical flat sensor implies an overall degradation of throughput and system performances to obtain the proper corrected image. With the recent development of curved sensor this can be avoided. This new technology has been gathering more and more attention from a very broad community, as the potential applications are multiple: from low-cost commercial to high impact scientific systems, to mass-market and on board cameras, defense and security, and astronomical community. We describe here the first concave curved CMOS detector developed within a collaboration between CNRS- LAM and CEA-LETI. This fully-functional detector 20 Mpix (CMOSIS CMV20000) has been curved down to a radius of Rc = 150 mm over a size of 24x32 mm^2 . We present here the methodology adopted for its characterization and describe in detail all the results obtained. We also discuss the main components of noise, such as the readout noise, the fixed pattern noise and the dark current. Finally we provide a comparison with the flat version of the same sensor in order to establish the impact of the curving process on the main characteristics of the sensor.

astro-ph.IM↗

arXiv2018

Design of optical systems with toroidal curved detectors

We consider using toroidal curved detectors to improve the performance of imaging optical systems. We demonstrate that some optical systems have an anamorphic field curvature. We consider an unobscured re-imaging three-mirror anastigmat as an example (f'=960 mm, F/5.3, FoV 4x4 degrees). By assuming that the image is focused on a toroidal detector surface and perform re-optimization, it becomes possible to obtain a notable gain in the image quality - up to 40 % in terms of the spot RMS radius. Through analytic computations and finite-element analysis, we demonstrate that this toroidal shape can be obtained by bending of a thinned detector in a relatively simple setup.

astro-ph.IM↗

arXiv2018

POLLUX: a UV spectropolarimeter for the LUVOIR space telescope project

The present paper describes the current baseline optical design of POLLUX, a high-resolution spectropolarimeter for the future LUVOIR mission. The instrument will operate in the ultraviolet (UV) domain from 90 to 390 nm in both spectropolarimetric and pure spectroscopic modes. The working range is split between 3 channels -- far (90-124.5 nm), medium (118.5-195 nm) and near (195-390 nm) UV. Each of the channels is composed of a polarimeter followed by an echelle spectrograph consisting of a classical off-axis paraboloid collimator, echelle grating with a high grooves frequency and a cross-disperser grating operating also as a camera. The latter component integrates some advanced technologies: it is a blazed grating with a complex grooves pattern formed by holographic recording, which is manufactured on a freeform surface. One of the key features underlying the current design is the large spectral length of each order ~6 nm, which allows to record wide spectral lines without any discontinuities. The modelling results show that the optical design will provide the required spectral resolving power higher than R ~ 120,000 over the entire working range for a point source object with angular size of 30 mas. It is also shown that with the 15-m primary mirror of the LUVOIR telescope the instrument will provide an effective collecting area up to 38 569 cm 2. Such a performance will allow to perform a number of groundbreaking scientific observations. Finally, the future work and the technological risks of the design are discussed in details.

astro-ph.IM↗

arXiv2018

Curved detectors for wide field imaging systems: impact on tolerance analysis

In the present paper we consider quantitative estimation of the tolerances widening in optical systems with curved detectors. The gain in image quality allows to loosen the margins for manufacturing and assembling errors. On another hand, the requirements for the detector shape and positioning become more tight. We demonstrate both of the effects on example of two optical designs. The first one is a rotationally-symmetrical lens with focal length of 25 mm, f-ratio of 3.5 and field of view equal to 72$^\circ$, working in the visible domain. The second design is a three-mirror anastigmat telescope with focal length of 250 mm, f-ratio of 2.0 and field of view equal to $4^\circ \times 4^\circ$. In both of the cases use of curved detectors allow to increase the image quality and substantially decrease the requirements for manufacturing precision

astro-ph.IM↗

arXiv2018

Advanced optical designs of curved detectors-based two-mirrors unobscured telescopes

In the present paper we consider a family of unobscured telescope designs with curved detectors. They are based on classical two-mirror schemes -- Ritchey-Chretien, Gregorian and Couder telescopes. It is shown that all the designs provide nearly diffraction limited image quality in the visible domain for $.4^\circ \times .4^\circ$ field of view with the f-number of 7. We also provide a brief ghost analysis and point on special features of the systems with curved detectors. Finally, the detector surface shape obtained in each case is analyzed and its' technological feasibility is demonstrated.

astro-ph.IM↗

arXiv2018

Spectrographs with holographic gratings on freeform surfaces: design approach and application for the LUVOIR mission

In the present paper we demonstrate the approach to use a holographic grating on a freeform surface for advanced spectrographs design. On the example POLLUX spectropolarimeter medium-UV channel we chow that such a grating can operate as a cross-disperser and a camera mirror at the same time. It provides the image quality high enough to reach the spectral resolving power of 126 359-133 106 between 11.5 and 195 nm, which is higher than the requirement. Also we show a possibility to use a similar element working in transmission to build an unobscured double-Schmidt spectrograph. The spectral resolving power reaches 2750 for a long slit. It is also shown that the parameters of both the gratings are feasible with the current technologies.

astro-ph.IM↗