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Earth-like planets, dark energy and variability of fundamental physical constants can be discovered by observing wavelength shifts in the optical spectra of astronomical objects1–5. These wavelength shifts are so tiny that exquisitely accurate and precise wavelength calibration of astronomical spectrometers is required. Laser frequency combs, broadband spectra of laser lines with absolutely known optical frequencies, are uniquely suited for this purpose6–13, provided their lines are resolved by the spectrometer. Generating such astronomical laser frequency combs (‘astrocombs’) remains challenging. Here, a microphotonic astrocomb is demonstrated via temporal dissipative Kerr solitons14–16 in photonic-chip-based silicon nitride microresonators17, directly providing a spurious-free spectrum of resolvable calibration lines. Sub-harmonically driven by temporally structured light18, the astrocomb is stabilized to a frequency standard, resulting in absolute calibration with a precision of 25 cm s–1 (radial velocity equivalent), relevant for Earth-like planet detection and cosmological research. The microphotonic technology can be extended in spectral span17,19–24, further boosting the calibration precision. A microphotonic astrocomb is demonstrated via temporal dissipative Kerr solitons in photonic-chip-based silicon nitride microresonators with a precision of 25 cm s–1 (radial velocity equivalent), useful for Earth-like planet detection and cosmological research.

KalAO is a natural guide star adaptive optics (AO) imager to be installed on the second Nasmyth focus of the 1.2m Euler Swiss telescope in La Silla, Chile. The initial design of the system is inspired on RoboAO with modifications in order to operate in natural guide star (NGS) mode. KalAO was built to search for binarity in planet hosting stars by following-up candidates primarily from the TESS satellite survey. The optical design is optimised for the 450-900 nm wavelength range and is fitted with SDSS g,r,i,z filters. The system is designed for wavefront control down to I-magnitude 11 stars in order to probe the same parameter space as radial velocity instruments such as HARPS and NIRPS. The principal components of the system are an 11x11 10.9 cm sub-apertures Electron Multiplying CCD (EMCCD) Shack-Hartmann wavefront sensor, a 140 actuators Microelectromechanical systems (MEMS) deformable mirror, a fast tip/tilt mirror, and a graphics processing unit (GPU) powered glycol cooled real-time computer. It is designed to run at up to 1.8kHz in order to detect companions as close as the 150mas visible-light diffraction limit. The real-time adaptive optics control is using the CACAO software running on GPUs. The instrument is planned for commissioning early 2021 in Chile if the covid restrictions are lifted.

Laser frequency combs (LFC) have been shown to provide exquisite precision that can be used in astronomical spectrograph calibration [1–5], for research fields such as exoplanets, cosmology and fundamental physics.

The study of Earth-mass extrasolar planets via the radial-velocity technique and the measurement of the potential cosmological variability of fundamental constants call for very-high-precision spectroscopy at the level of $\updelta\lambda/\lambda, Comment: 15 pages, 8 figures

The quest for extrasolar planets and their characterisation as well as studies of fundamental physics on cosmological scales rely on capabilities of high-resolution astronomical spectroscopy. A central requirement is a precise wavelength calibration of astronomical spectrographs allowing for extraction of subtle wavelength shifts from the spectra of stars and quasars. Here, we present an all-fibre, 400 nm wide near-infrared frequency comb based on electro-optic modulation with 14.5 GHz comb line spacing. Tests on the high-resolution, near-infrared spectrometer GIANO-B show a photon-noise limited calibration precision of

Synchronous-driving of nonlinear optical microresonator offers robust, deterministic and ultra- efficient generation of ultra-short temporal soliton pulses as well as high-repetition rate frequency combs. The soliton pulses are all-optically controlled and provide resolvable lines for astronomical spectrometer calibration or optical telecommunications.

High resolution spectroscopy enables the detection of atmospheres of exoplanets. To reach the required radial velocity precision of about 1 m/s, calibration with even more precise sources is mandatory. HIRES will employ several calibration sources, the most important ones are an Laser Frequency Comb (LFC) and Fabry-P´erots (FP). The LFC needs to be filtered with a set of FP. One possible solution is to illuminate this set of FP with a broadband light source and use them as calibrators, when they are not used for filtering the LFC. It has been demonstrated that passively-stabilized FP can perform better than 10 cm/s per night. We give an overview of the currently used FP in different surveys and compare their individual features. For the FP which may be used in HIRES we discuss different configuration. We show that the Finesse and FSR of the FP needs to be optimized with regard to the resolution of the spectrograph and we outline how we aim to fulfill the requirements of HIRES.

CHEOPS is an ESA Class S Mission aiming at the characterization of exoplanets through the precise measurement of their radius, using the transit method [1]. To achieve this goal, the payload is designed to be a high precision “absolute” photometer, looking at one star at a time. It will be able to cover la large fraction of the sky by repointing. Its launch is expected at the end of 2017 [2, this conference]. CHEOPS’ main science is the measure of the transit of exoplanets of radius ranging from 1 to 6 Earth radii orbiting bright stars. The required photometric stability to reach this goal is of 20 ppm in 6 hours for a 9th magnitude star. The CHEOPS’ only instrument is a Ritchey-Chretien style telescope with 300 mm effective aperture diameter, which provides a defocussed image of the target star on a single frame-transfer backside illuminated CCD detector cooled to -40°C and stabilized within ~10 mK [2]. CHEOPS being in a LEO, it is equipped with a high performance baffle. The spacecraft platform provides a pointing stability of < 2 arcsec rms. This relatively modest pointing performance makes high quality flat-fielding necessary In the rest of this article we will refer to the only CHEOPS instrument simply as “CHEOP” Its behavior will be calibrated thoroughly on the ground and only a small subset of the calibrations can be redone in flight. The main focuses of the calibrations are the photonic gain stability and sensibility to the environment variations and the Flat field that has to be known at a precision better than 0.1%.

Context. Nulling interferometry has been suggested as the underlying principle for the Darwin and TPF-I exoplanet research missions. Aims. There are constraints both on the mean value of the nulling ratio, and on its stability. Instrument instability noise is most detrimental to the stability of the nulling performance. Methods. We applied a modified version of the classical dithering technique to the optical path difference in the scientific beam. Results. Using only this method, we repeatedly stabilised the dark fringe for several hours. This method alone sufficed to remove the 1/ f component of the noise in our setup for periods of 10 minutes, typically. These results indicate that performance stability may be maintained throughout the long-duration data acquisitions typical of exoplanet spectroscopy. Conclusions. We suggest that further study of possible stabilisation strategies should be an integral part of Darwin/TPF-I research and development

Nulling Interferometry applied to the search and characterization of earth-like ex- oplanets requires to eliminate the star's contribution at a rejection level (Rej = collected en- ergy/residual energy)larger than 10 6 over a large bandwidth (6 to 18 µm). Nulling test-benches are in development in several laboratories so as to master such high a rejection. One approach relies on a Mach-Zehnder set-up with Achromatic Phase Shifters (APS). One APS concept is based on the focus-crossing property, providing an intrinsically achromatic phase shift by π. Using a confocal configuration for the focus-crossing approach, a Fresnel's diffraction effect de- grades the rejection. Usual optical engineering softwares fail in assessing rejection performance and an analytical approach is needed. We describe the bench optical configuration and the Fres- nel's diffraction effect as well as a possible way for correction. Then we describe the analytical method, based on Lommel's integrals, to evaluate the expectable rejection.

The field of exoplanet research is moving towards the detection and characterization of habitable planets. These exo-Earths can be easily found around low-mass stars by using either photometric transit or radial-velocity (RV) techniques. In the latter case the gain is twofold because the signal induced by the planet of a given mass is higher due to the more favourable planet-star mass ratio and because the habitable zone lies closer to the star. However, late-type stars emit mainly in the infrared (IR) wavelength range, which calls for IR instruments. SPIRou is a stable RV IR spectrograph addressing these ambitious scientific objectives. As with any other spectrograph, calibration and drift monitoring is fundamental to achieve high precision. Our goal was to build, test and finally operate a Fabry-P\'erot-based RV-reference module able to provide the needed spectral information over the full wavelength range of SPIRou. We adapted the existing HARPS Fabry-P\'erot calibrator for operation in the IR domain. After manufacturing and assembly, we characterized the FP RV-module in the laboratory. We measured finesse, transmittance, and spectral flux of the system. The measured finesse value of F=12.8 corresponds perfectly to the theoretical value. The total transmittance at peak is of the order of 0.5%, mainly limited by fibre-connectors and interfaces. Nevertheless, the provided flux is in line with the the requirements set by the SPIRou instrument. Once installed on SPIRou, we will test the full spectral characteristics and stability of the RV-reference module. The goal will be to prove that the line position and shape stability of all lines is better than 0.3 m s$^{-1}$ between two calibration sequences (typically 24 hours), such that the RV-reference module can be used to monitor instrumental drifts., Comment: 13 pages, 21 figures, accepted on 30-01-2017 in section 13. Astronomical instrumentation of Astronomy and Astrophysics

SPIRou is a near-IR (0.98-2.35μm), echelle spectropolarimeter / high precision velocimeter being designed as a next-generation instrument for the 3.6m Canada-France-Hawaii Telescope on Mauna Kea, Hawaii, with the main goal of detecting Earth-like planets around low mass stars and magnetic fields of forming stars. The unique scientific and technical capabilities of SPIRou are described in a series of seven companion papers. In this paper, the fiber links which connects the polarimeter unit to the cryogenic spectrograph unit (35 meter apart) are described. The pupil slicer which forms a slit compatible with the spectrograph entrance specifications is also discussed in this paper. Some challenging aspects are presented. In particular this paper will focus on the manufacturing of 35 meter fibers with a very low loss attenuation (< 13dB/km) in the non-usual fiber spectral domain from 0.98 μm to 2.35 μm. Other aspects as the scrambling performance of the fiber links to reach high accuracy radial velocity measurements (1m/s) and the design of the pupil slicer exposed at a cryogenic and vacuum environment will be discussed., Ground-Based and Airborne Instrumentation for Astronomy IV, July 1-6, 2012, Amsterdam, Netherlands, Series: Proceedings of SPIE

For the PRV instrument using simultaneous calibration technique such as with sources like thorium lamps, Fabry Perot Etalons or laser comb, it is essential that the instrument stays stable between the wavelength calibration frame and the actual scientific measurement. These instruments are usually in pressure and temperature controlled environments for example under vacuum. However this is not sufficient to reach the instrumental stability required to get to precision level of the ms -1 and below required to build the next generation PRV instruments. Another requirement is an as constant as possible illumination of the spectrograph to stabilize the line profile of the instrument. To achieve this, it is necessary to use a device that will scramble the light coming from the star to mitigate the effects of the atmosphere. In addition this device should not increase significantly the beam etendue, which is already a technological challenge for large telescopes. The common solution to this problem is to use optical fibers. Historically the solution has been to use circular fibers as they were the only one available. Recently for other purposes non-circular fibers have been developed and made available. They have been tested, and present an important improvement in the scrambling over the circular fibers. We will present in this paper the properties of the octagonal fibers used for the HARPS-N2 instrument and the achieved performance of its fiber train.

The Observatory of Geneva has designed, built and tested in collaboration with ESO a calibrator system based on a Fabry-Perot (FP) interferometer to explore its potential in the calibration of radial velocity (RV) spectrographs. Today, the RV technique has pushed th e planet detection limits down to s uper-earths but the reach the precision required to detect earth-like planets it is necessary to reach a precision around 1cm s -1 . While a significant part of the error budget is the incompressible photon noise, another part is the noise in the wavelength calibration of the spectrograph. It is to address this problem that we have developed this new device. We have obtained exciting results with the calibrator system demonstrated 10 cm s -1 stability over one night and 1 m s over 60 days. By further improving the injection system we are aiming at a 1 m s -1 repeatability over the long term. Keywords: extra-solar planets, radial velocity m easurement, high resolution spectrograph

The NULLTIMATE project developed and realized three concepts of achromatic phase shifters for nulling interferometry. One of the concepts is based on dispersive plates made of three materials which where fully characterized regarding their refractive index and thermo-optic behavior between 100K and 330 K. The other two concepts are based on mirror optics, one of which uses the phase shift of π when crossing a focus, the other the reversal of electric fields at reflection. An optical bench has been set up to test and characterize these phase shifters at wavelengths 2 − 2.4 μm with the option of changing to the 10 μm domain. We summarize the development of the achromatic phase shifters and report on the current status of the test bench.

We present in this paper experimental data on fibres and scramblers to increase the photometrical stability of the spectrograph PSF. We have used round, square, octagonal fibres and beam homogenizers. This study is aimed to enhance the accuracy measurements of the radial velocities for ESO ESPRESSO (VLT) and CODEX (E-ELT) instruments.

In the last six years, thanks to the very high radial velocity precision of the HARPS spectrograph, it was possible to detect 21 out of the 30 super-Earth (extrasolar planets masses below 20 times the mass of the Earth) discovered up to date. The radial velocity precision of the instrument is estimated around 80 cm/s on a single measurement. The main instrumental limitations are the wavelength calibration and the stability of the light injection. We address both factors and present the results of recent tests on the HARPS spectrograph. We have identified the laser frequency comb as the ideal wavelength calibrator, due to the width, density and flux of the lines, and to its intrinsic stability. The results from the recent tests that we performed on HARPS are encouraging. The accurate guiding of the telescope is critical to maintain a stable light distribution at the injection stage, where the light is sent into the spectrograph entrance fiber. To pursue this goal we are testing a secondary guiding system which is able to apply the guiding corrections twenty times faster than the primary guiding system.

One of the remaining limitation of the precise radial velocity instruments is the imperfect scrambling produced by the circular fibers. We present here experimental studies on new optical fibers aiming at an improvement of the scrambling they provide. New fibers shapes were tested: square and octagonal. Measurements have been performed of the scrambling performances of these fibers in the near field as well FRD measurements. These fibers show extremely promising performances in the near field scrambling: an improvement of a factor 5 to 10 compared to the circular fiber. They however show some strange behavior in the far field that need to be understood.

The radial velocity (RV) technique has pushed the planet detection limits down to super-earths. To reach the precision required to detect ear th-like planets it is necessary to reach a precision around 1cm.s -1 . Part of the error budget is due to noise in the wavelength calibration of the spectrograph. The Observatory of Geneva has designed, built and tested in collaboration with ESO a calibrator syst em based on a Fabry-Perot interferometer to explore its potential to improve the wavelength calibration of RV spectr ographs. We have obtained exciting results with the calibrator system demonstrated 10 cm s -1 stability over one night. By further improving the injection system we are aiming at a 1 m s -1 repeatability over the long term. Keywords: extra-solar planets, radial velocity m easurement, high resolution spectrograph 1. INTRODUCTION The radial velocity (RV) technique is so far the most powerful extra-solar planets discovery tool. With the current precision achieved by RV of 69 cm.s

Since its first light in 2003, the HARPS radial velocity spectrograph (RVS) has performed exquisitely well on the 3.6m ESO telescope at La Silla Observatory (Chile). It now routinely exhibits a measurement noise of 0.5 m/s or 1.7 10-9 on a relative scale. Despite innovative work by Lovis and colleagues [14] to improve the accuracy obtained with the calibration lamps used, there is evidence that still better performance could be achieved by using more stable wavelength standards. In this paper, we present two methods are aim at overcoming the shortcoming of present day calibrators and that could satisfy the need for a cm/s -level calibrator like we are planning on using on the 2nd generation instruments at the VLT and on the ELT instrumentation. A temperature-stabilized Fabry-Perot interferometer has the promise of being stable to a few cm/s and has very uniform line levels and spacings, while a laser comb has already achieved a precision better than 15 cm/s, despite using only one of the 72 orders of the spectrographs.

The achromatic phase shifter (APS) is a component of the Bracewell nulling interferometer studied in preparation for future space missions (viz. Darwin/TPF-I) focusing on spectroscopic study of Earth-like exo-planets. Several possible designs of such an optical subsystem exist. Four approaches were selected for further study. Thales Alenia Space developed a dielectric prism APS. A focus crossing APS prototype was developed by the OCA, Nice, France. A field reversal APS prototype was prepared by the MPIA in Heidelberg, Germany. Centre Spatial de Li\`ege develops a concept based on Fresnel's rhombs. This paper presents a progress report on the current work aiming at evaluating these prototypes on the SYNAPSE test bench at the Institut d'Astrophysique Spatiale in Orsay, France.

Nulling interferometry has been suggested as the underlying principle for an instrument which could provide direct detection and spectroscopy of Earth-like exo-planets, including searches for potential bio-signatures. This paper documents the potential of optical path difference (OPD) stabilisation with dithering methods for improving the mean nulling ratio and its stability. The basic dithering algorithm, its refined versions and parameter tuning, are reviewed. This paper takes up the recently presented results 1 and provides an update on OPD-stabilisation at significantly higher levels of nulling performance.

International audience; We revisit the nulling interferometer performances that are needed for direct detection and the spectroscopic analysis of exoplanets, e.g., with the DARWIN [European Space Agency-SCI 12 (2000)] or TPF-I [JPL Publ. 05-5, (2005)] missions. Two types of requirement are found, one concerning the mean value of the instrumental nulling function and another regarding its stability. The stress is usually put on the former. It is stringent at short wavelengths but somewhat relaxed at longer wavelengths. The latter, which we call the variability noise condition, does not usually receive enough attention. It is required regardless of telescope size and stellar distance. The results from three nulling experiments performed in laboratories around the world are reported and compared with the requirements. All three exhibit 1/f noise that is incompatible with the performances required by the mission. As pointed out by Lay [Appl. Opt. 43, 6100-6123 (2004)], this stability problem is not fully solved by modulation techniques. Adequate solutions must be found that are likely to include servo systems using the stellar signal itself as a reference and internal metrology with high stability.

International audience

The Darwin/TPF mission aims at detecting directly extra solar planets. It is based on the nulling interferometry, concept proposed by Bracewell in 1978, and developed since 1995 in several European and American laboratories. One of the key optical devices for this technique is the achromatic phase shifter (APS). This optical component is designed to produce a π phase shift over the whole Darwin spectral range (i.e. 6-18 μm), and will be experimentally tested on the NULLTIMATE consortium nulling test bench (Labeque et al). Three different concepts of APS are being simulated: dispersive plates focus crossing and field reversal. In this paper, we show how thermal, mechanical and optical models are merged into a single robust model, allowing a global numerical simulation of the optical component performances. We show how these simulations help us to optimizing the design and present results of the numerical model.

The Darwin mission is a project of the European Space Agency that should allow around 2015 the search for extrasolar planets and a spectral analysis of their potential atmospheres in order to detect gases and particularly tracers of life. The basic concept of the instrument is a Bracewell nulling interferometer. It allows high angular resolution and high dynamic range. However, this concept, proposed 25 years ago, is very difficult to implement with high rejection factor and has to be demonstrated in laboratory before being applied in space. Theoretical and numerical approaches of the question highlight strong difficulties : - The need for very clean and homogeneous wavefronts, in terms of intensity, phase and polarisation distribution ; - The need for achromatic optical devices working in a wide spectral range (typically 6 to 18 microns for the space mission). A solution to the first point is the optical filtering which has been successfully experimentally demonstrated at 10 microns using a single mode laser. We focus now on the second point and operate a test bench working in the near infrared, where the background constraints are reduced. We present this test bench and the first encouraging results in the 2-4 microns spectral range. We particularly focus on recent optical developments concerning achromatic component, and particularly the beam combiners and the phase shifter, which are keypoints of the nulling interferometry principle. Finally, we present the future of this experimental demonstration, in the thermal infrared, covering the real and whole spectral range of Darwin.

For fiber-fed spectrographs with a stable external wavelength source, scrambling properties of optical fibers and, homogeneity and stability of the instrument illumination are important for the accuracy of radial-velocimetry. Optical cylindric fibers are known to have good azimuthal scrambling. In contrast, the radial one is not perfect. In order to improve the scrambling ability of the fiber and to stabilize the illumination, optical double scrambler are usually coupled to the fibers. Despite that, our experience on SOPHIE and HARPS has lead to identified remaining radial-velocity limitations due to the non-uniform illumination of the spectrograph. We conducted tests on SOPHIE with telescope vignetting, seeing variation and centering errors on the fiber entrance. We simulated the light path through the instrument in order to explain the radial velocity variation obtained with our tests. We then identified the illumination stability and uniformity has a critical point for the extremely high-precision radial velocity instruments (ESPRESSO@VLT, CODEX@E-ELT). Tests on square and octagonal section fibers are now under development and SOPHIE will be used as a bench test to validate these new feed optics., to appear in the Proceedings conference "New Technologies for Probing the Diversity of Brown Dwarfs and Exoplanets", Shanghai, 2009