High-resolution spectropolarimetry is a useful astronomical technique, in particular to study stellar magnetic fields. It has been extensively used in the past but mostly in the visible range. Space missions equipped with high-resolution spectropolarimeters working in the ultra-violet (UV) are now being studied. We propose a concept of a polarimeter working with temporal modulation and allowing to perform Stokes IQUV measurements over the full UV + Visible range. The purpose of this article is to describe the polarimeter concept, two prototypes and the bench developed to perform on ground testing to establish the performances of this new polarimeter.
PLATO (PLAnetary Transits and Oscillation of stars) is the ESA Medium size dedicated to exo-planets discovery, adopted in the framework of the Cosmic Vision program. The PLATO launch is planned in 2026 and the mission will last at least 4 years in the Lagrangian point L2. The primary scientific goal of PLATO is to discover and characterize a large amount of exo-planets hosted by bright nearby stars, constraining with unprecedented precision their radii by mean of transits technique and the age of the stars through by asteroseismology. By coupling the radius information with the mass knowledge, provided by a dedicated ground-based spectroscopy radial velocity measurements campaign, it would be possible to determine the planet density. Ultimately, PLATO will deliver the largest samples ever of well characterized exo-planets, discriminating among their ‘zoology’. The large amount of required bright stars can be achieved by a relatively small aperture telescope (about 1 meter class) with a wide Field of View (about 1000 square degrees). The PLATO strategy is to split the collecting area into 24 identical 120 mm aperture diameter fully refractive cameras with partially overlapped Field of View delivering an overall instantaneous sky covered area of about 2232 square degrees. The opto-mechanical sub-system of each camera, namely Telescope Optical Unit, is basically composed by a 6 lenses fully refractive optical system, presenting one aspheric surface on the front lens, and by a mechanical structure made in AlBeMet.
The space mission Arago is proposed as a candidate to ESA’s Cosmic Vision M5 call by the UVMag consortium. Arago is dedicated to the study of the dynamic 3D environment of stars and planets. Thanks to a high-resolution UV and visible spectropolarimeter, the instrument will detect and characterize the magnetic fields of the stars, their environment and its impact on exoplanets. Scientific requirements impose a wide spectral range from 119 to 888 nm with a single full-Stokes polarimeter followed by two high-resolution spectrographs. To achieve these stringent specifications, a polychromatic concept of polarimeter has been studied and tested thanks to a R and T study funded by CNES. Using an optimized combination of Magnesium Fluoride plates followed by a polarization analyzer, it measures all four Stokes parameters with a constant efficiency over the spectral range. This is performed with a sequence of 6 sub-exposures acquired with different plate angles. The two orthogonal polarized beams coming out of the polarimeter feed two spectrographs. The UV spectrograph has a spectral resolution of at least 25000 over its spectral range, while the visible spectrograph works at least at 35000. Finally, to image the high-resolution spectra, a CCD detector and a MCP were chosen for the visible and UV arms of the instrument respectively.
This paper describes the complete optical design of Arago’s instrument, as proposed to ESA as an answer to its M5 call, from the 1.3-m diameter telescope to the detectors. The design of the polarimeter is presented as well as the unusual way of demodulating the polarization information, in order to have a polychromatic polarimeter working with the same efficiency from FUV to NIR. The optical design of the UV and visible échelle spectrographs and their detection chains are also presented, as well as the achieved performances.
The UVMag consortium will propose the Arago space mission to the ESA call Cosmic Vision M5. This mission aims at characterizing all kind of stars and their environment simultaneously, to better understand the cycle of matter in our galaxy. It carries a single instrument, a spectropolarimeter, acquiring data from 119 to 888 nm and enabling the determination of the magnetic field of stars thanks to the Zeeman effect. One of the key instrumental point of this project is the development of an efficient polarimeter over the large spectral range and in space. We chose to use a polychromatic temporal modulation to achieve a measurement of all four Stokes parameters: I the intensity, Q and U the linear polarization states, and V the circular polarization. The modulator is composed by several birefringent Magnesium Fluoride plates, optimized to achromatize the extraction efficiency of the Stokes parameters from the FUV to the NIR. This polarization modulator is followed by a polarization beam-splitter to analyze the state of the light. After the polarization analysis, the light goes through a high-resolution spectrograph. We present the theoretical optimization and design of the polarimeter and of the whole instrument, as well as the first laboratory results on this concept.
Developing an efficient and robust polarimeter for wide spectral ranges and space applications is a main issue in many projects. As part of the UVMag consortium created to develop UV facilities in space (e.g. the Arago mission proposed to ESA), we are studying an innovative concept of polarimeter that is robust, simple, and efficient on a wide spectral range. The idea, based on the article by Sparks et al. (2012), is to use polarization scramblers to create a spatial modulation of the polarization. Along the height of the wedges of the scramblers, the thickness of the birefringent material crossed by the light, and thus the retardance, vary continuously. This variation creates an intensity modulation of the light related to the entrance polarization state. Analyzing this modulation with a linear polarizer, and dispersing the light spectrally in the orthogonal spatial direction, enables the measurement of the full Stokes vector over the entire spectrum. This determination is performed with a single-shot measurement and without any moving parts in the system.
After a quick introduction to the concept and optical design, this article presents the tolerancing study of the optical bench using this spectropolarimeter. The impact of different error sources, such as, birefringence uncertainty or decenter of the wedges, is investigated.
UVMag is a project of a space mission equipped with a high-resolution spectropolarimeter working in the UV and visible range. This M-size mission will be proposed to ESA at its M4 call. The main goal of UVMag is to measure the magnetic fields, winds and environment of all types of stars to reach a better understanding of stellar formation and evolution and of the impact of stellar environment on the surrounding planets. The groundbreaking combination of UV and visible spectropolarimetric observations will allow the scientists to study the stellar surface and its environment simultaneously. The instrumental challenge for this mission is to design a high-resolution space spectropolarimeter measuring the full- Stokes vector of the observed star in a huge spectral domain from 117 nm to 870 nm. This spectral range is the main difficulty because of the dispersion of the optical elements and of birefringence issues in the FUV. As the instrument will be launched into space, the polarimetric module has to be robust and therefore use if possible only static elements. This article presents the different design possibilities for the polarimeter at this point of the project.