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.
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<sup>2</sup> . 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.
The daytime sky has recently been demonstrated as a useful calibration tool for deriving polarization cross-talk properties of large astronomical telescopes. The Daniel K. Inouye Solar Telescope and other large telescopes under construction can benefit from precise polarimetric calibration of large mirrors. Several atmospheric phenomena and instrumental errors potentially limit the technique’s accuracy. At the 3.67-m AEOS telescope on Haleakala, we performed a large observing campaign with the HiVIS spectropolarimeter to identify limitations and develop algorithms for extracting consistent calibrations. Effective sampling of the telescope optical configurations and filtering of data for several derived parameters provide robustness to the derived Mueller matrix calibrations. Second-order scattering models of the sky show that this method is relatively insensitive to multiple-scattering in the sky, provided calibration observations are done in regions of high polarization degree. The technique is also insensitive to assumptions about telescope-induced polarization, provided the mirror coatings are highly reflective. Zemax-derived polarization models show agreement between the functional dependence of polarization predictions and the corresponding on-sky calibrations.
The THEMIS solar telescope is building a classical adaptive optics (AO) system to be operating on the Sun in 2017. To make compatible its excellent dual beam spectropolarimetric features with the AO also requires a major refurbishment of the relay optics starting at the M2 and down to the spectrograph entrance. This paper presents the design parameters and expected performances of our AO system, and explains why and how we intend to control to a few percent the Mueller matrix of the whole optical path from the prime focus to the spectropolarimetric cameras. This project is co-funded by the European Union SOLARNET Project Ref.:312495, and the Centre National de la Recherche Scientifique.
The daytime sky has been recently demonstrated as a useful calibration tool for deriving polarization cross-talk properties of large astronomical telescopes. The Daniel K Inouye Solar Telescope (DKIST) and other large telescopes under construction can benefit from precise polarimetric calibration of large off-axis mirrors. Several atmospheric phenomena and instrumental errors potentially limit the techniques accuracy. At the 3.67m AEOS telescope on Haleakala, we have performed a large observing campaign with the HiVIS spectropolarimeter to identify limitations and develop algorithms for extracting consistent calibrations. Effective sampling of the telescope optical configurations and filtering of data for several derived parameters provide robustness to the derivedMueller matrix calibrations. Second-order scattering models of the sky show that this method is relatively insensitive to assumptions about telescope induced polarization provided the mirror coatings are highly reflective. Zemax-derived polarization models show agreement between predictions and on-sky calibrations.
This communication presents a family of spectrographs designed for the European Solar Telescope. They can operate in
four different configurations: a long slit standard spectrograph (LsSS), two devices based on subtractive double pass
(TUNIS and MSDP) and one based on an integral field, multi-slit, multi-wavelength configuration. The combination of
them composes the multi-purpose grating spectrograph of EST, focused on supporting the different science cases of the
solar photosphere and chromosphere in the spectral range from 3900 Å to 23000 Å. The different alternatives are made
compatible by using the same base spectrographs and different selectable optical elements corresponding to specific
subsystems of each configuration.
EST (European Solar Telescope) is a 4-m class solar telescope, which is currently in the conceptual design phase. EST
will be located at the Canary Islands and aims at observations with the best possible spectral, spatial and temporal
resolution and best polarimetric performance, of the solar photosphere and chromosphere, using a suite of instruments
that can efficiently produce two-dimensional spectropolarimetric information of the thermal, dynamic and magnetic
properties of the plasma over many scale heights, and ranging from λ=350 until 2300 nm.
In order to be able to fulfill the stringent requirements for polarimetric sensitivity and accuracy, from the very beginning
the polarimetry has been included in the design work. The overall philosophy has been to use a combination of
techniques, which includes a telescope with low (and stable) instrumental polarization, optimal full Stokes polarimeters,
differential measurement schemes, fast modulation and demodulation, and accurate calibration.
The current baseline optical layout consists of a 14-mirror layout, which is polarimetrically compensated and nonvarying
in time. In the polarization free F2 focus ample space is reserved for calibration and modulators and a
polarimetric switch. At instrument level the s-, and p-planes of individual components are aligned, resulting in a system
in which eigenvectors can travel undisturbed through the system.
EST is a project for a 4-meter class telescope to be located in the Canary Islands. EST will be optimized for studies of
the magnetic coupling between the photosphere and the chromosphere. This requires high spatial and temporal resolution
diagnostics tools of properties of the plasma, by using multiple wavelength spectropolarimetry. To achieve these goals,
visible and near-IR multi-purpose spectrographs are being designed to be compatible with different modes of use: LsSS
(Long-slit Standard Spectrograph), multi-slit multi-wavelength spectrograph with an integral field unit, TUNIS (Tunable
Universal Narrow-band Imaging Spectrograph), and new generation MSDP (Multi-channel Subtractive Double-pass
Spectrograph). In this contribution, these different instrumental configurations are described.
Measuring vector magnetic fields in the solar atmosphere using the profiles of the Stokes parameters of polarized spectral lines split by the Zeeman effect is known as Stokes Inversion. This inverse problem is usually solved by least-squares fitting of the Stokes profiles. However least-squares inversion is too slow for the new generation of solar instruments (THEMIS, SOLIS, Solar-B, ...) which will produce an ever-growing flood of spectral data. The solar community urgently requires a new approach capable of handling this information explosion, preferably in real-time. We have successfully applied pattern recognition and machine learning techniques to tackle this problem. For example, we have developed PCA-inversion, a database search technique based on Principal Component Analysis of the Stokes profiles. Search is fast because it is carried out in low dimensional PCA feature space, rather than the high dimensional space of the spectral signals. Such a data compression approach has been widely used for search and retrieval in many areas of data mining. PCA-inversion is the basis of a new inversion code called FATIMA (Fast Analysis Technique for the Inversion of Magnetic Atmospheres). Tests on data from HAO's Advanced Stokes Polarimeter show that FATIMA isover two orders of magnitude faster than least squares inversion. Initial tests on an alternative code (DIANNE - Direct Inversion based on Artificial Neural NEtworks) show great promise of achieving real-time performance. In this paper we present the latest achievements of FATIMA and DIANNE, two powerful examples of how pattern recognition techniques can revolutionize data analysis in astronomy.