European Solar Telescope Research Articles

Analysis and design of PI-based controllers for the axes of European Solar Telescope

European Solar Telescope (EST) represents the largest European research infrastructure projected in the field of solar astrophysics. It is a 4-meter class solar telescope currently in the design phase. Within this phase, an essential objective is to analyze the control strategy of the telescope axes to achieve an accurate sun trajectory tracking. This paper presents a cascade control scheme based on proportional-integral (PI) controllers specifically designed for the elevation and azimuth axes of EST. The control strategy, which employs nested position-velocity loops, is devised to adeptly track the sun’s trajectory while mitigating wind impact. The controller tuning adheres to criteria of relative stability margins and bandwidth constraints using the state-space representation derived from Finite Element Model of the EST structure. An end-to-end dynamics model has been proposed to evaluate the performance of the control strategy during a ten-minute simulation. Results evidenced the convenience of the proposed controllers, highlighting wind as the main source of the control error (0.78 arcsec and 0.14 arcsec in azimuth and elevation axes, respectively) compared to the sun tracking error. Future control strategy enhancements should target the reduction of wind-induced effects to meet stringent image stability requirements.

The European Solar Telescope

The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope, the German Vacuum Tower Telescope and GREGOR, the French Télescope Héliographique pour l’Étude du Magnétisme et des Instabilités Solaires, and the Dutch Open Telescope. With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems.

First Light of the Integral Field Unit of GRIS on the GREGOR Solar Telescope

An Integral Field Unit (IFU) based on image slicers has been added to the GREGOR Infrared Spectrograph (GRIS). This upgrade to the instrument makes possible 2D spectropolarimetry in the near-infrared by simultaneously recording the full Stokes profiles of spectral lines (in a given spectral interval) at all the points in the field of view (FOV). It provides high-cadence spectropolarimetric observations at the instrument’s high spatial resolution and high polarization sensitivity at the GREGOR solar telescope. The IFU is ideal for observing the polarized spectrum of fast-evolving solar features at high spatial and spectral resolutions. The high observing cadence opens the possibility of time-series observations. The analysis of observations to this level of accuracy is essential for understanding the complex dynamics and interactions of solar plasma and magnetic fields. The image slicer of the IFU has eight slices of width 100[Formula: see text][Formula: see text]m, covering a total FOV of [Formula: see text]. It was designed and built within the framework of the European projects SOLARNET and GREST, as a prototype for future instruments of the European Solar Telescope (EST) and was integrated into GRIS. After two commissioning campaigns in 2017 and 2018, the IFU was finally installed at the end of September 2018 and offered to all observers who use the telescope.

Optical design of an image-slicer integral field unit with two output slits

The optical design of an integral field unit (IFU) based on image-slicers represents an interesting design exercise because of the small size of the components and the critical parameters to reduce optical aberrations. When the IFU to be designed is also intended to substitute an IFU prototype that is already integrated into a spectropolarimeter, it becomes even more challenging. The main reason to upgrade the IFU prototype is to cover a larger field of view (FOV). One of the work packages of the European project “Getting Ready for the European Solar Telescope” (GREST) had as its objective to find the best option to design such an IFU. After analyzing the requirements and the design considerations, the optical design pointed toward an IFU that doubles the FOV, reaching a value of 40.5 arcsec2. The designed IFU is composed of two image-slicers coupled together, each one with eight mirrors of 1.8 mm × 100 μm that feed two slits to the spectrograph.

Canarian Observatories Meteorology; Comparison of OT and ORM using Regional Climate Reanalysis

The present work has been carried out as a support for site selection for The European Solar Telescope (EST), a 4 m telescope project that will be located in the Canary Islands Observatories. For such purpose, we present a comparative analysis of a long time-series of 17 yr of climate parameters with significance in astronomical site testing for Teide (OT) and Roque de Los Muchachos (ORM) observatories. The climate parameters have been obtained from a climate reanalyses database with a spatial resolution of 11 km × 11 km, which has been compared with local averaged data using Automatic Weather Stations (AWS) and radiosounding. The results show the reliability of using climate reanalysis databases for site-testing both at the heights of the OT and ORM observatories (boundary layer) and in the free atmosphere. Climate parameters analyzed are relative humidity, temperature, wind velocity, and wind direction. For the boundary layer, we have obtained full time-series, annual behavior, and statistical distributions for each parameter. Monthly averaged values are given by using a 17 yr time period. Statistics are given for two time periods: Summer (June to August) and non-summer (the rest). The summer period has been selected according to the most intense observational period along the year for the EST. Day/night variations of the climate parameters have been also analyzed. Both observatories show values in the range 6%–70% for relative humidity, 2.6°C–19°C for temperature, and 5–12 m s−1 for wind velocity, 95% of the time; median values are around 25%, 10°C, and 5 m s−1, respectively. The most frequent wind direction (wind blowing from) is West for both observatories (∼20%), with a second peak at Northeast (∼19%) for ORM and Northwest for OT (∼17%). Less frequent wind directions are South and Southeast (≲5% in both cases). No significant difference between OT and ORM in these climate parameters have been found. Using the relative humidity, temperature, wind, and velocity limits for EST we have estimated an operational time ≳98%. Our results confirm the quality of ORM and OT observatories with stable and predictable weather conditions.

Capabilities of bisector analysis of the Si I 10 827 Å line for estimating line-of-sight velocities in the quiet Sun

We examine the capabilities of a fast and simple method to infer line-of-sight (LOS) velocities from observations of the photospheric Si I 10 827 Å line. This spectral line is routinely observed together with the chromospheric He I 10 830 Å triplet as it helps to constrain the atmospheric parameters. We study the accuracy of bisector analysis and a line core fit of Si I 10 827 Å. We employ synthetic profiles starting from the Bifrost enhanced network simulation. The profiles are computed solving the radiative transfer equation, including non-local thermodynamic equilibrium effects on the determination of the atomic level populations of Si I. We found a good correlation between the inferred velocities from bisectors taken at different line profile intensities and the original simulation velocity at given optical depths. This good correlation means that we can associate bisectors taken at different line-profile percentages with atmospheric layers that linearly increase as we scan lower spectral line intensities. We also determined that a fit to the line-core intensity is robust and reliable, providing information about atmospheric layers that are above those accessible through bisectors. Therefore, by combining both methods on the Si I 10 827 Å line, we can seamlessly trace the quiet-Sun LOS velocity stratification from the deep photosphere to higher layers until around logτ = −3.5 in a fast and straightforward way. This method is ideal for generating quick-look reference images for future missions like the Daniel K. Inoue Solar Telescope and the European Solar Telescope, for example.

Revisiting the building blocks of solar magnetic fields by GREGOR

AbstractThe Sun is our dynamic host star due to its magnetic fields causing plentiful of activity in its atmosphere. From high energetic flares and coronal mass ejections (CMEs) to lower energetic phenomena such as jets and fibrils. Thus, it is of crucial importance to learn about formation and evolution of solar magnetic fields. These fields cover a wide range of spatial and temporal scales, starting on the larger end with active regions harbouring complex sunspots, via isolated pores, down to the smallest yet resolved elements – so-called magnetic bright points (MBPs). Here, we revisit the various manifestations of solar magnetic fields by the largest European solar telescope in operation, the 1.5-meter GREGOR telescope. We show images from the High-resolution Fast Imager (HiFI) and spectropolarimetric data from the GREGOR Infrared Spectrograph (GRIS). Besides, we outline resolved convective features inside the larger structures – so-called light-bridges occurring on large to mid-sized scales.

Recent advancements in the EST project

The European Solar Telescope (EST) is a project of a new-generation solar telescope. It has a large aperture of 4 m, which is necessary for achieving high spatial and temporal resolution. The high polarimetric sensitivity of the EST will allow to measure the magnetic field in the solar atmosphere with unprecedented precision. Here, we summarise the recent advancements in the realisation of the EST project regarding the hardware development and the refinement of the science requirements.

An Assessment of and Solution to the Intensity Diffusion Error Intrinsic to Short-characteristic Radiative Transfer Methods

Abstract Radiative transfer coupled with highly realistic simulations of the solar atmosphere is routinely used to infer the physical properties underlying solar observations. Due to its computational efficiency, the method of short-characteristics is often employed, despite it introducing numerical diffusion as an interpolation artifact. In this paper, we quantify the effect of the numerical diffusion on the spatial resolution of synthesized emergent intensity images, and derive a closed form analytical model of the diffusive error made as a function of viewing angle when using linear interpolation. We demonstrate that the consequent image degradation adversely affects the comparison between simulated data and observations away from disk center, unless the simulations are computed at much higher intrinsic resolutions than the observations. We also show that the diffusive error is readily avoided by interpolating the simulation solution on a viewing angle aligned grid prior to computing the radiative transfer. Doing this will be critical for comparisons with observations using the upcoming large aperture telescopes—the Daniel K. Inouye Solar Telescope and the European Solar Telescope.

Advanced DePFET concepts: Quadropix

Currently several development activities for the European Solar Telescope (EST) are ongoing, one of these is Getting Ready for EST (GREST), a venture for the development of new sensor and detector technologies for EST. Among other objectives, EST will perform time resolved, high precision polarimetric imaging. These measurements require detector systems that are capable of detecting even smallest changes in the polarization of light. At the MPG-HLL we developed a sensor concept for such a system, the Quadropix DEPFET. These devices offer in-situ storage of up to four independent images without reducing the sensors fill-factor. For polarimetric applications this properties can be used for polarimetry measurements at high modulation frequencies.

MuSICa: THE MULTI-SLIT IMAGE SLICER FOR THE EST SPECTROGRAPH

Integral field spectroscopy (IFS) is a technique that allows one to obtain the spectra of all the points of a bidimensional field of view simultaneously. It is being applied to the new generation of the largest night-time telescopes but it is also an innovative technique for solar physics. This paper presents the design of a new image slicer, MuSICa (Multi-Slit Image slicer based on collimator-Camera), for the integral field spectrograph of the 4-m aperture European Solar Telescope (EST). MuSICa is a multi-slit image slicer that decomposes an 80 arcsec2 field of view into slices of 50 μm and reorganizes it into eight slits of 0.05 arcsec width × 200 arcsec length. It is a telecentric system with an optical quality at diffraction limit compatible with the two modes of operation of the spectrograph: spectroscopic and spectro-polarimetric. This paper shows the requirements, technical characteristics and layout of MuSICa, as well as other studied design options.

A HIGH RESOLUTION INTEGRAL FIELD SPECTROGRAPH FOR THE EUROPEAN SOLAR TELESCOPE

This paper presents the proposal of a high resolution, integral field spectrograph that is currently being designed for the 4-meter aperture European Solar Telescope that will be located in the Canary Islands. This instrument is optimized to study the solar chromosphere and photosphere to allow the investigation of several phenomena concentrated within these two layers. It will be able to observe simultaneously a bidimensional field of view of 80 arcsec2 that is reorganized, using an integral field unit, into 8 long slits of 200 arcsec length by 0.05 arcsec width. It will have the capability to observe different layers of the Sun at the same time due to its multi-wavelength capability that allows the observation of 5 visible and 3 near-infrared wavelength intervals from 3900 to 23,000 Å, with a spectral resolution of about 300,000. The designed instrument is telecentric and presents an optical quality limited by diffraction.

New generation Multichannel Subtractive Double Pass for imaging spectropolarimetry – The S4I prototype

Imaging spectropolarimetry is a powerful tool in solar physics. Multichannel Subtractive Double Pass (MSDP) spectrographs can produce simultaneous 2D images in a number of wavelengths covering several line profiles with the following advantages: - Contrary to long slit spectroscopy, the spatial resolution can reach the diffraction limit. - Contrary to narrow band filters, full line profiles are recorded at the same time without any image motion and differential seeing effect. MSDP spectrographs use an entrance window instead of the usual long slit. A new generation MSDP using slicers made of mirrors instead of prisms is proposed for the future 4 m class European Solar Telescope. We describe the beam slicer prototype S4I (Spectral Sampling with Slicer for Solar Instrumentation) that will be tested soon at the 14 m spectrograph of the Meudon Solar Tower.

Adaptive optics development at the German solar telescopes

We present an overview of the past achievements and future developments of adaptive optics (AO) for the German solar telescopes. The basic design considerations as well as the differences in wavefront sensing between solar and stellar AO are presented. The AO systems at the 0.7m Vacuum Tower Telescope and the new 1.5m GREGOR Telescope, Tenerife, the image stabilization and active optics for the 1.0mSUNRISE balloon-borne telescope, and the ongoing multiconjugate adaptive optics (MCAO) development for GREGOR are described. The MCAO for GREGOR also serves as a test bed for the planned European Solar Telescope.

The EST project

AbstractEST European Solar Telescope is a pan-european project, presently in its Conceptual Design Study financed by the European Commission in the framework of FP7, involving 29 partners, from 14 different countries. The EST project is aimed at the realization of a 4-m class telescope, characterized by an optical design and a set of instruments optimized for extremely high resolution imaging and spectropolarimetric observations from near UV to NIR. EST will be four times larger than any existing high resolution solar telescope and it is designated with the highest priority among the ground-based, medium term (2016-2020) new projects in the ASTRONET Roadmap (Panel C). The EST instruments will measure fundamental astrophysical processes at their intrinsic scales in the Sun's atmosphere to establish the mechanism of magnetic field generation and removal, and of energy transfer from the surface to the upper solar atmosphere and eventually to the whole heliosphere. The conceptual Design Study started on February 2008 and will finish during 2011. EST will be operational at the same time as major ESA and NASA space missions aimed at studying solar activity.

EST Adaptive optics performance estimations

AbstractWe give a short overview of the Adaptive Optics (AO) and Multi‐conjugate Adaptive Optics (MCAO) system of the planned 4 m European Solar Telescope (EST). The optimization process of the AO / MCAO parameters is shown, including the parameters and layout of the Shack‐Hartmann wavefront sensor setup and the DMs. We show the expected performance of the AO and MCAO system (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

European Solar Telescope: Progress status

AbstractIn this paper, the present status of the development of the design of the European Solar Telescope is described. The telescope is devised to have the best possible angular resolution and polarimetric performance, maximizing the throughput of the whole system. To that aim, adaptive optics and multi‐conjugate adaptive optics are integrated in the optical path. The system will have the possibility to correct for the diurnal variation of the distance to the turbulence layers, by using several deformable mirrors, conjugated at different heights. The present optical design of the telescope distributes the optical elements along the optical path in such a way that the instrumental polarization induced by the telescope is minimized and independent of the solar elevation and azimuth. This property represents a large advantage for polarimetric measurements. The ensemble of instruments that are planned is also presented (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

S-DIMM+ height characterization of day-time seeing using solar granulation

To evaluate site quality and to develop multi-conjugative adaptive optics systems for future large solar telescopes, characterization of contributions to seeing from heights up to at least 12 km above the telescope is needed. We describe a method for evaluating contributions to seeing from different layers along the line-of-sight to the Sun. The method is based on Shack Hartmann wavefront sensor data recorded over a large field-of-view with solar granulation and uses only measurements of differential image displacements from individual exposures, such that the measurements are not degraded by residual tip-tilt errors. We conclude that the proposed method allows good measurements when Fried's parameter r_0 is larger than about 7.5 cm for the ground layer and that these measurements should provide valuable information for site selection and multi-conjugate development for the future European Solar Telescope. A major limitation is the large field of view presently used for wavefront sensing, leading to uncomfortably large uncertainties in r_0 at 30 km distance.

Open Principle for Large High-Resolution Solar Telescopes

Vacuum solar telescopes solve the problem of image deterioration inside the telescope due to refractive index fluctuations of the air heated by the solar light. However, such telescopes have a practical diameter limit somewhat over 1 m. The Dutch Open Telescope (DOT) was the pioneering demonstrator of the open-telescope technology without need of vacuum, now pursued in the German GREGOR. Important ingredients for this technology are primary beam completely open to natural wind flow, stiff but still open design by principal stiff overall geometries in combination with carefully designed joints and completely open-foldable dome constructions based on tensioned strong cloth. Further developments to large sizes are made within the framework of the design study for a European Solar Telescope (EST).

Large European Solar Telescope (LEST) - a Solar Telescope for High Spatial Resolution and Polarimetric Observations

In 1969 European solar physists formed the Joint Organization for Solar Observation (JOSO). The goals of JOSO were to find an, ideal site for a large solar instrument, and to create an observatory on this site equipped with a solar telescope for high spatial resolution and polarimetric observations. This instrument is referred to by the acronym LEST which stands for Large European Solar Telescope.