The European Solar Telescope (EST) will be Europe’s most ambitious ground-based infrastructure in solar physics. It will have a primary mirror with a diameter of 4.2m, which will make it the largest in Europe, and of identical aperture as the largest solar telescope available worldwide, namely the Daniel K. Inouye Telescope (DKIST) installed at the Haleakala summit, Hawai'i. EST will have two main characteristics devoted to maximising the scientific return. First, the telescope’s optical path will be polarisation-free using pairs of mirrors that compensate for instrumental polarisation. Second, the telescope will be designed together with a complete instrument suite with imaging and spectrograph instruments. EST will also bring many new technologies, such as a multi-conjugate adaptive optics system and integral field spectro-polarimeters. This contribution presents the conceptual design of the infrared (1 to 1.8 microns) integral field spectropolarimeter. The instrument will have an integral field unit composed of a mirror-based image slicer as input to a Czerny-Turner spectrograph. It will have a polarimeter to record the polarisation state of light on a dual-beam configuration to ensure high-precision spectro-polarimetry.
For more than ten years, the Gregor Infrared Spectrograph (GRIS) has been the infrared spectropolarimeter on the Gregor telescope since its inauguration in 2012. The instrument mainly performs polarimetric observations of the wavelength regions at 1083 and 1565nm, which have high scientific potential. There, observers can find spectral lines that provide information on the atmospheric parameters over a vast range of atmospheric layers and with high sensitivity to the magnetic field. Due to the high demand for GRIS and the versatility of the Gregor environment, we decided to upgrade the system once more, allowing it to become one of the instruments with the highest potential in any ground-based solar observatory. This time, the upgrades add two spectral channels for performing multi-wavelength observations in long-slit or IFU mode.
The Gregor Infrared Spectrograph (GRIS) is installed at the Gregor telescope at Observatorio del Teide, Spain. The instrument started observations in 2012 and underwent several upgrades in the following years. Currently, it is experiencing the most significant upgrade so far, which includes, among other things, adding two optical channels to perform simultaneous multi-wavelength observations. One of the elements we need to modify to optimise the performance of the instrument on all the new spectral channels is the Polarisation Module Unit (PMU). In this contribution, we want to present our theoretical analysis, laboratory experiments, and first-light results related to the new version of the PMU. This new version and many of the improved capabilities of the instrument will be offered to the community gradually during 2024.
The GREGOR Infrared Spectrograph (GRIS) can perform spectropolarimetric observations in the infrared. The spectrograph has a Czerny-Turner design and offered traditional long-slit observations since 2014. Later on, in 2018, it was upgraded with an Integral Field Unit (IFU) based on a slicer mirror, which has slicers of 100-μm width. The latter option makes GRIS a unique instrument in solar physics that, after more than 10 years of operation, is still at the forefront of the field, particularly in the infrared. A third upgrade took place in 2021, where optomechanical, control and software improvements have been conducted to introduce new optical channels that operate below 1 μm. These new spectral channels allow simultaneous observations with the previous infrared camera. Between these improvements, a new diffraction grating was installed to enable multi-wavelength configurations with high scientific potential. This new diffraction grating will be interchangeable with the previous one, offering flexibility between spectral resolution, simultaneity, and spectral coverage. Additionally, two more IFUs will be offered to the community. One IFU with a 70-μm width slicer mirror that has already been manufactured and tested at the telescope, and the second with a 35-μm width slicer mirror that is currently at development status. The three IFUs will be interchangeable and offer three different spatial resolution modes and fields of view. This work presents the final optical configuration of the upgraded GRIS and the installation procedure of the most recent upgrades.
Due to the complexity of scientific instruments, such as spectropolarimeters, managing instrument sequences can be challenging. To address this problem, a Finite-State Machine (FSM) approach has been used to manage solar observation sequences in the GREGOR Infrared Spectrograph (GRIS). FSMs provide a structured and visual representation of control logic, making them well-suited for managing intricate workflows. By using FSMs, both scientists and engineers can clearly define and modify instrument sequences, ensuring the precise coordination of various instrument components. In multiple optical channels spectropolarimeters, such as GRIS, FSMs can effectively synchronize the image acquisition across multiple channels, adjust exposure times, handle errors, and manage the selection of the scanning system. To streamline the implementation process, the CodeDesigner RAD tool was used to create diagrams that illustrate the execution order of the states belonging to a finite-state machine. CodeDesigner’s code generation feature automatically translates these diagrams into C++ code. This approach ensures the precise and reliable operation of the GRIS control software.
This paper describes the preliminary design of the Adaptive Secondary Mirror (ASM) for the European Solar Telescope (EST), as designed by TNO. This ASM will contain 1950 actuators over an optical aperture of Ø80cm. The mirror-shell of this ASM is concave with a radius of 2.156m and has a thickness of 2.5mm. To cope with the high optical heat-load, the mirror shell will be cooled actively by means of conduction through a small (0.3mm) airgap between the mirror shell and the cooled SiC supporting structure. One of the unique characteristics of this ASM are the highly efficient actuators by TNO, delivering 30N Peak-to-Valley of linear force range within a packaging of Ø15x40mm. These actuators are based on the proven electromagnetic actuation principle by TNO but have been significantly redesigned to improve manufacturability and integration. The actuators are interfaced to the mirror shell via a stiff carbon fiber strut assembly and are designed to be replaceable during off-telescope maintenance. For overall alignment of the ASM a hexapod is used, while a faster tip-tilt stage is used to compensate for tip-tilt disturbances up to 20Hz bandwidth. The optical performance of the ASM under various environmental conditions has been extensively studied with Finite-Element-Analysis. To verify the performances of the actuator and the cooling systems, an actuator and thermal- breadboard are being constructed.
The European Solar Telescope (EST) is a 4.2-m telescope which has been redesigned with a fully integrated Multi-Conjugate Adaptive Optics (MCAO) into the optical path right after the EST primary mirror. The current baseline configuration considers four altitude Deformable Mirrors (DM) conjugated to 5, 9, 12 and 20 km above the telescope entrance pupil and an Adaptive Secondary Mirror (ASM) conjugated to the entrance pupil. The wavefront sensing will be performed by a set of correlation-based Shack Hartmann wavefront sensors (WFS) combining an on-axis High-Order WFS (HOWFS) to be used either in Single Conjugate AO (SCAO) to drive the ASM as well as operating simultaneously with a Multi-Directional WFS (MDWFS) to drive the MCAO. Beyond the current baseline configuration, different alternatives are currently being investigated both in the wavefront sensing strategy by evolving from a HOWFS+MDWFS into possibly a single High Order Multi Directional WFS (HOMDWFS) and/or wavefront sensors operating at different observing bands.
The European Solar Telescope (EST) aims to become the most ambitious ground-based solar telescope in Europe. Its roots lie in the knowledge and expertise gained from building and running previous infrastructures like, among others, the Vacuum Tower Telescope, Swedish Solar Telescope, or the GREGOR telescope. They are installed in the Canary Islands observatories, the selected EST site. Furthermore, the telescope has a novel optical design, including an adaptive secondary mirror (ASM) that allows reducing the number of optical surfaces to 6 mirrors (plus two lenses) before the instruments’ focal plane. The latter, combined with a configuration of mirrors that are located orthogonally oriented to compensate for the instrumental polarisation induced by each surface, makes EST a reference telescope in terms of throughput and polarimetric accuracy. In its main core design, EST also includes a Multi-Conjugated Adaptive Optics (MCAO) system where the ASM compensates for the ground layer turbulence. The rest of the mirrors on the optical train correct for the atmospheric turbulence at different layers of the atmosphere. The MCAO guarantees that the large theoretical spatial resolution of the 4-metre EST primary mirror is achieved over a circular FOV of 60 arcsec. Those main elements, combined with a set of instruments with capabilities for spectropolarimetry, make EST the next frontier in solar ground-based astronomy. In this contribution, we will cover the main properties and status of all the mentioned sub-systems and the following steps that will lead to the construction phase.
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