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.
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 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.
The European Solar Telescope (EST), with its primary mirror of 4.2 diameter, will be the largest solar telescope available in Europe. EST will offer Integral Field Spectropolarimetry (IFS) by incorporating the EST spectropolariMeter Based on slicEr-mirrors for the near-infraRed (EMBER). This instrument is a high-resolution spectropolarimeter that allows for the analysis of a 2D field of view by using a slicer mirror-based Integral Field Unit (IFU) as input to the spectrograph. The slicer mirror, which is placed at the focal plane of the telescope, allows the observation of the integral field of view by slicing the entire field. After that, an optical system reorganizes the field of view and provides the spectrograph with an output slit composed of multiple slitlets. The spectrograph has a Czerny-Turner design and will cover the solar spectrum from 1 to 1.8 μm. Additionally, EMBER will offer spectropolarimetry observations with a dual-beam configuration. In this contribution, we present the conceptual optical design of the spectrograph and the IFU as a solution that meets the scientific requirements.
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.
The Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) has been developed for the third flight of the Sunrise balloon-borne stratospheric solar observatory. The aim of the SCIP is to reveal the evolution of three-dimensional magnetic fields in the solar photosphere and chromosphere using spectropolarimetric measurements with a polarimetric precision of 0.03% (1σ). Multiple lines in the 770 and 850 nm wavelength bands are simultaneously observed with two 2 k × 2 k CMOS cameras at a frame rate of 31.25 Hz. Stokes profiles are calculated onboard by accumulating the images modulated by a polarization modulation unit, and then compression processes are applied to the two-dimensional maps of the Stokes profiles. This onboard data processing effectively reduces the data rate. SCIP electronics can handle large data formats at high speed. Before the implementation into the flight SCIP electronics, a performance verification of the onboard data processing was performed with synthetic SCIP data that were produced with a numerical simulation modeling the solar atmospheres. Finally, we verified that the high-speed onboard data processing was realized on ground with the flight hardware using images illuminated by natural sunlight or an LED light.
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.
Once the design phase of the Multi-Conjugated Adaptive Optics (MCAO) testbed for the European Solar Telescope (EST) is completed, the system has been assembled and integrated at the Instituto de Astrofísica de Canarias (IAC). The aim of this highly configurable optical testbed is to study different solar adaptive optics solutions, including Single Conjugated Adaptive Optics (SCAO), Ground Layer Adaptive Optics (GLAO), and MCAO. The testbed will be used to evaluate the performance of different wavefront sensing and correction strategies. It consists of different subsystems that are being integrated with high precision by using optomechanical elements mostly developed at IAC. The testbed emulates a 4-m telescope with a Field of View (FoV) of 70 arcseconds. It currently operates as a SCAO configuration consisting of: an illumination system with point-like and extended source capability coupled with a configurable turbulence simulator based on Phase Screens (PS); a pupil-conjugated Deformable Mirror (DM) with 820 actuators; a high order Shack-Hartmann Wavefront Sensor (SH-WFS) with 33 subapertures across the pupil, which measures on-axis aberrations with a FoV of 10 arcseconds; and the science detector. The next steps include the integration of the elements that will provide GLAO and MCAO capabilities, such as two SH-WFSs with a FoV of 70 arcseconds, and two DMs conjugated to different atmospheric layers. This contribution describes the optomechanical components designed and the alignment procedure carried out to set up each subsystem and configuration to be studied.
Integral Field Spectropolarimetry (IFS) makes feasible the simultaneous measurement of a 2D field with the use of an Integral Field Unit (IFU). Slicer-based IFUs rearrange an input 2D Field of View (FoV) to an output long-slit that is used to feed a standard spectrograph. The spatial resolution reachable with this technique is determined by the slicer width. As a technology demonstrator for a future EST instrument development, here we describe an IFU for the GREGOR solar telescope. It is an evolution from the first IFU installed at GREGOR with 100 μm width slicer mirror. A novel technique was applied to produce a 35 μm-width metallic image slicer, which has been manufactured in collaboration with NAOJ and Canon Inc. for the SOLARNET project. The IFU transforms an input FoV of 4.4” x 2.1” into 16 mini-slits rearranged in two parallel output long-slits to feed the GRIS infrared spectrograph for simultaneous 2D field spectropolarimetric measurements in several wavelengths. A collimator and camera mirror concept was applied to control the pupil and stray light contamination, leading to a telecentric configuration with a 1:1 magnification. Because of the thin dimensions of the slicer mirrors, diffraction effects are not negligible. An study on the impact of the diffraction effects on the whole IFU and the spectrograph, has been carried out. Another important issue that has been taken into account in the design is the individual mini-slit tilt induced by the compact multi-mirror design. Different solutions were considered and the final design is presented.
THEMIS, a 90 cm solar telescope, offers spectropolarimetric observations in the VIS and NIR spectral ranges through a long-slit spectrograph. Now, an image slicer-based Integral Field Unit (IFU) is being developed as an evolution of the long-slit set-up. The IFU will allow observing a 2D Field-of-View (FoV) and several spectral regions simultaneously. Integral Field Spectroscopy mitigates very well the loss of information generated by the atmospheric seeing over a single long-slit and is more suitable for 2D evolutionary studies of solar phenomena. The IFU optical design inherits the concept of the IFU currently installed on the GRIS spectrograph at the GREGOR telescope and has been adapted to THEMIS. Also, we took advantage from the image slicers manufacturing process evolution since the GRIS IFU was created. First, the spatial resolution is increased by reducing the width of the slicer mirrors from 100 to 80 μm. Second, the FoV is increased by changing the slices number from 8 to 16. In terms of optomechanical integration, the slicer mirror unit is placed at the telescope focal plane. The available volume is limited and has constrained the IFU elements position and the mirrors focal length. The design strategy generates 16 different pupil planes that are superimposed and keeps the optical path constant to allow interchangeability between the new IFU and the long-slit mode. The most challenging condition was keeping the pupil plane position unaltered since the input beam is not telecentric. This contribution presents the IFU optical design and describes the challenges faced in design phase.
One of the main goals of the European Solar Telescope (EST), a 4.2-m telescope, is to clarify the roots of the magnetic processes taking place in the solar atmosphere. This goal has a top-level requirement: perform simultaneous spectropolarimetric measurements in multiple spectral lines. For this purpose, EST will be equipped with a set of instruments working simultaneously in diverse spectral ranges. In this regard, we are designing a Coudé Light Distribution (CLD) responsible for delivering the incoming solar radiation to each instrument. The CLD is formed by a series of optical elements like dichroic and intensity beam splitters, flat mirrors, and optical compensators that will be interchangeable to offer the solar community maximum flexibility for performing observations. In developing the CLD, we are paying great attention to controlling aberration effects generated by the different elements that constitute the light distribution system. Also, we are defining the CLD to reach a balance between throughput, image quality, and a compact distribution of the instruments in the Coudé room. Our aim is to describe in this contribution the current design of the CLD. The present design constitutes the basis of the CLD, with enough flexibility to improve it in the future, if indeed, and adapt it to the evolution of other sub-systems like the instruments, the adaptive optics, or the telescope structure to guarantee that it fulfils the science requirements.
The GREGOR Infrared Spectrograph (GRIS) can perform spectropolarimetric observations in 1.0-1.8 μm and pure spectroscopic in the band 2.0-2.3 μm. The spectrograph has a Czerny-Turner design and has allowed the operation in the traditional long-slit mode since 2014. And also, in IFU mode since was upgraded in 2018. A third upgrade took place in 2021, where optomechanical, control and software improvements have been conducted to introduce a new spectral channel that receives light below 900 nm. This new spectral channel allows simultaneous observations with the previous infrared camera. Additional improvements include replacing the old diffraction grating with a new one and acquiring a sensor for the new spectral channel. The new grating allows having different order separation and maximising the combination of spectral bands of interest between both channels. Also, a dichroic was placed after the focal plane of the spectrograph, and a reimaging system was introduced in the new spectral channel to match the spatial scale (arcsec/px) in the old and new detectors. The feasibility of operating both cameras simultaneously, and synchronised with the polarisation modulation package, has been already tested. This work describes the optical design for all the spectral channels. We also examine its optical performance, as well as discussing future upgrades in terms of additional spectral channels that are already in progress.
This contribution describes the software and electronic improvements implemented in the GREGOR Infrared Spectrograph (GRIS) installed on the Gregor telescope. It is located at the Teide Observatory, in Tenerife, Islas Canarias, Spain. As a demonstrator for an instrument for the European Solar Telescope (EST), this project aims to perform simultaneous spectropolarimetric observations in several spectral lines using several synchronized detectors that may operate at different synchronized frame rates. Throughout the article, the problems encountered in achieving the synchronization of two or more sensors and the solutions proposed to solve them are explained.
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.
For more than ten years, the GREGOR Infrared Spectrograph (GRIS) has been the main spectropolarimeter on the GREGOR telescope since its inauguration in 2012. The instrument mainly performs polarimetric observations of the wavelength regions at 1083 and 1565 nm, with 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, either in long-slit or IFU mode.
Polarization is a fundamental property of the light and is very useful to measure the magnetic field vector of the various features that can be observed in the solar atmosphere. Ideally, a solar telescope should not introduce any polarization to the incoming light that could mask the one coming from the Sun. However, some instrumental polarization is always introduced by the different optical components, because it depends on the coatings used, as well as on the incidence angle and wavelength. The calibration of these instrumental polarization is specially tedious and complicated if it varies with time (as is the usual case for telescopes, when the pointing changes in elevation and azimuth). The European Solar Telescope (EST) has been designed to minimize this spurious temporally-varying instrumental polarization. A numerical model based on geometrical ray tracing has been developed in combination with Zemax Optic Studio (ZOS), in order to estimate the Mueller matrices of the moving optical elements of the telescope. The Mueller matrices have been calculated as a function of wavelength and for different field of view (FoV) positions and telescope (azimuth and elevation) pointing, using generics coatings (aluminium for the primary mirror and silver for the rest of the mirrors). This paper shows the analysis and results of the Mueller matrices that have been obtained, leading to the confirmation that the telescope has an excellent polarimetric performance for all wavelengths, FoVs and pointing directions.
The Sunrise balloon-borne solar observatory carries a 1 m aperture optical telescope and provides us a unique platform to conduct continuous seeing-free observations at UV-visible-IR wavelengths from an altitude of higher than 35 km. For the next flight planned for 2022, the post-focus instrumentation is upgraded with new spectro- polarimeters for the near UV (SUSI) and the near-IR (SCIP), whereas the imaging spectro-polarimeter Tunable Magnetograph (TuMag) is capable of observing multiple spectral lines within the visible wavelength. A new spectro-polarimeter called the Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) is under development for observing near-IR wavelength ranges of around 770 nm and 850 nm. These wavelength ranges contain many spectral lines sensitive to solar magnetic fields and SCIP will be able to obtain magnetic and velocity structures in the solar atmosphere with a sufficient height resolution by combining spectro-polarimetric data of these lines. Polarimetric measurements are conducted using a rotating waveplate as a modulator and polarizing beam splitters in front of the cameras. The spatial and spectral resolutions are 0.2" and 2 105, respectively, and a polarimetric sensitivity of 0.03 % (1σ) is achieved within a 10 s integration time. To detect minute polarization signals with good precision, we carefully designed the opto-mechanical system, polarization optics and modulation, and onboard data processing.
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