The Origins and Development of the THEMIS (Télescope Héliographique pour l’Étude du Magnétisme et des Instabilités Solaires) solar telescope encompass a rich history of international collaboration, scientific ambition, and technological innovation.

In the latter half of the 20th century, solar physicists recognized that the Sun's magnetic field plays a crucial role in various solar phenomena, including sunspots, solar flares, and coronal mass ejections. However, understanding the intricate details of the Sun's magnetic field required advanced observational tools capable of high-resolution measurements.

While space-based telescopes offer the advantage of being above Earth's atmosphere, which eliminates atmospheric distortion, ground-based observatories like THEMIS provide sustained, long-term observations with more accessible maintenance and upgrades. The scientific community identified the need for a specialized ground-based telescope to complement existing space missions and fill gaps in solar magnetic field observations. In the 1970's, solar physicist from the Solar Astrophysics Department (Département d'Astrophysique Solaire) of the Paris Observatory (DASOP) had extensive experience studying solar magnetic fields through the Meudon magnetograph.

Unfortunately, the typical solar instrumentation (e.g. siderostat) introduced strong instrumental polarization, significantly limiting the instrument’s capabilities to measure the Sun's natural polarisation of light. To conduct a complete polarization analysis, it was either necessary to fully understand the polarization induced within the whole instrument optical path, or design an unprecedented polarization free instrument.

Hence, a comprehensive project for establishing a polarization-free solar instrument was conceptualised by Jean Rayrole, with significant contributions from Meïr Semel and Pierre Mein with the intend to measure magnetic fields with the highest precision. The Heliographic Telescope for the Study of Magnetism and Solar Instabilities, or more simply, THEMIS (acronym of Télescope Héliographique pour l’Étude du Magnétisme et des Instabilités Solaires) was presented for the first time to the Institut National d’Astronomie et de Géophysique (predecessor of INSU) in 1975.

The design phase focused on creating a telescope optimized for high-resolution spectropolarimetric observations. Key considerations included:

• Aperture Size: A 90 cm aperture was chosen to balance light-gathering power with the practicalities of ground-based operations.
• Polarimetry capabilities: A polarisation free telescope meant to have a polarimetric analyser at the first focus/image point (F1) of the telescope and being able to have an optical path that in which the polarisation state remains stable. Also advanced polarimetric instruments were integrated to measure the polarization of sunlight, which is essential for mapping the Sun's magnetic fields.

Developing specialized instruments, as well as the telescope and instrument control system was a significant aspect of the project's technological innovation. Sophisticated control systems were developed to manage the telescope's operations, including moving the dome and telescope for tracking the Sun, and coordinating data acquisition.

High-precision spectropolarimeters were designed to analyze the polarization states of light across different wavelengths, providing detailed information about magnetic fields. In THEMIS design, three first light instruments have been developed:

• a spectrograph, MTR: XXXX
• a spectro-imager, : XXXX
• an imaging spectrometer, MSDP: XXXX

The Teide Observatory on Tenerife, Canary Islands, was selected as the site for THEMIS due to its optimal geographical and climatic conditions for solar observations. The Canary Islands offer stable atmospheric conditions with minimal turbulence, essential for high-resolution imaging. The location provides relatively easy access for international teams, facilitating collaboration and maintenance.

Securing adequate funding was a critical step in the development of THEMIS. The project received financial backing from national science agencies of the participating countries, as well as from European Union research funds aimed at fostering international scientific collaborations.
* National Contributions:Each partner country committed a portion of the budget, covering aspects such as construction, instrumentation, and operational costs. * European Grants: Additional funding was secured through European Union grants, which supported specific components like advanced spectropolarimetric instruments and adaptive optics systems.

The THEMIS project was initiated as a collaborative effort primarily between three European countries: France, Italy, and Spain. This partnership leveraged the strengths of each nation's scientific institutions and expertise in astronomical instrumentation.

• France: The Institut National des Sciences de l'Univers (INSU), part of CNRS, took the lead in coordinating the project.
• Italy: The Instituto Nazionale di Astrofisica (INAF), part of CNR contributed significant expertise in solar physics and instrumentation.
• Spain: The Spanish National Research Council (CSIC) and other Spanish institutions, in particular the Instituto de Astrofísica de Canarias (IAC) provides the site and logistical support. provided support, particularly in site selection and infrastructure development.

An agreement was signed between the French CNRS and the Italian CNR. The construction cost of 87.5 million francs was covered with 80% financed by the CNRS and 20% by the CNR. The same funding distribution applies to the operation of the instrument. Observation time is allocated as follows: 60% for the French, 15% for the Italians, 20% for the Spanish, and 5% for “international time.” A Franco-Italian steering committee then determined the general policies for the telescope's operation and development.

Construction began in the early 1990s, involving meticulous planning and execution to assemble the telescope's complex components. The primary mirror and optical components were manufactured with high precision to meet the stringent requirements for spectropolarimetric measurements. Integrating various subsystems, such as adaptive optics, control electronics, and data acquisition systems, was a challenging but essential task to ensure seamless operation.

Before full-scale operations commenced, extensive testing was conducted to verify the telescope's performance. Ensuring precise alignment of optical components was critical for achieving the desired resolution and polarimetric accuracy. Spectropolarimeters and other instruments underwent calibration processes to ensure accurate measurements.

THEMIS achieved its “first light” in March 1996, marking the beginning of its operational phase. The initial scientific goals focused on: XXXX

Over the years, THEMIS has undergone several upgrades to enhance its capabilities:

• THEMIS Adaptive Optics: Upgrades to adaptive optics systems to further reduce atmospheric distortions, allowing for sharper images.
• Data Processing Improvements: Implementation of advanced data processing algorithms and high-performance computing resources to handle the increasing volume of observational data.
• Planetary observations: XXXX

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Despite advancements in space-based solar observatories, THEMIS remains a valuable asset due to its unique capabilities and the ability to perform long-term, high-resolution observations from the ground.