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 site selection for THEMIS was part of a larger project to search for optimal location for international solar observations. The Joint Organization for Solar Observations (JOSO) was funded in Catania, Italy, in 1969 with the aim to at establish a modern optical solar observatory with advanced instrumentation for high-resolution observation of solar atmospheric structures. Between 1969 and 1985, the JOSO was concerned with locating and testing observatory sites in which solar seeing allows a spatial resolution of the order of 0.1 arcsec for long periods of time. After the evaluation of about 40 locations, two sites on the Canary Islands were selected: Izana in Tenerife, and Roque de Los Muchachos, in La Palma, both at an altitude of about 2400 m. Those now hosts the majors European telescopes, such as the the Vacuum Solar Telescope (Germany), the Swedish Solar Telescope (Scandinavia), Gregor (Germany) as well as the projected site for the future European Solar Telescope (EST).

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.

The design phase, directed by Jean Rayrole, 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, IPM: the Italian Panoramic Monochromator ( Cavallini, A&A, 1998) performed solar bidimensional spectroscopy. On a square field $33''\times 33''$, this instrument allowed to obtain monochromatic images of the solar surface with high spectral resolution (R > 256 000 at 5500 Å), preserving all the spatial resolution delivered by the telescope (~0.2''). IPM consisted of a Fabry-Perot interferometer, used in telecentric mounting, in series with a Universal Birefringent Filter, used as an order sorter.
• an imaging spectrometer, MSDP: XXXX

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. Each partner country committed a portion of the budget, covering aspects such as construction, instrumentation, and operational costs.

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: Consejo Superior de Investigaciones Científicas(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 received its first light on March 16, 1996, and was inaugurated that same year by the King and Queen of Spain, along with French and Italian authorities.

xxx 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

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.

The Joint Organization for Solar Observations (JOSO), G. Godoli, Società Astronomica Italiana, Annual Meeting, 30th, Ravenna, Italy, May 1-3, 1986 Società Astronomica Italiana, Memorie (ISSN 0037-8720), vol. 57, no. 4, 1986, p. 631-654. Histoire des observations solaires de Meudon, by Regis Le Cocguen (in French)

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