[ The following text is attributed to Jean Arnaud, circa 1990]

THEMIS (Telescope Heliographique pour 1’Etude du Magnetisme et des Instability Solaires) is a new generation French-Italian solar telescope, built by INSU-CNRS (France) and CNR (Italy).
The main scientific goal for which it has been designed is the accurate determination of the vector magnetic field. This goal has led to those main requirements:

1. High spatial resolution
2. Accurate polarization measurement
3. Simultaneous observations in several spectral lines

1. The first condition comes from the fact that the largest part of the photospheric magnetic field is concentrated in very small structures.
2. The second one is obvious: an accurate determination of the vector magnetic field, using the Zeeman effect requires a precise measurement of the four Stokes parameters. This means a polarimetric accuracy of 10^-3 for precise determination of the direction of strong magnetic fields. Observations of weak magnetic fields (like intra-network fields in the photosphere) will require a polarimetric sensitivity of at least 10^-4
3. The third condition comes from several considerations:
   • Local physical parameters, like temperature and density, do affect the linear polarization. To separate those effects from the one of the magnetic field (and determine the vector magnetic field as well as the other parameters), one needs to observe several spectral lines with different sensitivities to the physical conditions.
   • A realistic determination of the variation with height of the physical parameters, as well as of the magnetic field flux tubes, requires the measurement of several sets of lines, properly chosen to scan different levels in the sun atmosphere
   • It will always remain magnetic elements of size beyond the attainable spatial resolution. For them, multilines Stokes polarimetry allows to get, in addition with the magnetic flux, the field strength and the magnetic field filling factor.

THEMIS is the first solar telescope to fill those three conditions.
For the first one, this is done thanks to the choice of the site, the quality of the optics, a sophisticated thermal control inside the dome and the building and a tip-tilt fast guiding mirror. The optical scheme of the telescope, is maintained axially symetrical up to the polarization analyser, minimizing the instrumental polarization and allowing to fill the second condition. The use of an echelle spectrograph allows simultaneous observations of several lines in a wide spectral range.

In addition to the multi-lines Stokes polarimetry, THEMIS is fitted out with other capabilities:

• Imaging spectroscopy by Multichannel Subtractive Double Pass Spectroscopy
• Very narrow-band imaging with an Universal Birefringent Filter followed by a Fabry-Perot.
• Fast full-disk magnetogram capability

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