1 Introduction

The French-Italian solar telescope THEMIS (Téléscope Heliographique pour l'Étude du Magnetisme et des Instabilités Solaires) opened its first observational campaign with polarimetric capabilities last summer. In this paper we present the results of the first reduction of data coming from the MTR polarimetry mode. Spectropolarimetric observations are at the origin of the special design of this telescope. Two main aspects characterize it: first, the telescope is free from systematic instrumental polarization, apart from some residuals, mainly expected at the entrance and exit windows. Second, it allows observation of up to 10 spectral domains simultaneously. These two characteristics distinguish THEMIS from any other solar telescope built to date.

The interest of a polarization-free telescope is self-evident. Nevertheless from the very impressive results obtained by ASP at the VTT at Sacramento Peak (Martínez Pillet et al. 1997; Westendorp Plaza et al. 1997) or ZIMPOL at the MacMath Telescope at Kitt Peak (Stenflo & Keller 1996) using telescope calibration, a polarization-free instrument avoids all the hard work this calibration involves (Skumanich et al. 1997). It will, in the future, constitute an advantage for seeking higher polarimetric sensitivities, which require analysis of the smallest instrumental residuals. With respect to the second characteristic, a lot of work has been devoted to the insufficiency of a single spectral line for a correct determination of magnetic field (and of the atmosphere parameters in general) as a function of depth (Rayrole 1967; Rayrole 1971; Semel 1981; del Toro Iniesta et al. 1990). There is a necessity of measuring simultaneously several sensitive lines in order to determine the magnetic field and its gradients along the line of sight unambiguously (as well as other atmospheric quantities). This is one of the multiple advantages that any multi-wavelength observation provides, even if limited to the visible (and neighbouring) spectrum, as in the case of THEMIS. Hence, this telescope appears as a promising instrument for the measurement of magnetic fields in the solar photosphere, the purpose of our observations.

THEMIS was, during this observing campaign, on the fine-tunning phase of its instruments. Our observations therefore provided much more information on what was happening in the telescope than on solar phenomena. Furthermore although many of the remaining problems should be solved for next campaigns, we consider that this first data reduction from the MTR mode of THEMIS still constitutes a good description of how the instrument behaves, how the data should be handled and what can be expected in the near future from this observing mode.

A brief description of our observations is made in next section. Sections 3 and 4 form the core of this work: the data reduction techniques are presented, the main defects are described, and several measures of observed noise are shown. Section 5 is devoted to illustrate the kind of data that the THEMIS MTR mode provides after reduction, together with preliminary longitudinal magnetic field charts.