The PILOT experiment will measure the polarized emission of interstellar dust grains, in the submillimeter range, at 240 and 550 μm. The scientific instrument will be on-board a stratospheric platform. A field of view of 0.8°×1° and a resolution of about one arc minute are needed to reach the scientific objectives. For this purpose, the use of an extended focal plane, a large diameter primary mirror and a re-imaging system is needed. For the off axis telescope, we must respect a combination of shape and angle between the primary and secondary mirror, in order to optimize the image quality and to minimize the instrumental polarization. The aim of my thesis work was the evaluation of the performances of this optical system and the prediction of their variations in flight conditions. All the optical elements, except the primary mirror will be cooled to 3 K. The variations of the primary mirror characteristics with the environment must be well known in order to obtain optimal performances during the flight. For the evaluation of these characteristics, I used tridimensional measurements, submillimeter characterization and modelling of the deformations in various environmental conditions. The method used for the primary mirror was applied to the lenses of the reimaging system too. The actual modelling of the optical system includes these results and show that the optical performances are nominal. Using this realistic modelling, an optimization of the positionning of each optical element was done for the integration of the instrument. The polarization degree of the dust grain emission is expected to be only a few percent. For the interpretation of the instrument signal, it is necessary to have a good knowledge of the instrumental polarization. Using modelling with Zemax software, I have determined the Stokes parameters for several positions of the optical axis of the half waveplate, different orientations of the incident polarization state and for a set of positions in the field of view. During the ground tests of the integrated instrument, I will verify and optimize these parameters.
