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The Full Disk Telescope is part of the Polarimetric Helioseismic Instrument on board the future Solar Orbiter ESA/NASA mission. It will provide full-disk measurements of the photospheric magnetic field vector and line-of-sight velocity, as well as the continuum intensity in the visible wavelength range. Along this mission, it is expected that thermal drifts will induce image focus displacements. Consequently, providing an autofocus system is mandatory to prevent image degradation. The refocusing system is based on an autonomous image contrast analysis and it allows for a lens displacement in order to locate the best focus position. The figure of merit chosen for the image quality evaluation is presented. The influences of attitude instability and mechanical uncertainties are considered in a refocusing process simulation. In addition, an engineering model of the mechanism is tested at flight operating conditions. To check its performance, an optical interrogation system is set up. Determination of accuracy and repeatability of the mechanism positioning is experimentally evaluated and discussed according to the ISO standard. The results show that the proposed refocusing system is sufficiently robust against the expected image shifts and mechanical instabilities. [ABSTRACT FROM AUTHOR]