The Full Disk Telescope is part of the Polarimetric Helioseismic Instrument on board the future Solar Orbiter ESA/NASA mission. The Full Disk Telescope will provide for full-disk measurements of the photospheric vector magnetic field 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 quality analysis and it allows for a lens displacement in order to locate the best focus position. In this document the system and the figure of merit chosen for the image quality evaluation is presented. The algorithm is a gradient based contrast and it is adapted to the object features. Moreover, the telescope is not equipped with image stabilization system and therefore attitude stability must be assessed. Jitter is modelled as a circular normal distribution of the pointing direction. A stability of 0.5 arcsec (σρ) during frame accumulation is simulated and its impact on image contrast evaluated. Considering this jitter, image contrast is only reduced to 99.8% the contrast of an unaffected image. Finally, to estimate the effect in the process performance, the mechanical uncertainties expected for the linear actuator were added to the defocus due to the lens excursion. The results show that the proposed refocusing system is sufficiently robust against the expected image shifts and mechanical instabilities.

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.