Product assurance for the PLATO Telescope optical unit

.We describe the main tasks of the Product Assurance process for the Telescope Optical Unit (TOU) of the ESA PLATO mission, that starts from the design phase and proceeds through all phases, up to the final product, with the aim of improving the likelihood of success of the mission. When dealing with the opto-mechanical components of the TOU, several aspects regarding safety and performance have to be analyzed and tracked. From the PA point of view, we focus in this paper on materials and processes selection that shall be suitable and robust enough for the space environment. Cleanliness and contamination control is needed to overcome loss of optical performance. Validations and qualifications on prototypes is fundamental to assess the reliability of the instrument for its purpose and for the lifetime of the mission.