A sub-micrometric thermal refocussing mechanism for High Resolution EO telescopes
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High resolution optical space telescopes for Earth Observation, with an Image Ground Sampling Distance of less than 0.5 meters, require extremely challenging thermostructural performances in terms of mechanical stability. The optical path needs to be extremely stable in time, coping with diurnal environmental modulation, spacecraft maneuvers, and long term degradation of the structural elements. In the framework of an high-resolution space telescope developed for the Italian Space Agency, a system architecture has been selected, which, besides an athermalization and active control of the structural support of the mirrors, is requiring an active refocussing system, installed on the secondary mirror of the telescope (M2). Scope of the Refocussing Mechanism (RFM) is to correct long term variations due to thermal optical properties degradation, moisture release and launch- induced deformations. The M2 RFM is hence required to correct steady displacements, with an accuracy of 100 nanometers and a total stroke of ±15 micrometers, with tight stability and tilt requirements. This paper describes the activities carried out during the design, manufacturing and testing of a breadboard of the RFM. The selection of a thermal device based on the Coefficient of Thermal Expansion of its components is justified. An overview of the design of the mechanism is provided. The thermal-vacuum test campaign is reported, focusing on the challenges of the positioning measurement methods, and on the characterization of the device properties and performances. Lessons learnt are reported, together with the identified improvements for the following project phases.