Different interferometric techniques are required to cover most of the scientific needs in the field of fluid dynamics science in microgravity research. The Fluid Science Laboratory (FSL), currently under upgrade for the Columbus Orbital Facility of the International Space Station (ISS), shall provide Holographic Interferometry, Digital Holography, Electronic Speckle Pattern Interferometry (ESPI) and Shearing Interferometry among other diagnostic tools. On earth, these highly sensitive interferometers are operated in a thermal and mechanical controlled environment. In opposition to the situation on ground the multi-user facility of the FSL has severe constraints for what concerns volume, mass, modularity, operational needs and its environment. This results in a three-dimensional modular drawer structure for the design of the optical-mechanical set-up, where performance limitations must be expected compared to systems on ground. In a rather uncontrolled thermal environment onboard the ISS this leads to misalignment due to thermo-mechanical changes of the Aluminum structure during experiment runs which finally result in interferogram distortions and therefore to significant measurement errors. In this paper we report about a misalignment detection- and active compensation concept developed on the basis of a thermo-mechanical and optical analysis of the set-up. The detection system is based on a simplified Hartmann-Sensor. It is able to separate wave front tilt and curvature errors due to misalignments of the interferometers itself from the effects caused by the experiment. The closed-loop compensation system uses optical components of the set-up driven by piezoelectric actuators. Due to its active approach this concept allows for the real time accessibility of the experimental effects in the framework of “Telescience.”
Extensive functional tests as well as representative thermal tests show the suitability of the proposed technique to compensate interferogram distortions due to thermal-mechanical deformations. Thus, it is able to ensure interferometric measurements with sub-wavelength accuracy onboard the ISS.