Alignment turning and assembly of the Sentinel 4 optical modules (2021)

Centering and positioning of the single optical elements within a lens system are essential for the quality of high precision optics. The positioning of the lenses have to be long term stable and shall withstand space environment to keep the performance of the optical system over years. This applies in particular to the optical modules for the Sentinel-4/Ultraviolet/Visible/Near-Infrared (UVN) sounder instrument /1/. In order to fulfill these requirements an appropriate opto-mechanical design and a deterministic assembly technique based on well-defined assembly interfaces are essential. For this reason, the application of the alignment turning technology is particularly suitable /2/. The basic approach is to reach the best possible coincidence between the optical axis of the lens and the mechanical axis of the lens cell and drop the lens stack in a barrel with a tight fitting. By machining the lens housing in respect to the optical axis the resulting ultra-precise references planes significantly simplify the alignment and integration of the objective. The assembly becomes deterministic – extensive alignments are not required. The mechanical and environmental specifications are achieved by the consistent compliance with: ultra-precise machined mounting interfaces, avoiding adjustment mechanisms, the application of a defined pre load during lens mounting. The S4 UVN optical modules requires a resulting decenter shift of each optical surface to the best fitting optical system axis less than 4 µm for the telescope, camera and collimator optics and less than 6 µm for the NIR optics. The total tilt of each surface to the best fitting optical axis has to be within 6 arcsec. The individual lens position (air distances) as well as the position of the lens stack in direction of the optical axis have to be within 5 µm, respectively. Aiming to the specified values the alignment turning technology was carried out on the proto flight models of the UVN instrument optics. Using a customized alignment turning machine the decentration of the housed lens were detected and adjusted in respect to the turning axis, reaching a minimum. After machining of the housing the vertex height as well as the geometry of the lens cell were measured in sub micron precision. For the passive joining of the alignment turned lens elements in to the lens tube without further adjustment steps a specific mounting device was realized. A newly developed clamping process that ensures a dedicated pre-load completes the assembly procedure. The alignment turned and assembled flight models were characterized using a centration measurement of the optical subsystems (telescope, collimator, camera, NIR spectrometer). The alignment status of the optical axis of all individual lens systems w.r.t. the measurement reference axis was determined within the required specifications. Further optical verification measurements confirm the achieved optical performance of the optical modules to the full extent. The paper deals with the alignment turning, the integration and verification of the lens assemblies for the S4 UVN Instrument.