First test results on SWIR subsystem optical breadboards: optical mounting characterization, first slit homogenizer, and immersed grating for space applications

In the frame of the ESA Sentinel-5 mission, as part of the Copernicus program, Airbus is the prime contractor for the S5 Instrument. As part of the S5 instrument, which is using the METOP SG satellite as a platform, Leonardo is developing the Short-Wave Infrared Spectrometer (SWIR-SS), comprising two parallel channels covering the wavelength ranges of 1589÷1676 nm (SWIR 1) and 2304÷2386 nm (SWIR 3). Major objective of the S5 is the monitoring of the Earth atmosphere by taking measurements of trace gases and aerosols impacting air quality and climate, with a swath width of ca. 2670 Km providing daily coverage of Earth atmosphere at an unprecedented resolution of 7x7 Km2 at nadir. Main characteristic of the SWIR-SS compared to other spectrometers is the high spectral position one orbit stability less than 1um and high spectral pixel resolution of 0.1nm. High stability and optical quality requires accurate optical elements mounting design and high resolution is reached by using new developments regarding Immersed Grating (grating immersed on a prism) and the implementation of a cutting edge 3D Slit Homogenizer system, positioned on the object plane of the spectrometer, to mitigate radiometric errors arising from scene heterogeneity. In order to demonstrate the system performance the optical design has been validated by means of flight representative optical assemblies. These breadboards are representing all relevant optical elements of the system. This paper presents the tests, which have been performed on the flight representative optical elements and the test results. As the Optical Element Mounting, the Slit Homogenizer and the Immersed Grating are major elements to fulfil mission requirements the development approach is described. Special emphasis for Lens mounting is put on the stability and WFE, for the Slit Homogenizer is put on the requirements regarding the slit width variation and the quality of the entrance edge while the major design driver for the Immersed grating is the optical quality. This paper presents the development approach of these two major optical elements and the validation test results.