Sentinel-5 short-wave infrared spectrometer optical design

In the frame of the Sentinel-5 mission, Leonardo is developing the Short-Wave Infrared Spectrometer (SWIR-SS), part of the UVNS instrument foreseen to be embarked on board of the MetOP-SG satellite. S5 instrument objective is to monitor the composition of Earth atmosphere by taking measurements of trace gases and aerosols impacting air quality and climate, providing daily coverage of Earth atmosphere at an unprecedented resolution. SWIR-SS baseline architecture is a pushbroom imaging spectrometer with two channels,[1589÷1676] nm and [2304÷2386] nm, with a spectral resolution of less than 0.25 nm. At the object space is located a Slit-Homogenizer, a special component which guides the optical path across the slit in order to mitigate radiometric errors arising from scene heterogeneity. Its behaviour introduces an astigmatism which is corrected at collimator level by making use of a cylindrical lens. Light is then guided towards the dispersers and is focused onto the detector by the cameras lens. Pupil anamorphism, smile and keystone due to dispersers, immersed gratings, are corrected by using a prism and combining the distortion/lateral color of collimator and focusing cameras in both optical channels. A low pass filter at the dichroic splits the wavelengths, while, for each channel, two successive coatings on front and back faces of the prisms select the band and mitigate outof- band straylight. The design has the purpose to keep the alignment of each subsystem simple, as no aberration compensations have been foreseen between collimator and focusing camera. It shows robustness, stability vs temperature and high optical quality.