Optical design of geostationary wide swath imaging spectrometer with freeform surfaces

Abstract

Geostationary (GEO) imaging spectrometer can provide spectral imaging data with high spectral resolution, high temporal frequency and wide coverage, and it is expected to be an important earth observation payload for the next-generation GEO satellite. For the GEO application, the imaging spectrometer is preferred to has ultra-long slit and compact size. Benefiting from the more degrees of freedom, freeform surfaces provide a feasible method to fulfil this requirement. The paper presents an optical design of the wide swath imaging spectrometer with freeform surfaces for the GEO remote sensing. The slit length is up to 240 mm. The designed imaging spectrometer is based on the Offner configuration, while both the mirrors and convex grating are freeform Zernike surfaces instead of traditional spherical surface. The introduction of the freeform surfaces can reduce the residual astigmatism induced by the long slit significantly. The root mean square radius of the spot diagram of the design is smaller than the Airy radius and the MTF is close to the diffraction limit at the Nyquist frequency. The size of the design is about 800mm×263mm×358mm. In order to make a comparison, the traditional Offner and Wynne-Offner spectrometers are also designed with the same optical parameters. Under the same imaging quality requirements, the size of the two spectrometers is about 5 times and 3 times larger than that of the proposed Offner imaging spectrometer with freeform surfaces respectively. Finally, the manufacturability of the freeform surfaces is analyzed and the tolerance analysis results are presented.