Cryogenic Refocusing of an Ultrawide FOV Long-Wave Infrared Imaging Spectrometer in a Geostationary Orbit

Imaging spectrometers in a geostationary orbit have unique advantages for various applications ranging from atmospheric to ocean remote sensing. To increase remote sensing range, we developed an ultrawide FOV LWIR imaging spectrometer. We compared several spectrometers and picked the "Offner + center single prism" form, which can simplify the system structure and increase cryogenic adaptability, making it ideal for ultrawide FOV designs. However, to reduce background radiation in LWIR bands, the imaging spectrometer was cooled to 123 K, causing the image plane to deviate from the focal plane. Therefore, we propose a cryogenic refocusing method based on an inclined slit in this paper that employs the full width half maximum (FWHM) of the spectral response function (SRF) as the refocusing evaluation function. By analyzing the effect of deviation on FWHM, the refocusing principle is well explained. The slit is tilted to find the minimal function value, so the deviation amount is calculated using differences in FOVs corresponding to the minimum FWHM at different temperatures. Experiments show that the FWHM is nearly 2.1 pixels after refocusing and that the actual spectral resolution is within 120 nm after spectral calibration. The method reduced the design time and provided references for optical systems experiencing refocusing issues in remote sensing imaging.