Out-of-field stray light correction in optical instruments: the case of Metop-3MI

In high performance optical instruments, post-processing methods are employed to reduce the stray light (SL) level below what can be achieved by design. Often, algorithms are limited to SL coming from sources located inside the field of view (FOV) of the instrument; however, SL could also come from sources located outside the FOV. We describe a method for correcting out-of-field SL, in particular for the case of the Earth observation instrument Metop-3MI. The proposed approach is a variant of the in-field (IF) method developed previously. We estimate the out-of-field SL by a linear combination of the SL kernels modulated by the input scene radiance. The correction is computed for fields on a regularly spaced polar grid, providing reasonable variations of the solid angle sustained around individual fields. Compared with the case of IF SL correction, a difficulty is that the out-of-field input radiance is unknown. We implement a mirroring technique, which is shown to be effective in most situations. This method will be used to correct the in-flight data of Metop-3MI, hence providing an SL level sufficiently low to fulfill the mission requirement.