1. A method of providing corrected values of gain and level coefficients of a set of correction coefficients for a scanning detector array, the scanning detector array comprising a plurality of detector channels, the method comprising the steps of: modifying first values of the gain and level coefficients using at least one frame of image data collected by the scanning detector array from out-of-focus multiple-temperature imagery; and determining updated values of the gain and level coefficients using a scene-based non-uniformity correction (SBNUC) routine applied to scene data corresponding to focused scene imagery.