1. An apparatus for increasing the resolution of a cryogenically cooled and vacuum-sealed infrared imaging detector, comprising: a two-dimensional detector array of photosensitive pixels arranged in a matrix, said detector array successively exposed to an image scene, to acquire multiple imaging samples of said scene; a masking filter, disposed between said detector array and the focal plane of said image scene and maintained at a fixed position with respect to said detector array, said masking filter operative to reduce the region of said pixels collecting incident radiation from said image scene for each of said imaging samples, such that only a portion of each pixel area of said image scene is imaged onto the corresponding pixel of said detector array; optics, disposed between said masking filter and the focal plane of said image scene, said optics operative to direct said incident radiation onto said detector array through said masking filter; shifting means, for successively shifting the optical path of said image scene relative to said masking filter, by a shifting increment equal to a fraction of the pixel width of said array pixels and corresponding to a fill factor reduction amount, to provide imaging of different sub-pixel regions in each of said imaging samples; and a processor, operative for reconstructing an image frame having a resolution greater than the intrinsic resolution of said detector, by a factor defined by said fill factor reduction amount, from said acquired imaging samples.