Alignment errors among components of an optical system may substantially degrade the image quality. Focus errors also affect system performance. The potential for serious degradation of image quality is substantial and requires that the tolerances for these errors receive significant attention early in system design. The image quality and reconnaissance performance of an all-reflecting Cassegrain is compared to an all-refractive optical system under conditions of zero and anticipated "real world" misalignments.
Eugene W. Cross,
"Alignment And Focusing Tolerance Influences On Optical Performance", Proc. SPIE 0354, Airborne Reconnaissance VI, (25 August 1983); doi: 10.1117/12.933980; https://doi.org/10.1117/12.933980