The principal applications of off-axis optical systems are to avoid obstruction of the primary aperture by secondary optical elements, instrument packages, or sensors, and to provide ready access to instrument packages or sensors at the focus. The engineering tradeoff of an off-axis optical system is an increase in image aberrations.
Source: From Federal Standard 1037C
"Image-Forming Optical System, Exposure Apparatus, and Device Producing Method" in Patent Application Approval Process
Dec 06, 2012; By a News Reporter-Staff News Editor at Politics & Government Week -- A patent application by the inventors Komatsuda, Hideki...