In a feasibility study the system analyst must assimilate many conflicting mission goals before selecting preliminary parameter values. If the analyst uses a computer to handle the data he may lose sight of some important interrelationships. It is therefore often preferable to keep to a paper-and-pencil level in the preliminary stages of a system design. In this paper such a level of analysis is developed for relating spatial and radiometric resolution to aperture and f/number in a large optical system. This "asymptote analysis" has been applied in a recent study of NASA's SEOS/LEST concept.