In this paper, the practical tradeoffs for designing submillimeter wavelength imagers based on optomechanical systems combined with focal plane arrays (FPAs) are presented. The architecture of these systems differs for operation at short and long ranges. General formulas to derive the effective field of view of diffraction limited quasi-optical systems in these two scenarios are shown. These formulas can be used to evaluate the performance of a specific optical system implementation. As an application example, we present the design of an optomechanical system that can operate at both ranges in a modular approach. The presented implementation achieves an effective field of view, which is 70% of the canonical one. The proposed solution consists of a linear FPA of eight active transceivers combined with a raster scan technique. The system for short-range scenario is a side-fed dual-reflector Dragonian architecture because of its good scanning performance when illuminated by an FPA. Thanks to the system's small aperture, the scanner is arranged after the primary mirror, without causing additional scan loss. The Dragonian system is then used to illuminate a confocal dual-reflector architecture to magnify its aperture, and can be used in the long-range scenario. The scanner in this case is before the main aperture and it has to be considered in the performance optimization of the optical system since it adds phase aberration loss.