As with any optical system, resolution is perhaps the single most important design consideration. For a telescope, on-axis resolution will depend on the figuring and alignment accuracy of the optical components, atmospheric turbulence, and diffraction. Diffraction effects arise from both the limiting primary mirror aperture and the central obscuration . The relative size of the primary and secondary mirror apertures are main considerations in the light collecting power and diffraction characteristics of the system. Light collection varies directly with the unobscured primary aperture area.
Diffraction depends both on primary mirror aperture and size of secondary obscuration.
Effective focal length of the telescope system will determine its geometrical imaging properties. Effective f-number will affect image quality. Since effective focal length is determined by the degree to which the primary mirror focal length is magnified by the secondary, flexibility between these focal lengths and the overall required package size is possible. Back focal length should be chosen convenient to the application. As discussed earlier, a wide range of choices are possible, including glass, ceramics, and metals.