Minimize obstruction of the light path. The deterioration is a direct function of the size of the obstruction. The effect may be minimized in a Newtonian or classical-Cassegrain telescope by making the secondary as small as possible. This in turn implies large focal length. Achieve optical excellence.
Optical excellence is the most important feature of any telescope.
Theoretically, this applies to any optical design, at any focal length. In practice, however, simple designs with few optical surfaces and long focal length work best. It should be remembered that the sensitivity of optical tests varies as the square of the focal length. Limit atmospheric turbulence. Many aspects of atmospheric turbulence lie beyond our control, but local turbulence can be ameliorated.