Target Sun & Earth
The Resolving Power of this telescope is 120 / (12 * 25.4) = 0.39 seconds of arc.  Here we note that some of the Near Earth Objects that have been found, have been found by good 14" telescopes, and that the atmosphere limits the viewing of ground based telescopes to a Resolving Power of 0.50 seconds of arc at the best of times. This can be resolved by a telescope with a Clear Aperture of 10". Most of the time, atmospheric seeing limits ground based telescopes to a resolving power of 3.0 seconds of arc, - which requires the resolving power of a telescope with a 2" clear aperture.  Thus, any ground base telescope not equipped with "adaptive optics", no matter what size it is, usually only has the resolving power of a telescope with a 2" clear aperture.
Telescopes equipped with a Self Guiding CCD camera and a Tip-Tilt mirror accessory, can achieve Adaptive Optics capabilities.  A telescope with a 10" Clear Aperture, equipped this way can resolve 0.7 arc sec. from the ground any time the sky can be seen. This is the equivalent of using a telescope with a clear aperture of 6.75" on the best "seeing" days.
The above clear apertures are considered as pure Useable Clear Apertures. Almost all Newtonian Telescopes use a Paraboloid for a main mirror. Paraboloids only mimic True Parabolic Reflectors, and always introduce coma error, so that the Usefull Clear Aperture is somewhat reduced. Combine this with the fact that the surface deformities of today's thin mirrors mounted on "floatation cells" further reduce the diffraction limited resolution of the telescope in which they are employed, and you will begin to appreciate my concern that using such equipment to detect comets and asteroids is largely a hit and miss affair.