[raymo]

There are so few bright enough objects to image with a small frac and a single sub that it would be just about an exercise in futility. A large frac [say130-150mm] would give some improvement, but really, as was already stated, aperture and/or stacked subs are the APer's friends. Here is a single sub of Eta Carina, 30 secs ISO 1600 with my 8" Newt.
The image of comet Leonard is not a good example for your situation, comets are often quite bright, and only come along occasionally.
Also, this single sub shows almost none of the comet's tail, which is what makes comet images speccy. Have a look at Gypsy's
great version in the deep space section,[ titled "my first comet"] which highlights what stacked subs provide.
raymo