amateur observing - Dish antenna as parabolic mirror for OPTICAL telescope?

I would have added this as a comment (not enough rep yet, I'm afraid)...
To elaborate on Andy's answer, the first reason is that the surface of the satellite dishes are too coarse to form any kind of image. Polished optical surfaces are smoothed to a polish (generally much smoother than one would achieve in polishing a car, though).

Other problems come from the precision of the form - the paraboloid of the satellite dish may have errors on the order of millimeters, because radio waves (which satellites use) don't care about that much error. In fact, satellite dishes are EXACTLY the same as radio telescopes, except the receiver cone is designed for frequencies used by communication satellites instead of frequencies of interest to astronomy.

Just for your interest, old satellite dishes ARE well-suited to function as rough solar collectors (for a solar oven) or as an acoustic telescope (position a microphone or speaker where the receiver cone normally lies)!