Hi Charlie:

No you are asking the right question. The problem is wavelenght and surface accuracy. What is parabolic for sound waves is probably eliptical or hyperbolic for light. The speed of light is about one million times that of sound and therefore about a million times shorter for a wave length (within scale). This just means that sound waves can be focused fairly well with just about any approximation of a paraboloid while light is more exacting. Most sound parabolics are f:0.5 or faster and even if the paraboloid was perfect the steep cone would introduce so much error that it would not image light properly. A crude paraboloid can collect sunlight and bring it to a small enough area to produce heat but still not be focusable for imaging.

Cheers,
-Rick