Hi Dale
Paraboloid surfaces are used because they reflect light back to a point. When we put our secondary in the way of that reflected light we are efectively taking that large surface area of the primary mirror and then making it more condensed for our eyes to see. Its the reason why our Austar and Foxtel satellite dishes are shaped they way they are - they take the incoming signal then reflect it back onto the actual antenna which is really only about the size of a matchbox (well .. ok maybe a bit bigger but not much !
)
Hope this helps,
Mathew.