how does one find the focal point of a 8 foot parabolic dish?

Answer 1

Just measure the depth of the dish and put it into the calculator.
http://www.stellafane.com/atm/atm_select_scope/atm_sag2fl_calc.htm

Answer 2

A parabola is the locus of points equidistant from a focus and a directorix (a 'base' line behind the parabola).

I have used this definition to create an 8' dish, before. The same definition can be used to find the focus (although the guy above's answer is simpler to do).

Set the dish up vertically and parallel to a vertical wall (side of your house/apartment) at a distance you estimate to be the focal distance. Using 4 pieces of string and a couple of friends, slide the dish out from the wall (or in toward the wall) until 4 pieces of string are of equal length. The 4 pieces are:
1. From the estimated focal point to the center of the dish (use tape to fix the string to the center).
2. From the center of the dish (minus the thickness of the dish) straight back to the wall.
3. From the estimated focal point to the edge of the dish
4. From the edge of the dish (same edge point as above) to the wall.

When all 4 lengths of string are equal, the estimated focal point is the real focal point.

To make sure the dish is parallel to the wall, use string to both edges of the dish, then when all 6 strings are equal, you have your focal point.

.

Answer 3

Place a long wooden pole along the axis of the parabola. Point the dish at the son. The point where the wood burns is the focal point.