Another close-up of a portion of the "ingredients" to test optics. This Foucault Tester resides in front of a precision 12.5 inch optical flat. The tester throws out light to the telescope mirror being tested at the other end of the optical tunnel. The light comes back from the mirror being tested to hit the big 12.5 inch optical flat mirror. The flat mirror then reflects the cone of light back to the primary mirror being tested and the return beam comes back to us by passing thru a hole in the back of the big optical flat. Since the light cone makes a double pass it is often called a "DOUBLE PASS TEST". Using a set up in this manor is rewarding because it is twice as accurate as the standard Foucault test methods, due to the double pass, and the light effectively "nulls" out the light cone. Meaning the zones really don't have to measured when using this apparatus. What we are really looking for virtually NO longitudal movement, as measured at the Foucault testing device. When this is experienced we have finished our telescope mirror!