What is the optical test "of Caustic".

Is’ born in 1939 and it serves to measure with extreme precision (impossible to Foucault test for reasons which we shall see here later looking at the title: REASONS WHY THE FOUCAULT TEST IS INACCURATE FOR LOW FOCALS RATIOS MIRRORS)) the radii of curvature of the zones of a parabolic mirror, or even more deformed figure from, a very short focal ratio (that is more inferior to F5) and a beyond the diameter 300 or