Foucault Shadowgrams


Here are some Foucault shadowgrams of my 10 inch f/6 mirror using my Foucault tester and my Sony Mavica digital camera. Each column shows the appearance of the full disk with the knife positioned in the middle of the corresponding zone of the Couder screen. Each row shows the results of applying the action in the first column to the state in the previous row. The numbers below each image are the deviations from the best sphere in thousanths of an inch as determined by the tester with the Couder screen. The last column shows the profile the deviations of the right half of the mirror relative to a sphere (a cheapo integration of the deviation numbers); note scale changes. (Click on an image for a 4x larger view. No image processing was done beyond that done by the camera itself. You may notice some faint square patterns in the images; these are digitization errors, not flaws in the mirror!)
Zone 1 Zone 2 Zone 3 Zone 4 Error Profile
4431-6-67
1/2 hour, tool on top (TOT), decentered 25%, pressure on outside edge
-2-4-1118
13 min, TOT, decentered pressure on zone 3
115-13-4
12 min, TOT, decentered pressure on zone 2
-2-6-210
35 minutes, MOT, center-over-center strokes
196-5-19
20 minutes, MOT, long slow W-strokes, hands on edges
-18386
10 minutes, MOT, long slow W-strokes, hands on center
-7-8-418
........... data below relative to a parabola ............
5 minutes, MOT, long slow W-strokes, hands on edge
-50-131450
6 minutes, MOT, long slow W-strokes, hands on edge
-6612-11
twice around (1 minute?), TOT, decenter slightly, light pressure on outside edge
524-11
twice around (1 minute?), TOT, decenter 50%, light pressure on outside edge
-426-3


Note: this test is very sensitive to air currents. Here is an example.

Bill Arnett; last updated: 1999 Mar 9