The Devil is in the details (or how I found a screw up!)

While doing my Wavelength analysis, I kept noticing a subtle triangular patter to the images. After doing some tinkering in Paint shop pro, I cam up with the below image. It’s a binary AND of the RED and BLUE images I took for the Wavelength Foucault tests.

In this picture I added 3 white circles that represent the position of the anchors I used to hold my pitch lap in place while fine grinding and polishing. Obviously I must think of something else! The anchors are deforming my mirror surface!

The anchors are 3/8 inch long pieces of Thick walled rubber vacuum tubing with a drywall screw down the middle. When installing them I would lay the tool down, position the rubber anchors around the tool, then screw them in place. I MADE SURE that the rubber was just slightly snug, just tight enough not to allow the tool to move very much, but loose enough to easily remove the tool to exchange it for a mirror, and do clean up tasks.

Note: All fine grinding and polishing have been done mirror on top since 12 Micron grit. My Focal length was a little long and I’ve been trying to tweak it.

Hopefully this mess will polish out once I conceive of a new method to hold the tool/mirror while I work on it.

(Notice the flat sides of the ‘round’ mirror and the corresponding shadows towards the middle!)

Update:

After many discussions on the ATM list I come to the following conclusions that resultsin my above rather ugly mirror.

1. The rubber cleats were converting the mechanical motion of the tool in to thermal energy, and then returning that heat to the tool and causing it to thermally expand. This resulting in three high spots on the tool, this in turn ground/polished the flat spots into the mirrors
2. Perhaps there was a rebound effect. The rubber cleats may have stored some mechanical energy and returned it to the tool causing it to ‘jump’ at the end of each stroke
3. My polishing stroke was all wrong!
1. My number one error was the miss understanding that a 1/3 Center of Center stroke did not mean that you moved the item on top 1/3 the diameter up, return to center and then 1/3 Down and back. 1/3 Center over center means you move the item on top 1/6 the diameter up, return to center and then 1/6 the diameter down and back! (ooopps)
2. My second error was that I was doing 10 or so strokes at each location on my way around the barrel, without rotating the mirror on each stroke. Seem you should rotate the item on top a few degrees between (or during?) each stroke…