polishing a 22" mirror

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eq platform

22" f/4.0 lowrider

making the 22" primary mirror

polishing

to grinding to figuring

For polishing I used a 40 cm plaster tool and 28° pitch. On the sides of the tool were shallow grooves to have a better grip when putting the tool on or off the mirror. I screwed four wooden blocks on the back of the tool, which allowed for better handling. Before warm pressing, the pitch was softened by heating the tool head over in a hot water bath in a big bucket. The grooves were formed with an aluminium bar. Later, I used a pad saw to deepen the grooves. The first fitting of the pitch lap was somewhat lengthy, as the thickness of the pitch was uneven in the beginning. It took therefore several cycles of warming, grooving, warming, and pressing to get a got fit of the entire surface.

Room temperature was around 18 to 20°C, which is actually too cold for 28° pitch. Still, it worked well, the lap had good contact, but the surface of the mirror was not really smooth in the Foucault image.

I used Ceri HPC as polishing agent in a rather thin suspension, roughly 1:8 in water. The ceroxide settles over time in the stock suspension, such that it needs to be stirred up before use. I added water to the mirror surface as soon as the movements of the lap became harder.

When I started to move the pitch lap for the first time, I immediately noticed that it will be a torture to keep this 40 cm lap running ! Therefore I increased the number of grooves and also put a thin mosquito netting between lap and mirror during pressing for micro facetting. This improved the movements somewhat, but it still was hard work. I polished between 45 minutes and 1.5 hours in a row. After that, I was always completely exhausted. The polishing strokes consisted, similar as the grinding strokes, of circular movements with varying overlap (like a rosetta pattern). The tool was rotated every once in a while and also the mirror was rotated on its support.

Foucault test was performed with a slitless tester and moving source.

Polishing took about 12 hours of hard work spread over two weeks.

I frequently tested the mirror for astigmatism, which was certainly present after fine grinding, and which was not due to a warped blank. This astigmatism could be detected optically, but not with a spherometer. I think such astigmatism is probably normal, usually goes unnoticed, and disappears during polishing.

Polishing progress was checked regularly by a microscope and by looking for stray light from a laser directed at the surface.

A more detailed diary of the polishing can be found here (German only).