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Blog Comments posted by Astrobits
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There is no need to achieve a perfect sphere before starting figuring. If you have a raised centre then work on the centre only. After all, you have to work on the centre of a sphere to get a parabola. If the centre is depressed ( but not too much ) then some of the figuring work has already started!
Finish the polishing first then work on the areas that your testing shows are not in the correct parabolic zone using smaller tools as necessary. It might be wise, before you finish polishing, to cast some smaller tool bases so that they will be ready for you when you need them.
Nigel
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It is possible that using a full diameter tool ( tool on top ) you are causing the centre of the tool to become depressed and hence lessening the polishing of the centre of the mirror. Try going to sub-diameter tools. I would never consider a full sized tool for a 20". All my 20" mirrors were made, tool on top, with 13" and smaller tools.
Have you felt the warmth of the mirror and tool after a bit of polishing?
Nigel
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finally a first test report
in crashtestdummy's Blog
A blog by crashtestdummy in General
Posted
I'd go for a smaller lap. 17" is a bit too close to a full dia lap for my liking. But whatever you use make sure that the first priority is a full polish and the figure is secondary until that full polish is reached.
I never tested my mirrors until they were fully polished. That way I avoided the temptation to call a parabolic figure with pits "done".
Nigel