Following the polishing of the 24 inch mirror, the parabolization process began.
Prior to the parabolization there was a lot of tools to be prepared. First of all the Offner null tester had to be finished. Then we also needed to make a mirror jig to stand up the big mirror in vertical position WITHOUT MIRROR FLEXING. In foreign countries atms who try to test their big + thin mirror had used a lot of peculiar method of supporting - from using piano wire to hang the mirror to floating the mirror on mercury. While the true effects of these scary methods were doubtful we just employed the most common method – use two pieces of plywood ( thickness 9 mm ) nailed them to 90 degree and just place the mirror vertically on its own edge. Due to the weight of the mirror it stands on its own edge rigidly. We also discovered that in such position the mirror will suffer from no flexing at all. ( Actually these are the experiences gained from the making of the 20 inch mirror ).
Before parabolization really took place, the surface figure had to be checked by simple Foucalt test to see how bad it was as the mirror had been polished by our crazy human fresh polishing machine.
There were two kinds of surfaces that we afraid most :-
1. Dog Biscuit – a highly irregular surface produced by rough handling during fine grinding, poor pitch lap or too little rotation during polishing. For small mirror a dog biscuit was not a real problem, as normal polishing continued the effect will slowly disappear. However for big mirror a dog biscuit is incurable and it need a restart at #2000 fine grinding.
2. Astigmatism – again this is incurable and need a restart at #2000. Astigmatism frequently occurs in big mirror. The reason is mainly due to poor supporting of mirror during polishing or wrong polishing method. For ultra thin mirror the risk of astigmatism is especially great. The remedy is to place the mirror onto a piece of thick carpet and use rotary stroke instead of 「W」 stroke. Frequently change the position of mirror relative to the carpet also helped to avoid astigmatism. Astigmatism could not be cured by polishing alone.
So what is the figure of our 24 inch mirror ?
Well, as indicated by the first Foucalt test, there is no astigmatism detected and the surface is smooth ( no dog biscuit ). We safely pass another crise.
The surface is indeed an oblate spheroid. So we can safely go ahead for parabolization right away. A parabolization lap of diameter of 9.5 inches with extra soft pitch will be poured. At the moment we still use the 14 inch lap for parabolization but the progress is very slow.
So far we reach the final stage. This stage may last for a long time and we are now being engage in a time consuming war.
Hopefully this war will soon be over.
Chan Yuk Lun
自製天文望遠鏡 牛頓望遠鏡 杜蘇式 卡式 設計和磨鏡方法<br>
佛科試鏡法 佛科刀片儀 光柵試鏡法 光柵片
佛科試鏡法 佛科刀片儀 光柵試鏡法 光柵片
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