24 in telescope - secondary mirror problem

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註冊時間: 週四 09 10月, 2003 21:06

24 in telescope - secondary mirror problem

文章 Chanlunlun » 週六 10 4月, 2004 01:18

Dear ATMs,

It has made well known that our telescope has a f/ratio of f/3.4, which requires a rather big secondary diagonal mirror. Classical wisdom tells that at such f/ratio at least a 25% secondary mirror should be used. ( C8 has a 33% secondary mirror ). That is to say we need a 6 inch minor axis elliptical diagonal mirror

That is a real problem because

1. There is no supplier who can supply such big diagonal mirror.
2. Even if such item exists, it is bond to be extremely expensive.
3. Such a diagonal weights too much, against our goal to design a light weight telescope.
4. If we choose to grind and figure the flat ourselves, we have no idea of how to cut it into elliptical shape, which means that we are forced to make a secondary mirror of circular shape of 8.4 inch diameter. It is a crazy idea.

So we solve the problem in an ugly but practical way. We make the focal plane of the final image lying INSIDE the entrance beam, in this way a standard 4 inch diagonal mirror is sufficient the purpose. We save a lot of money because we are using mass produced standard items.

Since the eyepiece and the focuser are inside the entrance beam now, there will be an extra light loss since they are blocking off the light. But don『t be panic. Simple calculation shows that using a 6 inch diagonal we have a light energy loss of 6.3%. Using a 4 inch diagonal plus an intruding focuser the total light energy loss will only be 4.8%. We not only solve the problem but also get some bonus of extra light energy.

There is only one drawback – an intruding focuser looks very ugly. At least it feels ugly to me.

Best regards

Chan Yuk Lun
Two ways to solve the problem. Left : 6 inch diagonal. Right : 4 inch diagonal + intruding focuser. Although it doesn』t seem to be, the right hand side solution blocks off less light than the left hand side one.
intruding_focuser.jpg (10.64 KiB) 已瀏覽 4346 次


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