#2000 fine grinding of 24 in mirror completed

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
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Chanlunlun
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註冊時間: 週四 09 10月, 2003 21:06

#2000 fine grinding of 24 in mirror completed

文章 Chanlunlun » 週二 24 2月, 2004 10:32

Dear ATMs,

The fine grinding of the 24 in mirror was carried on until last night ( 23rd Feb, Monday) where we reached a stage of finishing the #2000 grit. Up to here we regarded that the fine grinding process of the 24 inch mirror was fully completed.

The fine grinding of big mirror, especially from #600 onward, was a very terrible story. In the first place we had to pay particular attention to the status of the liquid film between the mirror and tool to avoid seizing. Actually in the first second we start the #600 fine grinding the mirror and tool stick together, due to the fact that the tool surface is not fully wet before we covered on by the mirror. The dry spots just 「welded」 together and we had to apply all our strength just manage to separate the two glass disks. Afterwards we were so scared that we wet every part of tool surface before we put on mirror and seizing never happened again.

If you think that for a mirror as big as 24 inch it must be very heavy in pushing it during the fine grinding stage then you are just plainly wrong. The situation was that the mirror, no matter how heavy it is, just floated on the tool and there was completely no friction during fine grinding. This was dangerous because if you just pushed and did not know how to stop you probably ended up with the mirror going straight to the floor. The fine grinding was the best demonstration of Newton』s first law : without any friction, the motion of an object ( the 24 in mirror ) will never stop.

Now we come the most scary part :- For every ten minutes we had to life up the 24 inch mirror, rest it onto a table, add new carbo and water ( very carefully, to make sure to wet all surface of the tool ) and then lift up the mirror again. THEN, holding the extremely heavy mirror on hands all your own, SLOWLY and VERY CAREFULLY lower the mirror on top of the tool, AVOIDING touching of the two glass disks too fiercely ( remember the mass of the 24 in mirror is very big, so is its momentum. Even a very slow impact on the tool surface will result in disaster of cracking the glass surface ) to crack the glass. Even a very small chip off the glass rim would produce a great regretting scratch on mirror surface. We had to carry out this scary action for every ten minutes, during the whole process of the fine grinding stage until we finished the #2000 carbo.

For every change to a finer carbo., everything had to be washed thoroughly clean. This was a very big job for we had to move the heavy mirror, tool, grinding table downstairs to clean with water jet and brought them up again. Moving the glass disks downstairs and upstairs was particularly scary for we have to pass through the playground and a flying football may ruin everything. Newspaper was very helpful. We tried to cover every exposing surface with new newspaper and this proved to be very practical. Anything dirty underneath got no chance to get in touch with the cleaned glass surface.

Despite all the difficulties we manage to finish all the fine grinding without a fault. Now the mirror surface is super smooth with not a single trace of scratch or hole. The final focal length of the mirror is 84.7 inch, or a f/3.53 curve.

Our next stage to polish the mirror. We are now trying to figure out a method of producing a 24 inch polishing lap. We cannot pour hot molten pitch onto the surface of the tool, the tool will certainly crack.

Do you have any suggestion for making the pitch lap? All comments are welcomed.

Best regards

Chan Yuk Lun
24-2-2004
附加檔案
reflection.jpg
The finished #2000 surface has a rather high reflectivity at a low angle.
reflection.jpg (56.36 KiB) 已瀏覽 9887 次
ultra_fine.jpg
The #2000 surface, the surface is so smooth that we can read the small characters on newspaper underneath.
ultra_fine.jpg (94.17 KiB) 已瀏覽 9887 次

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Wah!
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文章 Wah! » 週二 24 2月, 2004 12:43

好快手呀!
睇來今個月都可以完成喇吧?

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Wongsir
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Re: #2000 fine grinding of 24 in mirror completed

文章 Wongsir » 週二 24 2月, 2004 12:46

Chanlunlun 寫:Dear ATMs,

................
Do you have any suggestion for making the pitch lap? All comments are welcomed.

Best regards

Chan Yuk Lun
24-2-2004

Dear Mirror God,

As you said , the glass plate cannot stand the large quantity of hot pitch. Also making a perfect match with a 24" mirror is not easy.
So, I suggest an alternative way --- to make an individual pitch squares for the large mirror.
The pitch strips will be poured onto a wooden mold where it should be wet with polishing powder to prevent sticking.
Then the pitch strips can be cut into squares with a knife or an electric soldering iron, where soapy water can be used as lubricant.
At the mean time the surface of the tool is marked. Then the facet squares are heated either by lighter or hot plate, and pressed onto the tool afterwards. The extra square pitch around the edge is cut away.
For perfect adhesion, you have to heat up the whole pitch in hot water and then press with the mirror again.

Best wishes!
:D :D
Wongsir
24-2-2004

Chanlunlun
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文章 Chanlunlun » 週二 24 2月, 2004 19:43

Wah! 寫:好快手呀!
睇來今個月都可以完成喇吧?
Dear Mr. Wah!,

Thanks for your blessing. Polishing will not be time consuming.

The difficult part is still yet to come - the figuring of the mirror into the required paraboloid. We set no time limit for this stage and all depends on luck.

Hence I may not be that optimistic as you do. But one thing is for sure - the mirror cannot be finished within this month.

Best regards

Chan Yuk Lun
24-2-2004

Chanlunlun
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Re: #2000 fine grinding of 24 in mirror completed

文章 Chanlunlun » 週二 24 2月, 2004 19:50

Wongsir 寫: ..................
The pitch strips will be poured onto a wooden mold where it should be wet with polishing powder to prevent sticking.
..........................
Dear Mr. Wongsir,

Thanks a lot for your valuable opinion. I will do as told by you.

However there exists a question : how can I safely detach the moulded pitch strip away from the wooden mould? I think the hot pitch will stick hardly onto wood. It will be difficult to separate pitch from wood after solidification.

Look forward for your answer.

Best regards

Chan Yuk Lun
24-2-2004

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Wongsir
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Re: #2000 fine grinding of 24 in mirror completed

文章 Wongsir » 週二 24 2月, 2004 20:30

Chanlunlun 寫: Dear Mr. Wongsir,

Thanks a lot for your valuable opinion. I will do as told by you.

However there exists a question : how can I safely detach the moulded pitch strip away from the wooden mould? I think the hot pitch will stick hardly onto wood. It will be difficult to separate pitch from wood after solidification.

Look forward for your answer.

Best regards

Chan Yuk Lun
24-2-2004
Dear Mirror God,

You have to paint the surface of the wood with a layer of wet
polishing powder solution.
I think water and the "wet powder" may be helpful in
preventing the pitch stick to the wood.

Wongsir
24-2-2004

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Wah!
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文章 Wah! » 週二 24 2月, 2004 21:27

用木頭會有隙,可能食住瀝青.
用金屬板會唔會好d?例如月餅盒的蓋.
之後把凍了的瀝青塊放入冰格度,我估可以脫模吧?

Chanlunlun
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文章: 3831
註冊時間: 週四 09 10月, 2003 21:06

文章 Chanlunlun » 週三 25 2月, 2004 14:25

Wah! 寫:用木頭會有隙,可能食住瀝青.
用金屬板會唔會好d?例如月餅盒的蓋.
之後把凍了的瀝青塊放入冰格度,我估可以脫模吧?
Dear Mr. Wah!,

Thanks for your suggestion. The problem is , I don't know where to fond a moon cake box lid.

I also need many such lids. Will you please let me know where I can find them.

Thanks for your assistance.

Chan Yuk Lun
25-2-2004

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Wah!
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文章 Wah! » 週三 25 2月, 2004 14:32

呢層我就唔清楚喇......

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