24 inch telescope - sad news and how we overcome it

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
Chanlunlun
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24 inch telescope - sad news and how we overcome it

文章 Chanlunlun » 週五 21 5月, 2004 12:19

Dear ATMs,

We have been waiting, waiting…….and waiting. Finally we get the reply.

It was a sad news – the factory who had promised to deliver an aluminizing job for our 24 inch mirror could not fulfill his commitment. They had dismantled their vacuum chamber well before we finished figuring the mirror. The news was a shocking one and this could only mean that there was no way for our finished mirror to work.

We have no option but to switch to plan B – the long forgotten technique of depositing a layer of pure silver by chemical method. Silvered telescope mirror is only a legend – we heard of it but we never saw one. There are a lot of myth about mirror silvering – it is technically very demanding, it is expensive, it causes fierce explosion if the process is carelessly executed, the required chemical is extremely difficult to find, the silver layer obtained reflects very little light…………….

It seems that silver is a kind of devil. Isn』t that obvious? Otherwise why are all mirrors nowadays are aluminized ? But we have no choice since no firm can manage to aluminize such a big mirror. In the first place we have to collect all necessary chemicals :-

1. solid silver nitrate – it appears in all secondary school chemistry lab. Easy to obtain except that it is rather expensive. .
2. Solid potassium hydroxide – same as silver nitrate, easy to obtain in all secondary school lab.
3. Ammonium hydroxide – a must have in all school labs.
4. Dextrose – buy it in all super market in cheap cheap price.
5. A lot of distilled water – also readily available in all super markets.

It doesn』t seem that difficult to locate all the reagents. So we carried out a silvering experiment on a 10 inch mirror first, just as a pilot test before doing the real job on the 24 inch. The process is actually quite easy to handle, within the reach of average secondary school science students. The whole process only last for a few minutes ( preparation takes about half an hour ) and the silver layer obtained is a little bit yellowish in color instead of brightly white. In the first place the silver layer right after the process did not seem to reflect much ( that』s why the rumor ). However what we had discovered was that the silver layer was astonishingly strong and sticked firm on the glass, so firm that it could be directly polished by a chamois skin!! After such polishing the silver layer became very reflective and looked promising.

Now we have mastering the legendary technique of silvering a mirror. It has its own drawback and merit. On one hand we all expect that the life of silver layer will not exceed two years but on the other hand we can re-coat the mirror at any time we wish, all by ourselves. There will be no more waiting for reply from aluminizing firm. We can coat even bigger mirror by ourselves.

The only variable now is life-span of silver layer. Well, we have to wait and see.

Best regards
Chan Yuk Lun
20-5-2004
附加檔案
silvering_start.jpg
Start of silvering job. The solution are poured directly onto the mirror, which was surrounded by a 「dam」 made of newspaper to avoid liquid flowing out.
silvering_start.jpg (59.12 KiB) 已瀏覽 10956 次
not_OK.jpg
It doesn』t look good!!!! How comes the process becomeing so ugly?
not_OK.jpg (65.96 KiB) 已瀏覽 10956 次
flushing_1.jpg
After washing off the 「mud」, the underneath silver appears. The silver layer did not look reflective.
flushing_1.jpg (59.7 KiB) 已瀏覽 10956 次
最後由 Chanlunlun 於 週五 21 5月, 2004 21:46 編輯,總共編輯了 1 次。

Chanlunlun
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文章 Chanlunlun » 週五 21 5月, 2004 12:22

more pictures to come
附加檔案
silver_polish_1.jpg
The silver layer is astonishingly strong. It can withstand dry polishing by chamois skin. After polishing, the reflectivity is vastly improved.
silver_polish_1.jpg (45.51 KiB) 已瀏覽 10953 次
compare_2.jpg
The left one is a 7 inch mirror with standard aluminium coating. The right one is the test 10 inch silvered mirror. It seems that they both reflect with same ability.
compare_2.jpg (37.37 KiB) 已瀏覽 10953 次

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sodium
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文章 sodium » 週五 21 5月, 2004 12:29

咁磨鏡班d鏡咪唔使寄黎寄去?
Chan sir呀d chemical幾錢?

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Zephix
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文章 Zephix » 週五 21 5月, 2004 12:40

E? 今年可以試下喎~~~~睇落去唔係太難做~~

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Wah!
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文章 Wah! » 週五 21 5月, 2004 12:42

以香港的污染,不知道可以保持反射率多久呢? :roll: :?: :roll: :?:

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Zephix
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文章 Zephix » 週五 21 5月, 2004 12:44

Wah! 寫:以香港的污染,不知道可以保持反射率多久呢? :roll: :?: :roll: :?:
有冇可能o係鍍銀之後, 自製Coating?
我係一隻披住狼皮既羊
帶我睇星,唔好食我

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Wah!
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文章 Wah! » 週五 21 5月, 2004 12:47

石英coating就要搵真空室來蒸,咁即係回到鍍鋁的同一個問題.

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Wongsir
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文章 Wongsir » 週五 21 5月, 2004 13:19

Mirror God,

It's great !!
Its amazing!!:shock: :shock: :shock: :shock: :shock:

I did make a silver coating on my 6 inch mirror a long long time ago( 30 years :) ).
But I did not dare to polish it with a dry cloth.
Congratulation, you are really a man that would do it yourself with your hand not "child talk".

You are my hero. :wink: :wink:


Wongsir
21-5-2004

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Wah!
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文章 Wah! » 週五 21 5月, 2004 13:39

唔知24"幾時會"開光"呢?
我估摘個好天來開光會開心好多~~

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simonii
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註冊時間: 週五 19 12月, 2003 18:20

文章 simonii » 週五 21 5月, 2004 13:45

Really a very good try
acturally, we have done this experiment during our chemistry lesson,
This is actually the Tollen's Test.
I think the result of yellowish surface is the amount of excess Silver Nitrate is added, anyway, this is just my theoretical guess.

For the coating part, i have heard that using chloroform and some plastic can DIY a appreciable solution of coating layer which can protect the silver surface from the polluted air in HK.
But i have forgotten which kind of plastic it is. :wink: The sulphur compounds in HK is very terrible and it can much much lower the life of a silver surface :x

Looking forward to see a master piece of mirror

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simonii
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文章 simonii » 週五 21 5月, 2004 13:47

Ar.. I forgot something.
I saw the solution u used is a cloudy solution.
But it should be a clear solution.
Add more ammonia to dissolve the cloudy precipitate

Chanlunlun
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文章 Chanlunlun » 週五 21 5月, 2004 16:01

simonii 寫:Ar.. I forgot something.
I saw the solution u used is a cloudy solution.
But it should be a clear solution.
Add more ammonia to dissolve the cloudy precipitate
Dear Mr./Mrs/Miss Simonii,

Believe it or not, the cloudy mud you see are solid silver that are being precipitated out. If we add more ammonia, the solid silver will dissolve again and the whole process fails.

You can say that we are making a large scale Tollen's experiment. Don't expect clear solution, silver will spread to everywhere if solution leaks out. ( it did actually happened )

Best regards
Chan Yuk Lun
最後由 Chanlunlun 於 週五 21 5月, 2004 21:51 編輯,總共編輯了 3 次。

Chanlunlun
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文章 Chanlunlun » 週五 21 5月, 2004 16:05

sodium 寫:咁磨鏡班d鏡咪唔使寄黎寄去?
Chan sir呀d chemical幾錢?
Dear mr Sodium,

Our silvering experiment is fully back up by school. Silver nitrate is very expensive and you better coat the mirror by aluminium. Presuming that your mirror is not a big one.

Best regards

Chan Yuk Lun

Chanlunlun
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文章 Chanlunlun » 週五 21 5月, 2004 16:11

Wah! 寫:以香港的污染,不知道可以保持反射率多久呢? :roll: :?: :roll: :?:
Dear Mr Wah!,

We don't expect a very long life span for the silver layer. If it can survive one year we will be very happy.

If we can choose we absolutely vote for aluminium layer + silicone dioxide coating. But we have no option.

There is no way to aluminize a 24 inch mirror in HK, or in China.

Best regards
Chan Yuk Lun

Chanlunlun
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文章 Chanlunlun » 週五 21 5月, 2004 16:19

Wah! 寫:唔知24"幾時會"開光"呢?
我估摘個好天來開光會開心好多~~
Dear Mr. Wah!,

Since we have mastered the technique we can silver the 24 inch mirror at any time we wish. But we will wait until June, when clear weather appears.

It is essential to keep the silver layer as fresh as possible when telescope sees first light, at least we have the highest possible reflectivity.

Best regards
Chan Yuk Lun

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