The making of 30 in telescope - 28, mirror finished

夸克星 註冊時間: 2003-10-09 文章: 2207

Dear ATMs,

In my last message I still remember saying that we had completed the 99th step out of 100. As a matter of fact this last step had proved itself to be a very difficult one. Despite All the technologies at our disposal, figuring a mirror still remains very much a human art.

I had posted my last message on 19th September mentioning that we were very close to the finishing of optical work. It was not until 6th of December』s evening that we finally confirmed the figure of the 30 inch mirror to be truly parabolic. It took us two and half months, during which we had been working for 4 days a week, 3 hours each day, to work to completion of this last 1% of optical work.

What makes the work so difficult ? We have concluded the following points in case readers may find them useful in their future reference :-

1. Glass expansion – every time after even a short round of local figuring the pitch lap will heat up the glass sufficiently. This local heating effect was so small that we at all cannot feel it ( or test it by causal method ) but big enough to cause serious distortion ion the mirror figure. This distortion was not apparent when the mirror figure is far from parabola but when it approaches a null under the Offner tester the thermal effect became very pronounced. Hence, in order to obtain a more accurate result, we normally waited 24 hours before any optical test could be carried out. You could imagine how painstaking it was to proceed with the figuring bit by bit.

2. Deep curvature – it necessitated the use of very small lap for figuring. However since the radius curvature of adjacent zones differs by a great deal it meant that no matter how small the defects was the figuring of one zone inevitably affected the other. At last we needed to take care of the whole mirror surface no matter how small or how localized the defect was.

3. Huge amount of glass to be removed – a small defect it seemed to be shown by the Offner tester, a lengthy process was it required to be carried out to remove it.

We spent 17 hours to polish the mirror but 131 working hours to figure it. It takes surprisingly nearly 8 times as much time and labour to finish the figuring, consuming nearly 5 lbs of cerium oxide. The whole mirror, however, after the figuring, weights about 40 lbs. It is sufficiently light-weight enough to be held up by one man. Thanks to the ultra thin mirror configuration.

I would like to take this opportunity to express my thanks to all students participating in this 30 inch mirror making project. They have shown their endurance, hardworking and, most of all, patience. As with many other things in astronomy, patience is a virtue.

It would be wonderful to look through a giant telescope like the one we are going to finish. We are all very excited.

Best regards
Chan Yuk Lun
8-12-2005

壇主 註冊時間: 2003-06-28 文章: 2302

鏡神,

恭喜,恭喜!
全港第一大望遠鏡主鏡終於完成了.
Laughing

Wongsir

夸克星 註冊時間: 2004-12-13 文章: 6008 來自: NT

恭喜，相片入面幾個學生笑得好燦爛

紅巨星 註冊時間: 2003-07-26 文章: 450 來自: Hong Kong

Congratulation!
I wonder when this mirror will be first lighted Question

白矮星 註冊時間: 2003-12-19 文章: 607

wow!!
that's really great!!!!!
congrat

夸克星 註冊時間: 2003-07-02 文章: 5234

相信也是一項世界記錄!

夸克星 註冊時間: 2003-10-09 文章: 2207

starship 寫到:

Congratulation!
I wonder when this mirror will be first lighted Question

Dear Mr. Ship,

That depends on whether the mirror is planned to be aluminized or silvered. If the mirror could be aluminized the first light will not be far away.

However if we are forced to silver the mirror the first light will not take place until July next year.

Best regards
Chan Yuk Lun

夸克星 註冊時間: 2003-07-02 文章: 5234

等住聽鏡神今個星期六講嘅秘笈. Cool

中子星 註冊時間: 2004-06-18 文章: 1119 來自: 香港

恭喜曬啦..^.^""

原恆星 註冊時間: 2003-07-12 文章: 64

Congrad Chan Sir!~

Looking forward to seeing the final completion of this work of art

主序星 註冊時間: 2003-06-29 文章: 158

恭喜曬鏡神同皇天所有仝人....你們創造了神話.

星際雲 註冊時間: 2003-12-27 文章: 28

and hope to see that under a "real sky" ...

夸克星 註冊時間: 2003-06-28 文章: 9868

恭喜 chan sir! Cool

Subaru 寫到:

相信也是一項世界記錄!

我相信不只是一項.

紅巨星 註冊時間: 2003-07-26 文章: 450 來自: Hong Kong

鏡神, Chan sir -- "THE RECORD BREAKER" Laughing

夸克星 註冊時間: 2003-10-09 文章: 2207

Subaru 寫到:

相信也是一項世界記錄!

Dear Mr. Subaru,

The world record of the biggest ATM telescope at the moment is a 41.2 inch Newtonian, belongs to USA. I'm still far away from there.

The biggest ATM telescope in Japan is a 32 inch, still 2 inches bigger than us.

However 30 inch should be quite safe to hold the record of biggest ATM telescope in Hong Kong and in China.

Best regards
Chan Yuk Lun