#180 fine grinding on 24 in 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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#180 fine grinding on 24 in completed

文章 Chanlunlun » 週三 18 2月, 2004 15:10

Dear ATMs,

We already finished the #180 fine grinding of the 24 inch yesterday ( Tuesday, 17th Feb. ). We had arranged the mirror on the bottom and tool on top, with a 1/3 center to center stroke, in order to lengthen the focal length.

To fully filled up the concave bowl of the mirror we need 1.5 litres of water. That explained why we encountered very great seizing force between tool and mirror at the start of fine grinding. After about 10 imutes when the water film was completely built up between the tool and mirror, fine grinding became a very pleasurable job. The tool just moved on top of mirror with very little friction, you really would not believe that you were making a 24 in mirror.

For every five minutes we had to add new carbo and this had presented some problem for us. As the tool is very heavy we could not lift it up without danger and the separation between the tool and the mirror would also ruin the water film between them. Our solution was to slide the tool aside so that the center of mirror was exposed, add carbo around the circumference of the tool and carefully press down the hanging side of the tool so that a very little gap was open up to let in the new carbo. When new carbo had gotten in, fine grind resumed as usual.

Our experience told that if we replenished new carbo around every five minutes there would be no chance for the tool and mirror seizing each other. Hence this is a very useful piece of information when we carry out #600 fine grinding onward at later times.

After 2 hours of fine grinding with mirror on bottom, there was not the slightest lengthening of focal length observable. The focal length of mirror still remains at 82 inches, or f/3.4. I

I think we have to live up with such short focal length, anyway it is not at all a bad news since the zenith height of eyepiece is reduced.

Now we are making a big 「Epsilon」.

Best regards
Chan Yuk Lun
18-2-2004
附加檔案
add carbo.jpg
Procedure to add new carbo :- push the tool aside so that the center of mirror was exposed. Add in new carbo at the rim of the tool.
Press the hanging side of the tool slightly to allow a little gap between the tool and mirror. When new carbo diffuses i
add carbo.jpg (33.98 KiB) 已瀏覽 12309 次

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Wah!
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文章 Wah! » 週三 18 2月, 2004 17:46

點解好似有d崩咁o既?

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Subaru
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文章 Subaru » 週三 18 2月, 2004 18:06

Wah! 寫:點解好似有d崩咁o既?
我估只不過係濕咗嘅碎沙嚟.

Chanlunlun
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文章 Chanlunlun » 週三 18 2月, 2004 21:59

Wah! 寫:點解好似有d崩咁o既?
Dear Mr. Wah!,

You are correct. The edge of the tool ( and the mirror ) is chipped. At a price of HK$600 per piece, we have selected not to edge both the mirror and the tool to keep our cost low.

Best regards

Chan Yuk Lun
18-2-2004

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david_kmng
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文章 david_kmng » 週五 20 2月, 2004 00:07

Hi Mr Chan,

Then what is the cost of a routed piece of glass. I also want to grind a 20" mirror this summer. Where can I find the glass plate?

Regards,
David

Chanlunlun
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文章 Chanlunlun » 週五 20 2月, 2004 17:28

david_kmng 寫:Hi Mr Chan,

Then what is the cost of a routed piece of glass. I also want to grind a 20" mirror this summer. Where can I find the glass plate?

Regards,
David
Dear Mr. kmng,

An edged glass disk of 20 inch diameter cost about 1200 to 1500 HKD per piece. You need to buy 2 disks, one for tool and one for mirror. Hence prepare to pay for a maximum cost of HKD 3000.


Best regards

Chan Yuk Lun
20-2-2004

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david_kmng
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文章 david_kmng » 週五 20 2月, 2004 18:19

Thanks Mr. Chan,

Woo.. so expensive! Is it easy to round it myself. Let's say using sanding stone (which is a greenish grey brick for sharpening knives)

And where can I found the glass and what type of glass should I use?

Regards,
David

Chanlunlun
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文章 Chanlunlun » 週五 20 2月, 2004 18:25

david_kmng 寫:Thanks Mr. Chan,

Woo.. so expensive! Is it easy to round it myself. Let's say using sanding stone (which is a greenish grey brick for sharpening knives)

And where can I found the glass and what type of glass should I use?

Regards,
David
Dear Mr. kmng,

Well you can go ahead, but the task will be extremely laborous. Indeed is it that important for the glass disk to be edged ?

Best regards

Chan Yuk Lun
20-2-2004

Chanlunlun
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文章 Chanlunlun » 週五 20 2月, 2004 18:43

david_kmng 寫:Thanks Mr. Chan,

Woo.. so expensive! Is it easy to round it myself. Let's say using sanding stone (which is a greenish grey brick for sharpening knives)

And where can I found the glass and what type of glass should I use?

Regards,
David
Dear Mr. kmng,

You can order the glass from any glassware next to your door. We bought ours in teh shop next to the back door of our school.

By the way have you made a mirror by yourself before? I mean a mirror no smaller than 10 inch. If not, it is quite dangerous for you to carry out a 20 inch project solo. Just the weight of the glass disk will scare you so much that you prefer to choose to abondon the project.

Best regards

Chan Yuk Lun
20-2-2004

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Subaru
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文章 Subaru » 週五 20 2月, 2004 23:15

Congratulations to Chan sir』s birthday.

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david_kmng
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文章 david_kmng » 週五 20 2月, 2004 23:35

Happy birthday Chan Sir :)

I did a 4 inch mirror a long time ago. I donated a pair of 10 inch mirror to a astronomical society also long time ago.

Maybe in the summer I can find some partner :) Hope so.

Regards,
David

PS what is the density of glass? 2.5 times of water? (or what is the weight of a 20" mirror of 1 inch thickness?

howerjea
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文章 howerjea » 週六 21 2月, 2004 11:45

hi,
may i have a question for you
how do you have the second flat mirror for the 24" mirror,make one or buy one?
i have a glass cuter,i like to cut a rectangle mirror for my second flat mirror
but i just can cut the glass under 6mm think,i'm failure 6mm above
how about you think,thanks
it's miracle and great have someone also make the 60cm primary mirror in the same time and me do
but i began form last summer until now,i remember i bought 60cm/19mm about 600H$ per piece

Chanlunlun
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文章 Chanlunlun » 週一 23 2月, 2004 10:38

howerjea 寫:hi,
may i have a question for you
how do you have the second flat mirror for the 24" mirror,make one or buy one?
i have a glass cuter,i like to cut a rectangle mirror for my second flat mirror
but i just can cut the glass under 6mm think,i'm failure 6mm above
how about you think,thanks
it's miracle and great have someone also make the 60cm primary mirror in the same time and me do
but i began form last summer until now,i remember i bought 60cm/19mm about 600H$ per piece
Dear Mr. howerjea,

The secondary mirror from us is a 4 inch minor axis elliptical diagonal mirror from Meade, standard aluminium coating, thickness 1 inch ( !!! ) with Pyrex glass blank.

It is not advisable to use ordinary plate glass to act as a seconadry mirror since its surface is far from desire optical flatness. Serious astigmatism will be introduced into the finished telescope whwen in use.

If you need one, you either grind theflat yourself or ask some dealer to supplied a commercial one.

Best regards

Chan Yuk Lun
23-2-2004

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