8吋鏡 - 拋光/球面化時遇到的問題

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
回覆文章
cytangjoseph
星際雲
文章: 46
註冊時間: 週一 10 8月, 2009 15:59

8吋鏡 - 拋光/球面化時遇到的問題

文章 cytangjoseph » 週二 05 6月, 2018 02:09

焦內:
圖檔

焦點:
圖檔

焦外:
圖檔
發現中央有山, 及邊沿有低邊.

請問師兄師姊, 可以用甚麼方法修正?

Chanlunlun
夸克星
文章: 3833
註冊時間: 週四 09 10月, 2003 21:06

Re: 8吋鏡 - 拋光/球面化時遇到的問題

文章 Chanlunlun » 週二 05 6月, 2018 06:40

You have a huge amount of turned down edge, difficult to cure by pitch lap.

Best suggestion : go to #1200 fine grinding and do polishing again with hard pitch. What is the f/no. of your mirror ?

cytangjoseph
星際雲
文章: 46
註冊時間: 週一 10 8月, 2009 15:59

Re: 8吋鏡 - 拋光/球面化時遇到的問題

文章 cytangjoseph » 週二 05 6月, 2018 16:30

Hello Chanlunlun,

it is 8 inch, f/6

thanks for your tips!!

:D

cytangjoseph
星際雲
文章: 46
註冊時間: 週一 10 8月, 2009 15:59

Re: 8吋鏡 - 拋光/球面化時遇到的問題

文章 cytangjoseph » 週二 05 6月, 2018 16:34

So I think the turned down edge is top issue for me to tackle first.

Will be it like going back to #1200, and grind with 1/3 W stroke?

Chanlunlun
夸克星
文章: 3833
註冊時間: 週四 09 10月, 2003 21:06

Re: 8吋鏡 - 拋光/球面化時遇到的問題

文章 Chanlunlun » 週六 09 6月, 2018 07:37

Back to #1200, mirror on top using 1/3 center to center stroke.

cytangjoseph
星際雲
文章: 46
註冊時間: 週一 10 8月, 2009 15:59

Re: 8吋鏡 - 拋光/球面化時遇到的問題

文章 cytangjoseph » 週二 12 6月, 2018 13:17

Thanks Mirror god!!

Past few days, as a bit too busy [ or lazy ;) ]to go back to #1200 and making a new pitch lap, I spent a few more hours with the pitch lap I am now using:

some ways to try to improve the rolled edge:

1. more frequent cold press for good contact

2. hands placed in the center of the glass, and not on the edges.

3. Keep movemnts horizontal and slow. Push ( and pull) the glass with whole body weight ( back straight, bend your knees and ankles). As in Tai Chi

4. 1/3 COC. Alternate Mirror on Top, and Tool on Top

5. stop the polishing after 10 to 15 minutes, and take a break of 5 to 10 minutes, not to let the glass and pitch to warm up

焦內(inside focal poiint):

圖檔

圖檔

焦點 (focus):

圖檔


焦外 ( outside focal point):

圖檔

圖檔


So far the rolled edge is much reduced. Still some turned down edge.

Next I am going to tackle the remaining turned down edge, and the hill in the center

thanks!!

[edited for mixup of inside focal point and outside focal point pics]

Chanlunlun
夸克星
文章: 3833
註冊時間: 週四 09 10月, 2003 21:06

Re: 8吋鏡 - 拋光/球面化時遇到的問題

文章 Chanlunlun » 週五 22 6月, 2018 06:53

The mirror is not smooth. Cold press can do very little towards matching the surface of mirror to pitch surface.

You should hot press the pitch lap every time you start polishing again. Best of all is that the polishing should be done all in once, avoiding pressing the pitch lap from time to time.

Pitch, especially those bought from local, deteriorate upon repetitive heating and pressing.

cytangjoseph
星際雲
文章: 46
註冊時間: 週一 10 8月, 2009 15:59

Re: 8吋鏡 - 拋光/球面化時遇到的問題

文章 cytangjoseph » 週六 23 6月, 2018 02:08

Well noted Chanlunlun,

Thanks for reminding me, that to start a polishing session, it must begin with a good hot pressing first. I also do a cold press after a 15 minutes polishing, for 10 minutes or bit more.

Just updated the progress:

inside focal:
圖檔


outside focal:
圖檔

The edge is now more polished

But the turned down edge still remains :wink:

The centre region is now closer to sphere shape, and seems to be smoother.

Found that this pitch is better suited in lower temperature, around 20 deg C. As now is summer, in the 30+ degrees room temperature, need to have air conditioning on. Otherwise the edge region is very hard to get polished out

Kelvin W.
主序星
文章: 246
註冊時間: 週三 11 1月, 2017 22:33

Re: 8吋鏡 - 拋光/球面化時遇到的問題

文章 Kelvin W. » 週六 23 6月, 2018 11:46

[Good Job] 加油

cytangjoseph
星際雲
文章: 46
註冊時間: 週一 10 8月, 2009 15:59

Re: 8吋鏡 - 拋光/球面化時遇到的問題

文章 cytangjoseph » 週日 24 6月, 2018 00:15

Thank you.

Here is some update of today.

inside focal:
圖檔

focal:
圖檔

outside focal:
圖檔

圖檔


The turned down edge is much reduced. Now I am thinking what should be next.

To further smooth out the zones ( how ? ), or to going for figuring as parabola?

回覆文章

回到「自製天文望遠鏡 Amateur Telescope Making 」

誰在線上

正在瀏覽這個版面的使用者:沒有註冊會員 和 2 位訪客