The making of 30 in telescope - 47, Making Optical Flats

[Chanlunlun]

Dear ATMs,

The 30 inch telescope has a primary mirror of f/3, or in other words, 90 inches of focal length. The matching secondary diagonal, for normal focusing method, should be at least has a minor axis of 6.5 inches, assuming that we are using a very low profile focuser. Regrettably no such big diagonal mirror exists on the market and we have to make one for ourselves.

Today we originally planned to test run the 30 inch telescope at Water Mouth but due to weather factor we switched to plan B – to start grinding the secondary mirror for the 30 incher.

The method we are using is the classical A-B-C- method – that is, using three pieces of mirror blanks, A, B and C and grind them by the following sequences – A on B, B on A, C on B, B on C, C on A, A on C and so on. Each sequence last for exactly same amount of time ( 15 minutes in our case ). It is not difficult to imagine that using such grinding sequences the only possible outcome is three surfaces at absolute flatness.

Optical flats are precision optics and making one, and to optically test it, is already an interesting project. If everything is under control we expect to finish this project within a weeks』 time

Best regards
Chan Yuk Lun
20-5-2006

[observer]

或許離題, 外間的光學平面鏡似乎要比拋物面還要貴, 為甚麼呢?

[Wah!!]

磨砂面是鏡面還是鏡背?

[Chanlunlun]

或許離題, 外間的光學平面鏡似乎要比拋物面還要貴, 為甚麼呢?

Dear Mr. Observer,

I belief that the demand of optical flat in the market is very small, much smaller than the demand of paraboloidal mirror. That's why optical flat is very expensive to manufacture comparing to the paraboloid mirror.

Best regards
chan Yuk Lun

[Chanlunlun]

磨砂面是鏡面還是鏡背?

Dear Mr. Wah,

What you see in the picture is the grinded surface of the flats, i.e., the surface that does the business.

Best regards
Chan Yuk Lun

[simonii]

Dear Chan Sir,

Which number of grit are you using to make such optic flats?
I see a bottle of 320 on the table, are you starting from 320?

Best regards,
Simon

[kevinyau]

Mr Chan,

I would like to ask is there any reasons for using 3 glasses but not 4 or 5? In my limited concepts, it is likely to be more accurate if more glasses are used, isn't it?

Kevin @beginner

[Chanlunlun]

Dear Chan Sir,

Which number of grit are you using to make such optic flats?
I see a bottle of 320 on the table, are you starting from 320?

Best regards,
Simon

Dear Mr. Simonii,

Yes we start at #320. We regret that we haven't start at #600 otherwsie great time will be saved.

Best regards
Chan Yuk Lun

[Chanlunlun]

Mr Chan,

I would like to ask is there any reasons for using 3 glasses but not 4 or 5? In my limited concepts, it is likely to be more accurate if more glasses are used, isn't it?

Kevin @beginner

Dear Kevin,

It can be proved by mathematics that three blank is all that required to make absolute flat surfaces. ( in grinding stage,not inpolishing stage.)

Using more blanks is not a wise option since you will get the same outcome.

best regards
Chan Yuk Lun

[Chanlunlun]

Dear ATMs,

The flats were all grounded #2000 OK. They are just waitring to be polished.

For polishing we will design and fabricate a real polishing machine. It is just not a good idea to polish three surfaces by hand.

Best regards
Chan Yuk Lun

[c++]

Mr Chan,

I would like to ask is there any reasons for using 3 glasses but not 4 or 5? In my limited concepts, it is likely to be more accurate if more glasses are used, isn't it?

Kevin @beginner

註意，是不能用四塊對磨兒把他們都磨平的。如第一，第三塊凹，第二、四塊凸，麯率絕對值一樣，那么他們就會越磨偏離平麵越遠

理論一點討論：
假設三塊玻璃為A/B/C，他們麯率分別是CA,CB,CC
隻要A-B/B-C/C-A互相吻閤，有：

CA+CB=0
CB+CC=0
CC+CA=0
解之，有CA=CB=CC=0,他們隻能都是平的。

單如果用四塊（或任何偶數塊）
有方程組
CA+CB=0 ……1
CB+CC=0 ……2
CC+CD=0 ……3
CD+CA=0 ……4
四個變量四個方程，似乎也能解齣唯一解。 但畱意1-2+3，結果就是4
實質隻有三個有傚方程，有無數解，他們可以都是平的，也可以CA=CC,CB=CD,CA=-CB,且CA<>0 都是彎的

現在最頭痛的是，進入拋光階段， chan sir 是用什麼方法，保持麵型的精確性？

[Chanlunlun]

現在最頭痛的是，進入拋光階段， chan sir 是用什麼方法，保持麵型的精確性？

Dear Mr. C++,

For this we are making a polishing machine.

The best method to avoid error in polishing is to polish by mechanical force rather than by human.

Best regards
Chan Yuk Lun