24 inch mirror - 11 hours of parabolization

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Chanlunlun
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註冊時間: 週四 09 10月, 2003 21:06

24 inch mirror - 11 hours of parabolization

文章 Chanlunlun » 週四 25 3月, 2004 10:58

Dear ATMs,

At the end of the day the mirror had received a total of 11 hours of parabolization. The process is slower than what we had expected and it seemed that there still left a long way to march.

The following table perhaps gives you an idea of how immensely the speed of parabolization can vary for mirrors of different aperture :-

Mirrror //time needed//sphere to start with?//Remarks
4」 f/10// 0//yes//no parabolization needed.
4」 f/5.5//3 mins.// Yes//easily overdone
6」 f/8//5 mins//yes//easily overdone
8」 f/6//8 mins//yes//easily overdone
12」 f/6//10 mins//yes//easily overdone
20」 f/4//45 hrs.//no.//a very difficult task
24」 f/3.4//????//no//a war to fight

May be you will find it stunning that for mirror as much as 12」 aperture the parabolization is being so quick. From my own experience even for a 14」 f/6 parabolization can be finished within 10 minutes, provided that the surface is a good enough sphere to start with, the pitch lap is warm enough so that the it is adequately fluid, and you know where to stop.

For mirror of aperture bigger than 16 inches the amount of glass to be removed increases immensely and the precision required suddenly boosted. So for a 20」 mirror it takes much much much more time for the parabolization. Since it takes so long, there is no need to prepare a sphere before head and, strange as it is, there is no need even to fully polish the mirror before parabolization, since the long time involved will give you a perfect polish anyway.

So for our 24 inch, the journey is far from finish. In fact the adventure just barely begins.

Best regards
Chan Yuk Lun

24-3-2004
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Holding_mirror.jpg
Poor Mr. Sodium had just received an order to take the mirror to the test stand for Offner null test.
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profile_1.jpg
What we saw under the Offner tester tonight . We were using a red LED for the light source to avoid chromatic aberration of the nulling lens and that』s why you see all red in color. Notice that the mirror is under corrected ( of course ) with a turn down
profile_1.jpg (36 KiB) 已瀏覽 10157 次

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Wah!
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文章 Wah! » 週四 25 3月, 2004 12:57

拋物面的話,是否會經過null lens會見到均勻的光暗呢?

Chanlunlun
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註冊時間: 週四 09 10月, 2003 21:06

文章 Chanlunlun » 週四 25 3月, 2004 13:14

Wah! 寫:拋物面的話,是否會經過null lens會見到均勻的光暗呢?
Dear Mr. Wah!,

You are correct. When a paraboloid is reached, we will see the mirror surface darkens evenly.

This is the idea of null test.

Best regards

Chan Yuk Lun

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Wah!
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文章 Wah! » 週四 25 3月, 2004 13:30

咁null lens係以邊種波長的光而設計呢?

Chanlunlun
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文章 Chanlunlun » 週四 25 3月, 2004 15:34

Wah! 寫:咁null lens係以邊種波長的光而設計呢?
Dear Mr. Wah!,

My own Offner null tester is designed with the wavelength of 656.3 nm, however the LED ( light source ) purchased in Ap Liu street may not have exactly the same wavelength emitted but rather close, I presume.

Best regards

Chan Yuk Lun

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豆豸苗
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文章 豆豸苗 » 週四 25 3月, 2004 23:36

照咁計... 塊 12.5" f6 咪好易拋物面化0羅?? :?

Chanlunlun
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文章 Chanlunlun » 週五 26 3月, 2004 10:38

豆豸苗 寫:照咁計... 塊 12.5" f6 咪好易拋物面化0羅?? :?
Dear Miss BeanKitty,

Yes but No.

In order to quickly parabolize the mirror the following must be well prepared in prior, which includes

1. A graph of knife edge shift against radius of zones. Included in the graph you must also plot the upper ( + 1/8 wave ) and lower limit ( -1/8 wave ) for you to estimate the tolerance for your figuring.

2. Of course, a precision Foucalt tester which is able to read a knife edge shift down to 0.05 mm. The preparartion of this Foucalt tester may take you quite a while.

3. The mirror must be figured to a perfect sphere at first before any meaningful parabolization takes place. Depending on your luck and skill, You will find that figuring the mirror into a good sphere may be very time consuming.

4. As we are not able to get high quality optical pitch in Hong Kong, we must heat up the pitch to at least 50 degree centigrade otherwise there is no effect in parabolization. You will find that no matter how hard you try, the profile of the mirror still remians unchange if you are working on a cold pitch lap. What you need is to think of a way to produce a room/ enclosure which can maintain a constant temperature of 50 degree centigrade lasting at least for 15 minutes.

5. If you are not able to create a constant temperature room, heat up the pitch lap by hot gas gun, quickly parabolize the mirror while the pitch is still hot. The drawback is that in this way the lap will cool down really fast, your parabolization procedure has to be very quick, so quick that before the lap cools down you finish your parabolization. Furthermore you have to stop at the right moment, otherwise you will end up at a hyperbola instead of a parabola. It will be a painful experience to reduce a hyperbola back into a parabola.

Above are my own experience after parabolize hundreds of mirror, ranging from 4 in to 20 in. These are the 5 basic requirements for quick parabolization. If you are well prepared, you are able to parabolize the mirror in 10 minutes, even for a 12 inch mirror. ( assume f/6 not f/4 )

Good luck.

Best regards

Chan Yuk Lun

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