檢測望遠鏡光學部份,赤道儀機械組件,目鏡配件和拍攝器材 CCD等...
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註冊時間: 週四 09 10月, 2003 21:06


文章 Chanlunlun » 週四 21 6月, 2007 16:29

Dear Mr. So,

In the first place I would like to beg for your forgiveness for my late response. Your mirror had arrived Hong Kong early this year and WongSir demanded that the mirror must be handled with great care, avoiding upsetting your great support.

You had mentioned about using the auto-collimation test for figuring this mirror. This test is well-known for its extreme precision and a mirror, under such superb test, could easily reach an error not exceeding 1/20 wave P.V. I firmly believed that what I could do was just to reconfirm your result.

Due to the lack of a sizeable standard optical flat, I decided to test the mirror by the elementary Foucalt knife-edge test. Though primitive the method was, great precautions were engaged in order to minimize the errors as mush as possible. These included the use of a big laboratory with all its windows closed aiming to stabilize its environmental temperature. All data were sampled only after a long wait until not a single trace of air turbulence was observed over the mirror. Finally all single data was an average of three samples to make sure for reliable results. Nevertheless the results obtained was quite departed from yours, even under numerous tests, day after day, experiment after experiment. Basically the profile of your mirror is as follows :-

1. There is a shallow “depression”, diameter 4 to 5 cm , lying at center of the mirror. This is by no means surprising since the central portion of the mirror was blocked during the auto-collimation test. There was just was no way to tell how good or how bad the mirror was at this region.

2. Ignoring central portion, the mirror is basically a good paraboloid,. However test after test the results obtained only indicated a 1/6 to 1/8 wave P.V. accuracy. A 1/20 wave P.V. result was never achieved no matter how many times the experiment was carried out.

I belief that I am the one who brought about all these troubles. It is a well known fact that the Foucalt test could accomplish good precision only when the mirror under test is of sufficient long focal length. For fast mirrors, upon knife edge cutting, shadow transient was too quick for average human eye to capture equal grayness accurately. Moreover observer’s subjective judgement also leads to rapid growth of random errors. Mr. So’s mirror is a f/5. With such short focal length together with my poor eyesight it was not surprising that the test was not at all carried out competently.

Now we have a situation here which would, regretfully, lead to the disappointment of both Mr. So and WongSir. In order to fulfill my mission I made my decisive move to make another mirror with accuracy also of 1/20 wave P.V. and hopefully this reference mirror could be AB tested against Mr.So’s, giving a more sounding result. To me who equips with only mediocre ATMing technique the making of a mirror of such precision means a colossal job taking great length of time. It is not until recently that the reference mirror has been finished and with an error envelop of, despite my greatest effort spent, not better than 1/16 wave P.V. It deems that I am a worse ATMer comparing with Mr. So.

To truly fabricate an AB test we need to take care of a lot of recourses – such as two mirror testing rigs, two equatorial mounts and two pieces of identical diagonal flats from the same factory with equal dimensions. The list could be exhausting and preparation job is still not fully ready at the moment. However the individual star test of each mirror has been carried out. Under a magnification of 500X both mirrors performed very nearly identical. If we have to make the preliminary judgement between them all we can say at the moment is that the reference mirror gave a little more sharper and detailed image while the contrast of Mr. So’s mirror was a bit higher. Nevertheless during these two independent observations atmospheric turbulence were not exactly the same and the result was believed to be far from final. At the moment we are still waiting for arrival of all required equipment and hopefully we will come to a decisive conclusion shortly.

Nowadays commercially available astronomical telescopes selling at low prices are pouring into the market and consequently ATMing is a declining hobby both in its quantity and quality. ATMers with superb technique is rarely seen and Mr. So is definitely one of those. I am being inspired, and honored, to have a chance to see your excellent mirror and it is my hope that we will still enjoy the luck to have your continual offering of opinion to improve our ATMing technique.

Looking forward to receive your response, if my humble message worth your notice.

Yours truly,

Chan Yuk Lun
20th June 2007.

( Chinese translated version are following in the same thread)
Mr. So’s mirror under shop test.
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Mr. So ‘s mirror on the test rig, ready for star test.
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The reference mirror (right) and Mr. So mirror (left) and the Couder screen for paraboloid measurement.( bottom )
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The Foucalt test profile of the reference mirror. It has a P.V. error of about 1/16 wave.
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The Foucalt test profile of Mr. So’s mirror. It has a P.V. error of about 1/7 wave.
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文章: 3839
註冊時間: 週四 09 10月, 2003 21:06

文章 Chanlunlun » 週四 21 6月, 2007 16:31


蘇先生寄來之鏡早於年初收妥,黄隆先生囑咐吾人務必認真處理,免負蘇先生心意。今得蘇先生親製精品,誠惶誠恐, 測試結果, 恐有錯漏,未敢及早公佈,還請見諒。

憶蘇先生所述,此鏡乃出自準直法(Auto-collimation Test)修琢而成,查此法精度甚高,鏡面誤差輕易者不過1/20 波長P.V. 。吾人再度測試,力之所及,不外重新肯定蘇先生之結果矣。

由於吾等缺乏大型標準平面鏡,故衹能使用簡單之佛科試鏡法 (Foucalt Test) 。雖曰土法上馬,然亦沿用嚴謹態度處之。一曰使用倘大之實驗室,並緊閉所有窗户以靜其氣流,數據盡皆靜侍直至氣流擾動低至不能察覺之時才正式取樣。再者每數據皆經三次取樣,索其平均,以達最可靠之結果。然而經多番測試,甚至於不同日子進行同一試驗,結果仍無法與蘇先生所述相符。基本上鏡面呈現如下特性:

1. 於中心處直徑約4-5cm乃一下陷之凹宂。此不足怪,蓋於自準直測試中,鏡面中心為不可見者,此部精確度自然無法控制。

2. 略去上述部份不談,拋物面形狀確屬良好。然而多番測試,得出精確度約為1/6-1/8波長P.V., 奈何無法得出1/20波長P.V.之結論。

結果分歧,錯在我方。眾所周知,佛科刀口儀之於短焦鏡面,刀片入切,其光暗變化甚速。平凡眼睛實難以精細判讀,再計及主觀因素,逐令隨機誤差 (Random Error)急劇上揚。蘇生之鏡焦比為5,焦距頗短,加上本人老眼昏花,自然力有不逮。







陳王麟 謹識

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文章 苏鲁锭 » 週五 22 6月, 2007 20:59


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文章 昆仑 » 週日 24 6月, 2007 07:54


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來自: 北京

文章 BJDF » 週日 24 6月, 2007 09:21

看陈先生写国文也是很有趣 :lol:

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文章 苏鲁锭 » 週日 24 6月, 2007 12:24

昆仑 寫:今有幸一睹两位大师的杰作,实在是高.陈先生可否将两镜拍摄的星点图放到网上,让众多天文爱好者学习欣赏.谢谢!

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文章 苏鲁锭 » 週日 24 6月, 2007 12:31


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文章 Chanlunlun » 週一 25 6月, 2007 08:45

昆仑 寫:今有幸一睹两位大师的杰作,实在是高.陈先生可否将两镜拍摄的星点图放到网上,让众多天文爱好者学习欣赏.谢谢!
Dear Mr/Miss QingLun,

Unfoertunately we don't have sufficient equipment to make such pictures. There are difficulties :-

1. The therorectical Airy disc diameter of and 8 inch mirror is about 0.6 arc sec which demands very steady atmosphere for shooting, in order to see the disc and surronding rings.

2. After shooting the picture must go through some image processing which, unfortunately, cover up the minute difference between the two mirrors ( we are talking about a difference of 1/16 wave to 1/20 wave!!! )

3. Finally, and most importantly, we don't have such a good equatorial mount that have motor vibration smaller than 0.5 arc sec ( Even a Takahashi EM-1, EM-1s cannot do it, I don't know whether a EM-11, EM-200 could do it but we don't have two such units identical ) making comparision impossible. All previous star tests were carried out by using hand slow motion tracking to avoid motor vibration. Not shooting is possible undersuch arrangment.

We do hope that a true AB star test, visually, could be carried out shortly, to arrive at final conclusion.

Best regards
Chan Yuk Lun

文章: 1954
註冊時間: 週二 13 9月, 2005 14:35
來自: Hong Kong

文章 TONGKW » 週一 25 6月, 2007 21:15

陳先生致昆仑先生/小姐 (粗略翻譯)
(1). 壹只8英吋口徑反射鏡其理論上的艾里斑直徑大約有0.6弧秒,此需要十分穩定的大氣來進行拍攝使能夠見到光斑及週圍的環圈。
(2). 拍攝之後的影像需要經過圖像處理因而不幸地會掩飾2只反射鏡之間的微小差異(我們所談及只是1/16波至1/20波的差異 !!!)
(3). 最後及最重要是我們没有這麽良好的赤道儀其馬達的掁動小於0.5弧秒(甚至高橋廠的型號 EM1 及EM1s 也不能做到,我不知道型號EM11及EM200可否做得到但我們没有2台這類相同的赤道儀)因而使這比較不可能得以辦到。所有以前的星點檢驗均是用手動微調追蹤來避免馬達的掁動因而拍攝在該情況之下是不可能。
最後由 TONGKW 於 週二 25 11月, 2008 23:24 編輯,總共編輯了 1 次。

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