半年前用土法製造一個 null 儀器, 不久前再經 Subaru 和Wah 改良,
於9日25日用來測試C8 的不同光譜下的誤差.詳細結果可參巧subaru在論鏡9月26日一文.
現在,我想把怎樣改動和為什麼要改變理由說一下.
改動
1)光源
舊版試鏡器:
由一枝激光筆做光源,放在10"拋物面(卡式)焦點製造平行光
改良版:
用白光LED做光源,仍然放在10"拋物面(卡式)焦點製造平行光
2)拍攝光柵像
舊版試鏡器:
再用光柵或刀片放在折射鏡後焦點前後不同位置,
影像則投影在一磨沙紙上,再用數碼相機拍攝成像.
改良版:
用光柵放在折反射鏡後焦點前後不同位置, 影像則直接用數碼單鏡相機DSL拍攝成像.
理由
因為很多廠制鏡,尤其是折射鏡在製造時,會遍向矯正某一波長的誤差,
所以祇用紅色雷射光束測試,可能有遍差.
當然這次我們採用白色光源是想驗測望遠鏡是否對全部可見光的表現一樣,
是量度不同光譜下的誤差,白色光源是必然之選.
光柵像仍然是分折主鏡面的主要方法,因為投影在磨沙紙再拍攝後的成像質素很差,
所以改用數碼單鏡相機直接拍攝,效果比較好.
但像質仍然未如理想,後來Wah把*裂隙光源改為點光源, 拍攝出的光柵像非常清晢.
臨時的點光源祇是用釘在一張錫紙上造一小孔,
散射光源(diffuse light)則用一塊膠片以幼金鋼砂打磨而成.
相信日後以針刺個點光源--- **1/100~1/200 吋小孔,效果會更理想.
**小孔太大,成像會因干涉現象而濛糊不凊.
*裂隙光源
很多時候,試鏡者會把光柵蓋在光源上,活著許多條裂隙同時存在,以增加光度.
裂隙光源可次提供較強的光,但現在用上了強光LED代替小燈泡,基本上已解決光度問題.
裂隙若果和光柵並不是平行的話,光柵像反差和亮度會降低.
改善方法,就是把一塊光柵蓋在光源上和同時當作觀察光柵用,這樣便可確定裂隙光源和光柵平行.
(這個 Null 試鏡系統並不適用)
光柵試鏡法
http://www.atm-workshop.com/ronchi-test.html
土法製造 null 試鏡法儀器 ---( 1 )
http://www.hkastroforum.net/viewtopic.php?t=1507
Subaru在論鏡9月26日一文
文章主題: 折反射鏡的Spherochromatism
http://www.hkastroforum.net/viewtopic.p ... &start=130
土法製造 null 試鏡法儀器 ---( 2 ) 改良版
土法製造 null 試鏡法儀器 ---( 2 ) 改良版
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最後由 Wongsir 於 週四 17 3月, 2016 16:53 編輯,總共編輯了 20 次。
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- 夸克星
- 文章: 3847
- 註冊時間: 週四 09 10月, 2003 21:06
Dear WongSir,
Congradulation! A new null method certainly increases the accuracy of optical testing.
However the error of the test is still limited by the quality of the paraboloid mirror used and the quality of the parallel emergent light beam. For instance if you see over or under correction in C8 we are still not quiet certain whether this over/under correction comes from the paraboloid mirror, from the C8 or from the slight un-collimated parallel emergent beam.
So it is best to use a true auto-collimation null test for testing telescope - it is being sensitive and reliable. Of course a big enough optical flat is a rather expensive item.
However if many could be benefitted from this flat, it deserves to buy it.
Best regards
Chan Yuk Lun
Congradulation! A new null method certainly increases the accuracy of optical testing.
However the error of the test is still limited by the quality of the paraboloid mirror used and the quality of the parallel emergent light beam. For instance if you see over or under correction in C8 we are still not quiet certain whether this over/under correction comes from the paraboloid mirror, from the C8 or from the slight un-collimated parallel emergent beam.
So it is best to use a true auto-collimation null test for testing telescope - it is being sensitive and reliable. Of course a big enough optical flat is a rather expensive item.
However if many could be benefitted from this flat, it deserves to buy it.
Best regards
Chan Yuk Lun
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