2011-2 Astrocafe光學工作坊 - 室內人工星點測試

[社長]

應早前提出，我一直有意推行一些小型工作坊，讓各Astrocafe會員親身體驗我們經常提到的APO、ED、普消折射鏡的成像差距。
viewtopic.php?f=14&t=639

昨日我們搞了一個小型聚會，實行了部份環節。可能因時間倉卒關係，只有Xiaoer及David能出席。

測試工具：
50um 紅綠藍光人工星點
LOMO 80/600 APO折射鏡 (一般會稱為TMB 80/600)
高橋FC76 折射鏡
Sky Watcher Equinox 80ED 折射鏡
Sky Watcher 70/500普消折射鏡
Pentax XO 2.5mm 目鏡
高橋 Hi Or 2.8mm 目鏡
TMB Supermonocentric 4mm 目鏡

測試方法：
在約24米外，即焦距40x左右，配合高倍率目鏡，分別以紅綠藍三光譜人工星光檢視成像表現。

LOMO 80/600 - 三枚玉OK4玻璃APO鏡
在藍及綠光的第二衍射環是幾乎看不見的，完美的教科書類星像表現。可能人眼白天對紅光敏感度關係，紅光的第一以外衍射環較明顯。這鏡三個波段的成像都比下述三台鏡結實緊湊，說明了頂級APO鏡為何往往可以在每毫米口徑2x放大率以上發揮，有點像超級跑車，其真正性能是在高速上體現，高修正度三枚玉空氣頭APO折射鏡，實際上以超口徑性能發揮，去挑戰更大口徑的其他種類望遠鏡，本身價值在此，至於是否理順其高昂售價，則見仁見智。

高橋FC76折射鏡 - 雙枚玉螢石鏡
在人工星點測試下，我們會發現其紅綠藍光譜下，其第一及二衍射環較容易察覺，其中藍端表現差距較大，紅光表現接近但仍稍遜。除去有限測試距離下造成的少量殘餘球面像差，某程度上顯示FC76的修正度稍遜於LOMO 80/600。由於這情況說明埃里盤上有部份光源被推至衍射環上，所以也引證了跟LOMO鏡星空觀測下的對比度差距(另一部份原因可能是其無鍍膜鏡頭)。以這鏡計，直正的色差修正差距是在比藍光更短波長的紫光上，而我們所用的50um人工星點並無這波段。
viewtopic.php?f=22&t=743&p=7367#p7367

Sky Watcher Equinox 80ED 折射鏡
紅綠光星點表現跟上述兩鏡表現接近但有容易看出的分別。問題最大在藍光，埃里盤及衍射鏡已不完整成型，在那波段已不能叫diffraction limited了，這亦是通俗叫法的ED鏡跟APO鏡的最重要差距。亦因此我一直指出，若作Ha光譜太陽觀測，如日珥等，這類ED折射鏡表現已十分理想。但若作Cak波段觀測，還是要選用一台在紫光譜成像優秀，相對長焦比的APO折射鏡，否則會嚴重影響成像質量。

Sky Watcher 70/500 普消折射鏡
僅以綠光檢視，發現光軸有偏差，這點可以用方法校正，說明了數百元一台的望遠鏡，造工上不像高檔APO那麼嚴謹。僅綠光而言，第一、二及以外的衍射環都十分明顯，相信這鏡就算校正光軸後，綠光的球差修正也明顯遜於上面兩鏡，也直接限制了其可用高倍率。亮起紅及藍光星點，需要重新調焦，而且距離頗大，上面三台折射鏡則無這必要，說明普消折射鏡的色差修正差距，唯一改良方法還是要把焦比儘量拉長。

結論：
三片或兩片APO、ED、普消折射鏡都有其歷史上的地位及今天的存在價值，純粹追求高性能，或是平衡的性價比，或是廉價入門鏡，哪款望遠鏡適合什麼人，其實很視乎個人需要及實際情況，不可一概而論。

望遠鏡光學本身並無神秘之處，不外乎用料造工及設計。一個簡單個多小時的工作坊，配合適當工具加上導師講解便一目了然，並不用花長時間去討論或尋求名師指點，掌握基本要素後，選望遠鏡便不會犯方向上的大錯誤。

Astrocafe日後還會推行更多類似工作坊，讓同好增進光學知識。

[benben88]

A pity I could not join the function

[社長]

Ben, 其實是我們安排不當，本來打算測試完後，天黑便上天台做星空測試，但沒考慮到部份人已開始上班。

下次我們會再搞，我的辦公室天台可以開放至晚上8時半，有需要可以跟保安說延後一下，那便不用到處跑了。

這些小工作坊或許不是什麼大學問，但讓大家很直觀地認識不同望遠鏡的光學性能。有機會一定要來，很有興趣看你的Pentax O。

另外應會今個月內找到一個TMB Mono 4給你。

[David]

我最肯定看到的是在藍光下，Lomo 80/600的第一衍射環初時很難看到，要十多秒讓眼睛適應後才看到；高橋的第一衍射環一看便看到了。Equinox 80ED 藍光的衍射環完全化開，好似中央Airy Disk周圍有個光暈 (Halo)般。

不同價錢和設計的鏡，分別真的很大。

[funoooo]

正！
小兒是否己決定要支 lomo? 如果她要，我就放棄。
如果不要，就由我拿下吧！
君子不奪人所好，更何況是淑女！

[willis]

正！
小兒是否己決定要支 lomo? 如果她要，我就放棄。
如果不要，就由我拿下吧！
君子不奪人所好，更何況是淑女！

Ha! Ha! These two scopes had been posted for sometime. I put them in my wish list for a long time. If I haven't seen their performance, I wouldn't have such desire to buy them. Now I've got a new 80/480 lens & used tube. u still have chance to buy the used 80/480 lens & a new tube. However, if u want the 80/600, u may need to ask xiaoer. As I know she is very interested in it. So which one do u want? I guess u need to think about the telescopes u have now. What focal lengths u owned etc.

[funoooo]

正！
小兒是否己決定要支 lomo? 如果她要，我就放棄。
如果不要，就由我拿下吧！
君子不奪人所好，更何況是淑女！

Ha! Ha! These two scopes had been posted for sometime. I put them in my wish list for a long time. If I haven't seen their performance, I wouldn't have such desire to buy them. Now I've got a new 80/480 lens & used tube. u still have chance to buy the used 80/480 lens & a new tube. However, if u want the 80/600, u may need to ask xiaoer. As I know she is very interested in it. So which one do u want? I guess u need to think about the telescopes u have now. What focal lengths u owned etc.

Willis,

You know I have a lot telescope. I will buy it for outing......... The 130 is too heavy for outing. I will let it to stay at my roof. ( My roof condition isn't too bad. Last lunar new year I use 130 to take few photo without filter for 3 minute, the sky glow isn't very worse. I think the condition is similar to PTC. I will post some without processing photos for your reference. Next time, please come and join wth me. At least, you have a place to sleep for overnight shooting. ) If I get it, I will happy. If not, I am also very happy. I can save the money.

Anyway, let Xiaoer to make the decision first.

[willis]

Willis,

You know I have a lot telescope. I will buy it for outing......... The 130 is too heavy for outing. I will let it to stay at my roof. ( My roof condition isn't too bad. Last lunar new year I use 130 to take few photo without filter for 3 minute, the sky glow isn't very worse. I think the condition is similar to PTC. I will post some without processing photos for your reference. Next time, please come and join wth me. At least, you have a place to sleep for overnight shooting. ) If I get it, I will happy. If not, I am also very happy. I can save the money.

Anyway, let Xiaoer to make the decision first.

So u want the 80/800 instead of 80/480. It seems that the focal length of 80/600 is quite close to ur 130. My advice is that 480mm is a good choice. But it costs more. About ur roof, I would be happy, if I can image at ur roof. Let's organize it next time.

[社長]

can I come? I can give you free lessons on different things

I think Xiaoer will take the 80/600, the only one you can consider is 80/480, and you need an inexpensive adaptor to install your existing Feather Touch motor for 3.5" model.

[社長]

我最肯定看到的是在藍光下，Lomo 80/600的第一衍射環初時很難看到，要十多秒讓眼睛適應後才看到；高橋的第一衍射環一看便看到了。Equinox 80ED 藍光的衍射環完全化開，好似中央Airy Disk周圍有個光暈 (Halo)般。

不同價錢和設計的鏡，分別真的很大。

我看這個測試並不代表品牌高低，

三片空氣頭APO，還要用上LOMO OK4玻璃，那是必勝無疑的，除非LOMO或LZOS換上螢石，正如Dr Massimo Riccardi所說，或會有丁點改善。

兩片空氣頭APO用上螢石(及相應Mating element)，比三片的差丁點是正常的，這台鏡證明高橋當代的工藝如何高。而Equinox則是兩片空氣頭用FPL53，而且焦比又短至f6.25，相對差一點也是合理。

所以我只能說，這三台鏡都在自己的光學配置及工藝上正常發揮。唯一例外是Xiaoer之前的那台三片空氣頭ED，明顯是造工問題了。

[funoooo]

can I come? I can give you free lessons on different things

I think Xiaoer will take the 80/600, the only one you can consider is 80/480, and you need an inexpensive adaptor to install your existing Feather Touch motor for 3.5" model.

Why not, you are welcome also.
但不要期望過高！始終是近林村，而且自己屋苑光害都很大。
現 post 3 張於 6/2/211 拍的 testing 相。
第一張無 Filter 拍了 3min. (450D ISO800)

第二張 P2 Filter 拍了 3min10sec (450D ISO800)

第三張 P2 Filter 拍了 7min21sec (450D ISO800)

全部單張出！

只是在 Photoshop 內將紅綠藍三個 Channel 調至 Alignment.
因拍時正在 set 鏡，所以拉線到"鬼五馬六"。不過主要比大家看 Skyglow 情況而已。
在 Histogram 上，紅綠藍 Channel 約在靠右邊 1/4 至 1/5 之間。

當晚 setup 完器材後就起雲。無景！所以沒有其他用 P2 Filter 拍的片。可惜！

其實林村附近都黑及有很廣寬草地，不過那裡村民比較"皮氣"大，不大歡迎外來人。
曾經有拍車在林錦公路旁，當晚被人燒車！
如果不是這樣，我已介紹人入來觀星了。

[社長]

最後那張都不錯啊。

其實若以林村環境計，拍窄頻還是很好的，看我這張：
viewtopic.php?f=17&t=621

若你住林村的，不如日後搞些船灣淡水湖星空派對?

[willis]

三片空氣頭APO，還要用上LOMO OK4玻璃，那是必勝無疑的，除非LOMO或LZOS換上螢石，正如Dr Massimo Riccardi所說，或會有丁點改善。

Have u ever seen the flourite TMB 80/480 or TMB 80/600? I think the second hand prices of these two original TMB design scopes are very high. It is kind of collection...

[社長]

我懷疑那兩款鏡是否生產過.....

[willis]

我懷疑那兩款鏡是否生產過.....

I once saw a thread in CN that there were TMB 80/480 & 80/600. From Thomas Back last post in CN, it implied that there are TMB 80/480 & 80/600.

By Thomas Back:
80mm f/6 and f/7.5 fluorite triplet apochromats (only when
it was a fluorite triplet, later, LOMO stopped using fluorite,
and went to OK-4 SD glass, and used their own design).

http://www.cloudynights.com/ubbarchive/ ... in/1163855

But my APM-LOMO's logo is TMB APO...

[社長]

黑白照片的? 拿回去才兩天..... 千古，千古......

[社長]

我也來一張，我的203APO 千古.....

[benben88]

Look forward next event and the arrival of the ep.

Ben, 其實是我們安排不當，本來打算測試完後，天黑便上天台做星空測試，但沒考慮到部份人已開始上班。

下次我們會再搞，我的辦公室天台可以開放至晚上8時半，有需要可以跟保安說延後一下，那便不用到處跑了。

這些小工作坊或許不是什麼大學問，但讓大家很直觀地認識不同望遠鏡的光學性能。有機會一定要來，很有興趣看你的Pentax O。

另外應會今個月內找到一個TMB Mono 4給你。

[willis]

黑白照片的? 拿回去才兩天..... 千古，千古......

No! It is a color photo. Look at the color of the focuser.

[StevenTam]

近日好帖真多，還得找日空閒仔細看看～
我很希望知道ED鏡加Fringe Killer filter和Moon & Skyglow (Neodymium) filter或最新Semi-APO Filter的效果... 不知會否舉行類似的測試？