論鏡 2
無意中在在cloudynights forum 看到一段相當有趣的討論, 大意係在ED / APO 折射鏡橫行的年代, 長焦普消折射鏡有什麼存在價值......
現引用其中的一些正反意見:
正面意見:
"Compared to ED's (F/9ish), the attraction of long achromats (F/15ish) is:
1. Some of the most distructive turbulence is often right above the ground, and also from the observer's body heat. Having the objective/entrance pupil towering as much as possible above (as is the case with "long" scopes) avoids some of this.
2. "Long" scopes (=high F-ratios) have BIG tolerances in focus travel.
3. Because with "long" scopes it's possible to use medium focal length eyepieces to achieve the high magnifications one needs for planetary, double star and small Deep Sky Objects observation, one can enjoy the long eye relief these longer focal length eyepieces provide (compared to the short eye relief of the short focal length eyepieces one needs to use with the "short" scopes to achieve these same high magnifications).
4. It's easier to grind high F-ratio lenses (shallow curves) to a high precision (=high quality lens) than low F-ratio lenses (steep curves).
5. As you already mentioned, more and more aberations creep in as the focal ratio decreases (even IF the short lens is manufactured to the same high precision than the long lens).
6. ED glass (which is only used for one lens of the two- or three-element objective) typically has a quite different thermal expansion coefficient compared to its/their (one or two) conventional glass mating element(s).
During falling/rising temperatures, this leads to much more problems with under-/overcorrection than with the doublet of an Achromats, which uses conventional glasses for BOTH of its elements with quite similar thermal expansion coefficients. The Achromat is thus less prone to under-/overcorrection. This issue becomes more severe as scope apertures increase.
7. It's typically easier to effectively baffle long scopes than it is to do that for short ones. This gives the long scopes a contrast advantage.
8. Aestetics (for some): Long scopes look like REAL scopes (Sigmund Freud looms in the background!).
9. to n. : you name it...
Greetings,
Ralph"
反面意見:
"Long achromats are not really so much cheaper, than same size, but faster APOs. To mount them equally good to faster APOs, one need to use more massive and, so, more expensive mount. Saving on a mount can be used for apochromatization of an objective. This is valid at least for 4-5" range."
當然, 在香港呢個特殊環境, 仲要考慮搬運同存放問題.
現引用其中的一些正反意見:
正面意見:
"Compared to ED's (F/9ish), the attraction of long achromats (F/15ish) is:
1. Some of the most distructive turbulence is often right above the ground, and also from the observer's body heat. Having the objective/entrance pupil towering as much as possible above (as is the case with "long" scopes) avoids some of this.
2. "Long" scopes (=high F-ratios) have BIG tolerances in focus travel.
3. Because with "long" scopes it's possible to use medium focal length eyepieces to achieve the high magnifications one needs for planetary, double star and small Deep Sky Objects observation, one can enjoy the long eye relief these longer focal length eyepieces provide (compared to the short eye relief of the short focal length eyepieces one needs to use with the "short" scopes to achieve these same high magnifications).
4. It's easier to grind high F-ratio lenses (shallow curves) to a high precision (=high quality lens) than low F-ratio lenses (steep curves).
5. As you already mentioned, more and more aberations creep in as the focal ratio decreases (even IF the short lens is manufactured to the same high precision than the long lens).
6. ED glass (which is only used for one lens of the two- or three-element objective) typically has a quite different thermal expansion coefficient compared to its/their (one or two) conventional glass mating element(s).
During falling/rising temperatures, this leads to much more problems with under-/overcorrection than with the doublet of an Achromats, which uses conventional glasses for BOTH of its elements with quite similar thermal expansion coefficients. The Achromat is thus less prone to under-/overcorrection. This issue becomes more severe as scope apertures increase.
7. It's typically easier to effectively baffle long scopes than it is to do that for short ones. This gives the long scopes a contrast advantage.
8. Aestetics (for some): Long scopes look like REAL scopes (Sigmund Freud looms in the background!).
9. to n. : you name it...
Greetings,
Ralph"
反面意見:
"Long achromats are not really so much cheaper, than same size, but faster APOs. To mount them equally good to faster APOs, one need to use more massive and, so, more expensive mount. Saving on a mount can be used for apochromatization of an objective. This is valid at least for 4-5" range."
當然, 在香港呢個特殊環境, 仲要考慮搬運同存放問題.
CN212卡式焦點的光軸調較
卡式鏡的光軸調較
早前購入咗支8.3”卡式反射鏡, 它的鏡面精度很高, 之前和友人到石澳用這支鏡睇土星, 比一般SCT清晰許多, 也沒有Spherochromatism問題.
但是初時用佢放高倍做star test, 發覺就算用廠方提供的工具和方法調較副鏡的光軸, 就算已經把Airy Disc較成跟衍射環同心, 所得影像的衍射環還有點斷斷續續, 相信支鏡並未被tune到最佳狀態.
到網上找資料及自己的光學書本, 只揾到一些片面的指引, 卻找不到完整的方法和概念來調較卡式鏡的光軸. 在沒有其他辦法之下, 只好自己動動腦筋, 想想是什麼問題. 我相信是傳統卡式鏡的副鏡由於是雙曲面關係, 主鏡及副鏡的光軸同軸要求會比用球面副鏡的SCT或DK來得敏感. 如果主鏡的光軸跟副鏡不同軸, 只靠調較副鏡的傾斜度去把光線夾硬調回目鏡座中心的話, 光路的距離便會有所不同, 形成衍射環的斷斷續續現象.
經過這些思考, 便想試一試這個理論對不對, 於是把望遠鏡拆開, 看看主鏡的光軸可否調較, 並進行多次的微調及實地測試, 總算掌握了卡式鏡的光軸調較及理論, 現在跟大家分享一吓這個方法, 這個方法適用於傳統卡式鏡及RC. 由於SCT及DK的副鏡是球面, 一般不必用到這麼複雜的方法, 可見Celestron的創辦人Tom Johnson絕對是一位奇才, 開發了可以集體生產並且易於調較的SCT.
步驟:
1. 拆下副鏡
2. 把一支single beam laser collimator 放進目鏡座, 以目鏡座的光軸為基準
3. 把另一支holographic laser collimator 放在距離主鏡2f的位置, 並且把它的中心對準由第一支single beam laser collimator射出來的beam上, 再把這支holographic laser collimator的中心光線射到第一支single beam laser collimator的出口, 這樣放在2f上的holographic laser collimator便跟目鏡座同軸
4. 調較主鏡的光軸, 使holographic laser collimator投射出來的grid, 從主鏡反射回holographic laser collimator的中心
5. 經過這樣的調較, 主鏡的光軸便會跟目鏡座同軸
6. 裝回副鏡
7. 調較副鏡架, 使放在目鏡座的single beam laser投射在副鏡的中心
8. 利用廠方提供(要另外購買的!)的卡式鏡collimation tool, 把副鏡的傾斜度作初步的調較
9. 重複第7到第8個步驟, 直至副鏡的中心位於光軸上及光學面跟光軸成90度
10. 最後利用star test放高倍觀看diffraction pattern, 作副鏡的細微調較
11. 完成
早前購入咗支8.3”卡式反射鏡, 它的鏡面精度很高, 之前和友人到石澳用這支鏡睇土星, 比一般SCT清晰許多, 也沒有Spherochromatism問題.
但是初時用佢放高倍做star test, 發覺就算用廠方提供的工具和方法調較副鏡的光軸, 就算已經把Airy Disc較成跟衍射環同心, 所得影像的衍射環還有點斷斷續續, 相信支鏡並未被tune到最佳狀態.
到網上找資料及自己的光學書本, 只揾到一些片面的指引, 卻找不到完整的方法和概念來調較卡式鏡的光軸. 在沒有其他辦法之下, 只好自己動動腦筋, 想想是什麼問題. 我相信是傳統卡式鏡的副鏡由於是雙曲面關係, 主鏡及副鏡的光軸同軸要求會比用球面副鏡的SCT或DK來得敏感. 如果主鏡的光軸跟副鏡不同軸, 只靠調較副鏡的傾斜度去把光線夾硬調回目鏡座中心的話, 光路的距離便會有所不同, 形成衍射環的斷斷續續現象.
經過這些思考, 便想試一試這個理論對不對, 於是把望遠鏡拆開, 看看主鏡的光軸可否調較, 並進行多次的微調及實地測試, 總算掌握了卡式鏡的光軸調較及理論, 現在跟大家分享一吓這個方法, 這個方法適用於傳統卡式鏡及RC. 由於SCT及DK的副鏡是球面, 一般不必用到這麼複雜的方法, 可見Celestron的創辦人Tom Johnson絕對是一位奇才, 開發了可以集體生產並且易於調較的SCT.
步驟:
1. 拆下副鏡
2. 把一支single beam laser collimator 放進目鏡座, 以目鏡座的光軸為基準
3. 把另一支holographic laser collimator 放在距離主鏡2f的位置, 並且把它的中心對準由第一支single beam laser collimator射出來的beam上, 再把這支holographic laser collimator的中心光線射到第一支single beam laser collimator的出口, 這樣放在2f上的holographic laser collimator便跟目鏡座同軸
4. 調較主鏡的光軸, 使holographic laser collimator投射出來的grid, 從主鏡反射回holographic laser collimator的中心
5. 經過這樣的調較, 主鏡的光軸便會跟目鏡座同軸
6. 裝回副鏡
7. 調較副鏡架, 使放在目鏡座的single beam laser投射在副鏡的中心
8. 利用廠方提供(要另外購買的!)的卡式鏡collimation tool, 把副鏡的傾斜度作初步的調較
9. 重複第7到第8個步驟, 直至副鏡的中心位於光軸上及光學面跟光軸成90度
10. 最後利用star test放高倍觀看diffraction pattern, 作副鏡的細微調較
11. 完成
SCT的極限?
近來又很多同好討論不同望遠鏡的光學表現, 為什麼高級折射鏡可以做出那麼完美的影像, 其實看一看它們的像差修正圖, 就可以了解得到.
圖一為一支8" SCT的修正圖, 可見它的Spherochromatism也不少, 在實際觀測或拍攝上我也可以察覺得到.
圖二為一支頂級的5" ED triplet, 像差修正得非常之好, 色差比SCT還要低很多.
很多時, 用Aprochromat看高倍, 影像非常整潔, 而用大口徑的SCT觀測, 由於多一點的energy是分佈在diffraction rings上, 較易受seeing影響, 所以通常沒有Aprochromat那樣清晰, 但在seeing好的時候, 大口徑的SCT就可以看到更多細節, 不過就要用多一點的眼力才分辨得到.
近來又很多同好討論不同望遠鏡的光學表現, 為什麼高級折射鏡可以做出那麼完美的影像, 其實看一看它們的像差修正圖, 就可以了解得到.
圖一為一支8" SCT的修正圖, 可見它的Spherochromatism也不少, 在實際觀測或拍攝上我也可以察覺得到.
圖二為一支頂級的5" ED triplet, 像差修正得非常之好, 色差比SCT還要低很多.
很多時, 用Aprochromat看高倍, 影像非常整潔, 而用大口徑的SCT觀測, 由於多一點的energy是分佈在diffraction rings上, 較易受seeing影響, 所以通常沒有Aprochromat那樣清晰, 但在seeing好的時候, 大口徑的SCT就可以看到更多細節, 不過就要用多一點的眼力才分辨得到.
- 附加檔案
-
- 圖二, 5" APO
- APO130.jpg (27.82 KiB) 已瀏覽 21123 次
-
- 圖一 8" SCT
- SCT203.jpg (38.62 KiB) 已瀏覽 21123 次
ε-180ED vs FSQ106ED:
這兩台是差不多價位及焦距的astrographs, 一台光圈達f/2.8, 另一台的field illumination比較平均.
終於有機會影同一個object去比較一吓, 由於拍攝的時間不一樣, 這並不是一個嚴肅的測試, 相機是QHY8 CCD. 用ε-180ED影的時候還影埋SN2011dn.
可見這兩台astrographs各有專長, ε-180ED的f/2.8焦比的確有其高速優勢, 對於香港變化快的天氣來講, 可以用比較短的時間去達到高一點的s/n ratio, 焦點對於温度變化也來得比較穩定. 而FSQ106ED的操作相對地簡單很多, flat field也比較容易處理, 很適合用來影mosaic, 解像力也相當高.
這兩台是差不多價位及焦距的astrographs, 一台光圈達f/2.8, 另一台的field illumination比較平均.
終於有機會影同一個object去比較一吓, 由於拍攝的時間不一樣, 這並不是一個嚴肅的測試, 相機是QHY8 CCD. 用ε-180ED影的時候還影埋SN2011dn.
可見這兩台astrographs各有專長, ε-180ED的f/2.8焦比的確有其高速優勢, 對於香港變化快的天氣來講, 可以用比較短的時間去達到高一點的s/n ratio, 焦點對於温度變化也來得比較穩定. 而FSQ106ED的操作相對地簡單很多, flat field也比較容易處理, 很適合用來影mosaic, 解像力也相當高.
誰在線上
正在瀏覽這個版面的使用者:Google [Bot] 和 14 位訪客