A design of powerful and fast 3 mirrors reflecting telescope

[oldsaltfish]

強力快速三片反射式望遠鏡設計 (第二版)
A design of powerful and fast 3 mirrors reflecting telescope(Second edition)


I have done a modification again
又修改了一下,
1.no lens, no chromatic aberration
不使用透鏡,完全無色差.
2.improve the speed to F/2.67, very fast now!
相對口徑提高到了2.67
3. No more need to make some change when photo on the focus without eyepiece since no lens on the way to focus,can use barlow too.
因為沒有透鏡,直接焦點攝影時,不用改動相機,也可以用增倍鏡了.
4.The tube length is only 420mm now, much shorter than 750mm before
鏡筒長度大大縮短,現在是420mm,原來是750mm
5. smoothly enhanced illumination
照明略有改善,特別是邊緣

Others
其它:
Now there is a hyperboloid, a concave ellipsoidal surface and a convex sphere surface need to be done.
現在是一個雙曲面,一個凹橢球面,一個凸橢球面















第一版FYI.(Original version)
Mirror 1 is the major paraboloid with contain coma
鏡1是有彗差的拋物面主鏡
Mirror 2 is a spherical surface which will introduce spherical aberration to the system, but no coma.
鏡2是球面,產生球差,但不會產生彗差,
Mirror 3 is an ellipsoidal surface that can correct spherical aberration and coma, but the field is curved
鏡3是個橢球體,校正了系統的球差和彗差,但還存在像場彎曲
Mirror 4 is just a flat mirror like Newtonian telescope's secondary mirror, to give convenience for eyes.
鏡4只是一塊平面鏡,就像牛頓式望遠鏡的副鏡一樣,方便觀測
Lens 1 smooth the curved field for eyepiece or photo.
透鏡1讓像場成為平面以供目視或照相


Strongpoints:
優點
1.It has a F/3 fast speed;
F/3的大相對口徑
2.A large excellent image field, 60mm diameter,or 4.6 degree, with little chromatic aberration;
很大的優秀圖像範圍,60毫米直徑,也就是說4.6度,色差極小
3.easy to grind. mirror 1 is a concave paraboloid, mirror 3 is a concave ellipsoid, both of them can be tested using a knift edge+SIXTEST program, mirror 2 is a convex sphere so that it can be tested while mirror is still a sphere, using "Newton's circlet"
容易磨製.鏡1是凹拋物面,鏡3是凹橢球面,都可以用刀口加SIXTEST軟件檢驗,鏡2是凸球面,可以在鏡3還是球面時用牛頓環檢驗.

Here are shortcomings:
缺點
1. semi illumination
只有一半照明
2. very difficult to adjust axis
校準光軸困難
3. obstruction area is 21.8%, just acceptable
阻擋面積為21.8%,可接受
4. need to make some change(as it is not long enough from the lens 1 to the focus, or negative) when photo on the focus without eyepiece,can't use barlow too.
直接焦點攝影時,必須改動相機(因為從透鏡1到焦點,也就是底片位置的距離不夠長),也不能用增倍鏡.


References:
參考資料
http://www.ast.cam.ac.uk/history/mmmt/

[Wah!]

用四片反射鏡,我有以下考慮:
- 鏡片研磨/鍍膜精度要求也很高吧?!
- 鏡片放的位置誤差對於成像的影響有多大?有沒有計算過?
- 鏡片的反射率要求也不能太低,否則減光利害.
- 主鏡外的三片副鏡都相對地很大,固定它們會否困難?
- 整支鏡望遠鏡的重量相較同口徑牛頓鏡相差多少?
- 光學面多且大部份不是球面,如果不是自行研磨,訂製一套鏡子的價錢可能不低.
- 副鏡對於反差和繞射的影響有多少?

如有說錯請指正.

[oldsaltfish]

thanks in advance!
-凹鏡精度容易通過刀口+軟件檢驗,凸鏡要等凹鏡做好後一起檢驗,或代加工.
-位置可以微調,比一般的是要痛苦一些.
-用銀膜.98.3%的反射率,經過四次反射後是93.4%,比單層鋁膜還要高.
-不可避免的重,幸好鏡筒短.
-對於我個人來說,如果全訂製就沒太大意義了,自製有成就感.

[Wah!]

你的設計完成,會何時動工呢?
到時記得分享一下你的製作過程呀!

[oldsaltfish]

正在做主鏡,因為鏡子大,進度很慢.

[Wah!]

你做的主鏡有多大?
有沒有相片可以分享?

[oldsaltfish]

250mm(10"),無相.

[Subaru]

用銀膜.98.3%的反射率,經過四次反射後是93.4%,比單層鋁膜還要高.

我以前試過為牛頓式反射鏡鍍過銀膜, 有以下問題:
1. 單用家用設備及化學方法, 很難鍍得均暈;
2. 銀面會被氧化, 氧化銀是黑色, 非常之影響鏡面的反射力.

[oldsaltfish]

解決方法很簡單,送廠鍍銀+SiO2保護層.