折返 Catadioptric system + 副鏡遮蔽率

[peter]

https://www.telescope-optics.net/obstruction.htm
Central obstruction effects

[peter]

C9.25

 9.25” aperture Schmidt Cassegrain is the second largest of Celestron's main range of Schmidt Cassegrain Telescopes. The clear aperture is 235mm (9.25”) with a 35% Central Obstruction (by diameter.)

折射 18cm  vs  C9.25 
https://www.facebook.com/photo/?fbid=7456834841011654&set=gm.6862599433796828&idorvanity=244730338917137

[peter]

Mason解說
Peter Lin 35％CO是直徑比，集光面積只減少不到13％，C925剩下的面積和口徑219mm的無遮蔽系統差不多，就算再扣掉修正片的2個折射面和主副鏡反射的損失，集光力也不比20公分折射鏡差，中央遮蔽主要的影響是MTF上中頻部份的反差下降，這對星點和月面這些反差高得不得了的目標來說，完全可以忽略，對於反差低的行星表面細節而言，就是看得到或者有看沒有到的差別了，所以那些中央遮蔽動輒50％以上的RC或CDK，不管多高檔，都沒什麼人拿來看或拍行星，基本上徒勞

[peter]

RHO  350  f/3 ; Central Obstruction, 56% by diameter



https://familystar.org.tw/index.php?option=com_smf&Itemid=45&topic=38918.0

[peter]

TED

http://s00639.blogspot.com/2015/03/blog-post_25.html

當光學系統存在像差沒如此理想, 甚或有內反射存在時, 就會變成下圖的 b 與 c 圖. 這個圖叫作 CTF (contrast transfer function), 也有人稱它為 MTF (mudulation transfer function), 或 OTF (optical transfer function). CTF 很好用, 因為把受量測的光學系統結果畫上去, 就可以與理想狀況比一比, 從而得知其品質狀況

例如很多人講: 反射鏡或折反射鏡前有次鏡或支架遮蔽, 因此 "對比變差", 那麼, 有多嚴重?


n 是主鏡的被遮蔽率, n=0 就是無遮蔽, n=0.4 就是 40%, 蠻多折返鏡都接近這個數字. 大多數情況, 看線1, 2;

 相同解像力下, 有遮蔽的鏡子的對比都會比無遮蔽的差, 符合大家講的.
不過以 40% 遮蔽率為例, 高解像力情況下 (ex. 120 lp/mm 以上),
40% 中央遮蔽的鏡子的 contrast 竟然高於無遮蔽的鏡子?

線 1,2 與線 3,4 這兩組都是描述同口徑, 同焦比下中央無遮蔽與 40%中央遮蔽兩隻鏡子 performance 的 CTF.
有何不同? 這要回到 contrast 的數學定義,

如果目標透過光柵儀器的量測, 其能量分佈正弦圖, 波峰到波谷是從 0~1, 對應 contrast 就是 0% ~ 100%,
那麼我們就要看線 1,2. 比如說: 雙星.

如果目標的能量分佈量出來, 最高就只有 10%, 那麼我們就看線 3,4. 比如說: 對比很低的行星表面細節.

至於線5, 6 是人眼能辨識的最小 contrast 門檻.


像: 看雙星時, 在雙星目標 contrast 20% 以下… 這樣講比較抽象, 因為想像不到 contrast 20% 是怎樣?; 倒過來講, 在放大倍率超過某種程度 (到達 resolution > 120 LP/mm) 後, 40% 中央遮蔽鏡子目視的 contrast 會稍稍比無中央遮蔽的鏡子好 (A>B; E>F). 這就是標題寫的那個 "反射或折反射鏡的副鏡遮蔽竟也有好處",

而 "中央遮蔽的鏡子airy disk 盤面比較小

[peter]

https://starizona.com/blogs/tutorials/observing-theory


https://www.telescope-optics.net/obstruction.htm

[peter]

http://www.astrophoto.fr/obstruction.html

[peter]

副鏡遮蔽率 低 折返  CFF 

21%
https://www.facebook.com/photo/?fbid=1552656532299791&set=a.179815806250544

[peter]

AI GOOGLE

=
central obstruction, such as the secondary mirror in a Newtonian or Schmidt-Cassegrain telescope, reduces image contrast and peak light intensity by redistributing light from the central Airy disk to the surrounding diffraction rings, which can cause a slight degradation of fine detail and a faintly darkened center of the image. The severity of these effects depends on the size of the obstruction relative to the main mirror, with effects becoming more pronounced and noticeable with higher percentages of obstruction.
Effects on Image Quality

    Reduced Contrast:
    The main effect is a reduction in overall image contrast, as light that would normally be concentrated in the central Airy disk is spread out into the diffraction rings.

Loss of Detail:
This loss of contrast makes it harder to distinguish fine details, especially on planets and the Moon.
Faint Shadowing:
At higher magnifications or with moderately contracted pupils, a very faint shadowing or "hole" may be visible in the center of the image.
Increased High-Frequency Aberrations:
A central obstruction can increase the impact of atmospheric "seeing" (high-frequency turbulence) by amplifying the effect of high-frequency aberrations in the image.

Factors Influencing Severity

    Obstruction Size:
    The larger the central obstruction, the greater the loss of contrast and the more noticeable the degradation.

Aperture Size:
The effect of a given central obstruction is more pronounced in smaller-diameter telescopes compared to larger ones.
Viewing Conditions:
While the effect of a central obstruction can be more noticeable during the day or with a contracted pupil, it is generally less apparent at night.

Practical Implications

    Acceptable Levels:
    Obstructions below 20-25% of the primary mirror diameter are generally considered acceptable and have a minimal impact on image quality.
    Tolerable Levels:
    Effects become somewhat more noticeable around 30% obstruction, but many instruments with 33-37% obstructions are still widely used for observing planets and the Moon.
    Optical Quality:
    The overall quality of the telescope's optics and the atmospheric seeing conditions are often more significant factors in image quality than the central obstruction size itself.

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