失敗的Omega星團

[社長]

到台灣後首張開光照，以21.9低緯度，趁奧米加星團過中天時，在頻密掠過的雲隙之間、100米外一群青年不斷放的煙花、天空上不時飄過的孔明燈下，爭取10分鐘拍攝了這張照片，但最後還是逃不過人造衛星的襲擊

心灰意冷，加上照片質量太低，免得花時間處理。導星期間發現LVI上的XY Chart來回波動無衰減，尤以一軸為甚，當時視寧度不致於太差，拍攝環境無風，所以遁內部問題查找，發現該指向緯軸極不平衡，所以這張照片追蹤並不合格。而且奧米加星團過中天時仰角不高，大氣也嚴重影響影像解像度。

儀器：APM-TMB 105/650 APO折射鏡、TMB 3.5"平場鏡、Sky Watcher HEQ5 Pro赤道儀
導星：第一代LVI導星器、9x50尋星鏡加2x巴羅透鏡、有效焦距350mm
相機：Canon 5D Mark II Modified、IDAS LPS P2 光害濾鏡、ISO 1600 單張曝光10分鐘。
地點：台灣墾丁鵝鑾鼻
處理：無平場、無暗場(室外26攝氏度)、僅Photoshop curve & histogram stretching

後來發現，其實3分鐘都差不多了，有點浪費時間，也累積了低仰角視寧問題造成星點漲大，稍後或許試用Photoshop的收細星點技巧騙騙眼球。今晚再接再厲...

1. 全圖resize出圖
2. 中央1800x1200裁切，50%出圖

[Simpleasure]

开光照已很精彩。单张10分钟，omega的漂亮已经跃然而出。
期待社长后面的精彩呈现。

[willis]

In fact the photos are not bad! I processed a bit in PS. The stars are pin sharp and of good color.By the way, is the 3.5" TMB flattener zero power?

[社長]

謝謝樓上兩位，現已移師墾丁貓鼻頭拍照.....

Wilis, yes, the TMB flattener is zero power, as Thomas Back don't want to degrade image quality.

[willis]

謝謝樓上兩位，現已移師墾丁貓鼻頭拍照.....

Wilis, yes, the TMB flattener is zero power, as Thomas Back don't want to degrade image quality.

社長, pls comment whether I am correct or not. Most people think that field flattener/reducer can provided completely flat field. But up to my understanding in most of the situation, the trade-off for edge performance is to sacrifice a bit the centre performance,

[社長]

社長, pls comment whether I am correct or not. Most people think that field flattener/reducer can provided completely flat field. But up to my understanding in most of the situation, the trade-off for edge performance is to sacrifice a bit the centre performance,

Willis, 我不是光學設計師，這方面很皮毛，但有幸跟業內領導性人物溝通，例如APM的負責人Markus Ludes，又或者近年光學設計紅人Dr Massimo Riccardi等。或許我把他們跟我說過的組織一下：
viewtopic.php?f=6&t=92

平場鏡，不管是直焦、增焦或減焦，其實都是一種平衡。若主鏡是一個色差很嚴重的普消折射鏡，又或者是一片短焦球面反射鏡，或許你可以不惜工本用很多片特種玻璃及光學面去營造一個高修正度及平場的成像圈，但會得不償失。

倒過來，若主鏡的成像本身就是很好，平場鏡的設計及工藝會簡單得多，在極端的例子如下，例如據我在意大利聽到，有一台科研級的1.5米口徑的RC是用單透鏡貼近焦平面工作的。

當然大部份例子是兩者之間，那便是一種取捨的藝術了。例如現時我要求Dr Massimo Riccardi把最近的APM通用型0.75x平場減焦鏡的修正，從紅光稍移至藍光一點，因為我寧可420nm好些，而放棄706nm波段。所以一台攝星鏡，很多時候目視的表現都沒怎麼樣，甚至可能很弱，而一台目視的頂級望遠鏡，例如APM的APO折射鏡，加了平場鏡後就犧牲了軸上成像質量交換了更大而修正度高的像場。以我的TMB 3.5"平場鏡計，最佳修正度是在軸外1度左右的。

所以你在軸上軸外修度的看法在現實絶大部份情況中是對的。

但我也想指出，平場的定義，若一個光學系統在軸上及軸外的最佳修正度焦點都在一個平面上，那是絶對的平場，問題是那個"最佳"的修正度有多高而已。Dr Riccardi最新為APM設計的四片代APM-LZOS 160 F5.5及140 F5攝星儀，軸上全波段可達至貼近APM-TMB的目視水平(但不能等同)，但軸外的拍攝修正度則可超越。

[David]

It is nice to hear that the cause of the problem seems to have been identified.

Wish you have better weather soon.

[社長]

It is nice to hear that the cause of the problem seems to have been identified.

Wish you have better weather soon.

這裡的視寧度並不好，所以星點仍是比在香港拍的稍大一點。好處是光害少一點，可能是西貢大浪東灣的情況吧，若到海拔高一點的地方會好得多，但這幾天的天氣真不好說。

高興的是，Henry的出來的照片都追蹤精確的，那回來後和Willis及你出去實習一下吧。

[David]

What are that series of red dots under the satellite trail?

[社長]

1樓那是飛機，不是人造衛星....

這三天有點頭昏腦脹了

[社長]

再來一張，今早拍的，跟1樓同樣設備，疊加5張，累計曝光10分鐘，25%出圖。

相機出了點問題，影像仍有點瑕疵....

僅以DSS疊加及Photoshop拉曲線，沒有心情處理其他了....

[社長]

樓上的中央部份，1800x1200裁切，50%出圖。星點比1樓的幼細些，可能是視寧度稍好吧，但相機有點問題.....

注意TMB 3.5"平場鏡並非以軸上修正度最高，因為其成像圈有80mm，最優化區域為軸外1度。所以這個星團的解像度仍有改善空間。

Copy of Omega.JPG (168.39 KiB) 被瀏覽 78404 次

[社長]

In fact the photos are not bad! I processed a bit in PS. The stars are pin sharp and of good color.By the way, is the 3.5" TMB flattener zero power?

Willis, would you mind processing my 2nd trial and share the results? thx!

[willis]

Thanks 社長 for inviting me to process his photos again. What I presented here is what I have learnt from experience astrophotographer and my experience of PS skills in ordinary photography. In Astrophotography, I am a true newbie
Different DSO has different character. The PS functions used are bit different. To me I think Levels & Curve are most important. Omega is relatively less colorful compared with the previous one M8 & M20.
Here are the steps in Photoshop:
1. Levels: Since Omega is less colorful, I used the gray eyedropper to align all 3 RGB channels in one go. Just point the gray eyedropper to a dark background point. There are three eyedroppers on the right hand side of the Levels menu. The gray eyedropper is at the middle. Then the 3 RGB channels will be aligned. U may fine tune different channel manually afterward, which I didn’t do it this time.
2. Reduce Noise: Use navigator to magnify the image to about 300-400%. set the Tolerance to about 20-30 and use the Magic Wand Tool to point at a dark or noisy point. Apply strong noise reduction.
3. Smart Sharpen: In step 2, u have selected the area for noise reduction. Now nverse select the area. Go to Smart Sharpen and adjust the parameters Amount & Radius. In this case I used 40% for Amount and 2 for Radius.
4. Curve: Different celestial objects got different curve shape. In this case I used 3 tags to shape the curve. The bright part of the curve is quite close to the original straight line. The dark part of the curve is suppressed in order to make a dark background.
This is no best way to process the photos, but what I presented here is what I have done in this particular Omega.

[xiaoer]

Great pics~~
但星點好像有點橢圓？

[社長]

Great pics~~
但星點好像有點橢圓？

yes, 珊看到問題所在了，所以我說這是失敗之作，一來相機的焦平面不準直，可能是新造的接環有問題，另外當晚在墾丁貓鼻頭公園陣風很強，所以在100% scale下，星點並不呈正圓點。

後來架起觀星帳篷就好些(廣東南十字星會那種)，我會進一個來用。

所以自動導星如何準確，有些外在因素例如視寧度及強風還是不可能解決的。

[社長]

Thanks 社長 for inviting me to process his photos again. What I presented here is what I have learnt from experience astrophotographer and my experience of PS skills in ordinary photography. In Astrophotography, I am a true newbie
Different DSO has different character. The PS functions used are bit different. To me I think Levels & Curve are most important. Omega is relatively less colorful compared with the previous one M8 & M20.
Here are the steps in Photoshop:
1. Levels: Since Omega is less colorful, I used the gray eyedropper to align all 3 RGB channels in one go. Just point the gray eyedropper to a dark background point. There are three eyedroppers on the right hand side of the Levels menu. The gray eyedropper is at the middle. Then the 3 RGB channels will be aligned. U may fine tune different channel manually afterward, which I didn’t do it this time.
2. Reduce Noise: Use navigator to magnify the image to about 300-400%. set the Tolerance to about 20-30 and use the Magic Wand Tool to point at a dark or noisy point. Apply strong noise reduction.
3. Smart Sharpen: In step 2, u have selected the area for noise reduction. Now nverse select the area. Go to Smart Sharpen and adjust the parameters Amount & Radius. In this case I used 40% for Amount and 2 for Radius.
4. Curve: Different celestial objects got different curve shape. In this case I used 3 tags to shape the curve. The bright part of the curve is quite close to the original straight line. The dark part of the curve is suppressed in order to make a dark background.
This is no best way to process the photos, but what I presented here is what I have done in this particular Omega.

Thanks Willis for the improved version of Omega, I love it.

Let me digest what you did and what does it means......

One question, is it possible to process the star sharper but less contrasty, such that the edge of star images have a gradient hence look more realistic?

Post processing is very important, and thanks for sharing all these

星雲、星團都試過了，下次給你一張星系處理一下

[willis]

What about this? Just follow the same steps, but with different parameters and curve. Say Tolerance is 10, Amount is 20%, Radius 1 & slightly different curve.

[David]

I prefer the background not-so-dark. The last big picture feels better to me.

[社長]

I prefer the background not-so-dark. The last big picture feels better to me.

agree, the last version is more 3D-like and close to visual impression from telescopes >150mm aperture.

thanks Willis.