12/9~12/10 再煲 M42

[funoooo]

Scope: FSQ 106

Mount: Skywatcher HEQ5

Filter: P2
Camera: Canon 450D (Modified)

Guiding System: LVI II
Guiding Scope: 350mm guide scope (A modified 9x50 finder scope)

ISO 400
120 sec x 20
300 sec x 15

Have a Dark frame, Flat frame, and Bias frame

Place: Dongguan ChaShau

今次再煲 M42，但用了 P2 filter，而且改用了 ISO 400。今次再一次調較，慢慢地改。 這次應該是暫時最好的。暫時有一段時間都不會再拍 M42 啦~~~ 改拍其他 object。

[社長]

funooo 上面三張的個別數據如何? 拍得十分好的，有丁點意見，或許可以說說，但要了解拍攝數據才可確認。

- 以上三張的曝光時間?

- 導星是哪一顆?

- 對焦不是完美但十分的理想，已在一般人的水平之上了，你是用魚骨板及人手調的，對吧?

很高興看到你進步很快的，導星也是無師自通，拍數百毫米焦段的深空，不會盲目追求或鼓吹兩三萬元的赤道儀。

[funoooo]

funooo 上面三張的個別數據如何? 拍得十分好的，有丁點意見，或許可以說說，但要了解拍攝數據才可確認。

- 以上三張的曝光時間?

- 導星是哪一顆?

- 對焦不是完美但十分的理想，已在一般人的水平之上了，你是用魚骨板及人手調的，對吧?

很高興看到你進步很快的，導星也是無師自通，拍數百毫米焦段的深空，不會盲目追求或鼓吹兩三萬元的赤道儀。

多謝社長! 我仍是初哥一名, 有很多事需要改進.

1. 三張都是同一張照片, 只是在 PS 上不同調較. 按 Universe 建議用了 ISO 400, 120 sec x 20, 300 sec x 15

2. 參宿一

3. 用 Canon 450D Live view 放大 10x 對焦.

今次更深明白 light 及 air pollution 影響, Orion 未升上 45 度以上. 根本拍不到什麼! 而用 P2 FILTER 在 ISO 400 情況最多 5 分鐘已是極限. 我都想了很久, 在高污染環境下很難再突破. 所以暫時不再拍它. 如果要再拍就會用 H alpha filter 去拍 L-channel 得 Detail, 再 combine 現有相去加 Color. 不過要先買 H alpha filter 先.

[社長]

funoooo 我在深空拍攝方面也沒有太多分享，後期處理也屬幼稚園水平。不過希望以僅有知識幫你一把吧。

若你的被導星是參宿一，即腰帶上的一顆星，而非參宿七，那相片左下角的星點輕微拉線便不是場旋問題，當然若樓上是三張獨立曝光不同的照片，因場旋造成的拉線也會按比例而不同。若有人沒問清楚曝光或被導星便認為你對極不準而導致場旋拉線，很明顯是缺乏了拍攝星空的基本認知了。

造成像場一角星點拉線有很多原因，除了外在因素例如場旋外，我建議你看看之前用這鏡拍的照片是否在同一地方有同樣問題，例如一般Consumer級的Canon單反，也不能保證焦平面是準直的，除非是Industrial Grade，但我看不應該在你的APS片幅有這明顯問題，另一個可能性是接環沒有鎖緊，甚至望遠鏡光軸有輕微問題，這在FSQ85上稍為多見，但FSQ106是好得多的，特別是其調焦筒粗壯得多，一定比Televue的好。

要搞清楚這點，仍是在拍完照片若發現有類似問題，應把相機轉180度後再拍一次，其他如赤道儀、導星等不要動，若星點拉線也在同樣那幾粒星，即跟相機無直接關係的，便代表望遠鏡有問題，反之便是被轉動部份的內在問題。

所以若以右側的星點來看，你無師自通玩LVI導星已上手了，解決了很多人掙扎經年甚至希望花兩三萬元搞個高精度赤道儀最後也解決不了的問題，所以這套三千多元的LVI實質上幫你省了不少無謂的錢，因為這是一套你可以轉移到任何一套帶導星口的赤道儀上，所以每一套赤道儀都可用最廉價的便可以，反正NEQ6 Pro可以承20公斤以上的350牛反，足夠大部份人的需求，才1萬元水平，而非迷信機械精度的造法，拍星野又找套上萬元的，承望遠鏡又找套三萬元的。

你的調焦很有水平了，若有數碼化的器材輔助會好丁點，當然這點你已有了答案...

曝光及仰角也是要隨機應變，而對於濾鏡，從P2到V4會有更強的抗光害效能，但Ha我個人有些保留，因為單反配Ha始終表現很怪的。

[willis]

社長， Which software can help to calculate the angle between two celestial objects? Or one needs to calculated by hand. I know the field rotation is small in funoooo's case. I want to do some calculation.

[社長]

社長， Which software can help to calculate the angle between two celestial objects? Or one needs to calculated by hand. I know the field rotation is small in funoooo's case. I want to do some calculation.

Willis, you just need to understand the angle of guide star from the farthest edge of the camera field, which you can look up from a star map, or measure from The Sky software. Then you need also the focal length of imaging telescope, exposure time, and polar alignment error and declination of your imaging target, then you can calculate the length of worst case star trails on your image due to field rotation.

this is the mathematic model, sorry I surrender....
http://www.stargazing.net/yizen/fieldrotation.html
http://celestialwonders.com/articles/po ... curacy.pdf

for me, an online calculator is good enough for the purpose.
http://celestialwonders.com/tools/rotat ... rCalc.html

if you keep everything the same but changing the guide star angle, it clearly illustrate that the farer from the the guide star the longer the star trails will be. That's why I say someone is too early to conclude funoooo have field rotation even before checking what guide star he is using. Because the star trails will be in circular form rotating around the guide star, now clearly the trailings are on the opposite side that it "should be" - clearly not due to field rotation, unless funoooo is actually guide on 心宿七 not 心宿一 which is on the left side of the picture.

this is the data I used to evaluate funoooo's case,
Alignment Error 5 arcmins
Guide Star Angle 5 degrees
Focal Length 530 millimeters
Time minutes 3 minutes
Declination 0 degrees

it show that with a not accurate yet reasonable polar alignment, e.g, 5min, funoooo will only get <1um star trails on his worse part of the image, which is still much less than 1 pixel of his camera, which should not be noticeable even at 100% scale.

That's why you still get acceptable result in Taiwan last time even imaging M31 for 10min and guide on Jupiter at 40 degree away, the movement of Jupiter is actually more a problem than polar alignment. Of course such case is undesirable, the only reason I told you to do was you don't have a dew-remove on your guidescope, so only Jupiter works that night, right?

sorry to say, if someone want to attack LVI guider and off-axis guiding technique, he should better prepare himself studying the basics of polar alignment and guiding technique, otherwise, keep attacking other people with wrong assumption and misunderstanding of the basics, or even worse, attacking other people with image resolution that he can never reproduced with the same focal length or even longer, will only make himself a joke.