willis 寫:I found that for a reasonable 5nm narrow band Ha image, usually takes at least 10 mins. I guess for 3nm narrow band Ha may at take at least 15 mins!
社長 寫:willis 寫:I found that for a reasonable 5nm narrow band Ha image, usually takes at least 10 mins. I guess for 3nm narrow band Ha may at take at least 15 mins!
我對你的論據有點保留。
不管3或5nm,目標波段的通光率都接近100%,那同樣曝光例如10分鐘,哪款的訊噪比及訊息強度會高些? 當然最暗/深能拍到什麼,又是另一會事。
社長 寫:- 在香港拍Ha或SII窄頻有很大發揮空間,不少人初拍Ha時都有些喜悅,但OIII便吃力得多了,所以黑暗天空還是沒有替代品的。另外有些天體,因為本身實在太暗,以我的理解,就算是Ha,在香港仍是十分十分吃力,至少我見不到有很類近深度的香港土產照片:
社長 寫:這帖移至相機及配件版,比較貼題。
Raymond TSE 寫:For Tibert, Tai Chung Hill top, Yin Na Hill, East Dam, you would be better off with 5nm (even 7-8-13nm would do quite well) but in more urban areas like HK you should use 3-4.5nm.
willis 寫:What do u mean by "但OIII便吃力得多了"?
willis 寫:社長 寫:這帖移至相機及配件版,比較貼題。
According to ur chinese description "Equipment (Eyepieces & Accessories) - 各類目鏡、濾鏡、平場鏡等光學配件 ". "3nm Vs 5nm ...." should be placed in ""Equipment (Eyepieces & Accessories)..."
willis 寫:社長 寫:willis 寫:I found that for a reasonable 5nm narrow band Ha image, usually takes at least 10 mins. I guess for 3nm narrow band Ha may at take at least 15 mins!
我對你的論據有點保留。
不管3或5nm,目標波段的通光率都接近100%,那同樣曝光例如10分鐘,哪款的訊噪比及訊息強度會高些? 當然最暗/深能拍到什麼,又是另一會事。
What I mean here is that when we look at the histogram, usually we will judge the exposure is enough if the histogram is shifted to the right for about 25% in DSLR imaging. For example, ISO 1600, F4, without any light pollution filter, it may take 30s for histogram to shift about 25%. Perhaps with P2, it may take about 2 mins for the histogram to shift about 25%. So I guess the narrower the band width, the more time it takes to shift the histogram to shift the same amount. Of course, I have no CCD imaging experience, I don't know whether CCD imaging has DSLR like histogram after each shot.
社長 寫:willis 寫:社長 寫:這帖移至相機及配件版,比較貼題。
According to ur chinese description "Equipment (Eyepieces & Accessories) - 各類目鏡、濾鏡、平場鏡等光學配件 ". "3nm Vs 5nm ...." should be placed in ""Equipment (Eyepieces & Accessories)..."
無喎,你再睇真D?
社長 寫:willis 寫:社長 寫:willis 寫:I found that for a reasonable 5nm narrow band Ha image, usually takes at least 10 mins. I guess for 3nm narrow band Ha may at take at least 15 mins!
我對你的論據有點保留。
不管3或5nm,目標波段的通光率都接近100%,那同樣曝光例如10分鐘,哪款的訊噪比及訊息強度會高些? 當然最暗/深能拍到什麼,又是另一會事。
What I mean here is that when we look at the histogram, usually we will judge the exposure is enough if the histogram is shifted to the right for about 25% in DSLR imaging. For example, ISO 1600, F4, without any light pollution filter, it may take 30s for histogram to shift about 25%. Perhaps with P2, it may take about 2 mins for the histogram to shift about 25%. So I guess the narrower the band width, the more time it takes to shift the histogram to shift the same amount. Of course, I have no CCD imaging experience, I don't know whether CCD imaging has DSLR like histogram after each shot.
我在西藏拍窄頻,通常單張曝光30分鐘,就算在月夜下拍攝,減Dark後天空背景亮度只有100多而已,相對於65536峰值,還有很遠距離。我想以Ha本身的性質,是很難看到一個天體本身的Histogram推至右邊去(除了M8、M42這些吧),那訊噪比可能更影響Stretch出來的結果。
而我的SII是3nm、OIII也是3nm,但Ha用5nm不用3nm,主怕有反效果,Cloudynights上有些相關討論,例如:
http://www.cloudynights.com/ubbthreads/ ... in/5640617
Quote :
As mentioned, 3nm H alpha cuts out NII which is common in many planetary and some other emission nebula. This changes the look from what you get with 5nm that picks up both. Nothing wrong with just the H alpha without NII just that it is "different". If LP is an issue then 3nm will help. Otherwise I'd go 5nm. The slight difference in star size can easily be controlled in processing with a bit of deconvolution or other means.
I have an issue you likely don't. I sometimes use Ha for very distant galaxies strong in H alpha. Even 5nm can be too narrow with redshift moving the H alpha line out of the passband! Happened to me even with my 6nm filter. Since my skies are very dark I'm thinking of replacing it with something wider for that reason. Obviously not an issue for typical narrow band imaging.
http://www.cloudynights.com/ubbthreads/ ... in/5760045
Quote :
Except for the HII regions, galaxies are very dim in HA light so I doubt going to 5nm will be of much help. What it does do is widen the bandwidth so you can pick up the HA in more highly redshifted galaxies. While nearby ones like M51 are fine in either 3 or 5nm as redshift increases the HA frequency moves out of the passband.
社長 寫:當然,進入數碼時代,十多年下來,我發現不少愛好者初踏足窄頻攝影,發現Ha的效果跟以前截然不同,北美星雲、馬頭、M42、玫瑰幾乎是必到之處,看到那種對比度層次感的提升,誤以為自己的攝影水平大躍進。
正如拍行星月面,從菲林到webcam,然後到天文攝像頭,又或者從菲林到數碼單反,然後到冷凍CCD,每一次儀器更新換代,都會湧現一批新的"大師",但下一輪的新技術新器材推出市場,原有一批"大師"又被淹沒了。
若你滿足於那種層面,那就不用太深究了。
Raymond TSE 寫:Try to buy par-focal narrowband filters with the other LRGB, since you would not have to adjust focus much for each filter.
If you use off-axis guider, par-focal will be more important, as you don't have to adjust the focusing of the guider when you change from LRGB to
narrowband.
I find the same problem during remote imaging with my 7x2 inch FLI filter wheel where I use 1 brand of LRGB and another brand
CS 4.5nm narrowband, since usually there is no way to adjust the focus of the guider remotely.
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