M31,M45,M42

[aa]

這張M31是40分鐘的，平場就是用同樣的檔案

這張 M31 有夠水...

[cuteip]

Cuteip 告訴你個殘酷的事實，8300的快門並沒有那麼快喔!
我的測試至少要90ms才不會高過快門簾的速度
這張是新機的時候實拍的，使用標準光板色溫5000去拍的90ms

快門速度我是依據畫面中的VALUE
8300最高採樣值應為25000
所以取一半差不多為12500
當然也有人說取約1/3
效果則好像還沒人比較過

照上面這張平場
我最早也是這樣拍
不過應該是過曝了吧!
這要套進去疊會對嗎?

The target ADU selected should be about 33% of the saturation level of your camera. This will give the most accurate and noise free raw flats which will result in the best master flat once stacked. Going too high can result in pixels outside the linear range of the CCD and too low can result in poor signal-to-noise in the flat. The allowable error percentage allows you to control how close to the target ADU count the flats must be - exposures outside this range will not be saved, rather a new exposure time will be calculated and the exposure retaken. Making this too restrictive may result in a lot of exposures being thrown away.

以上文章取自http://winfij.homeip.net/maximdl/skyflats.html

平場的ADU應設為相機飽和水準約為 33%
st8300
Full Well Capacity    ~25,500 e-
照這個算法應為 25500*0.33=8415

以上作法應該是錯誤的
在下面的回文我有做解釋

後來我是使用
Sky Flats Assistant MaximDL Plugin
設定好ADU及曝光時間範圍後
會自行找出適合ADU的曝光時間再進行平場拍攝
如光源過亮 也會告知

[cuteip]

下面這篇文章
有針對各種ADU做不同的測試

•The OTA was a Takahashi BRC-250 with a ST-10XME CCD.

ST-10XME CCD
Full Well Capacity 77,000 e-
1.3 electrons/ADU
算出來pixel saturation point of the ST-10 is 59,213 counts
這篇介紹中
最後原作者是採用ADU 35000來作平場
換算了一下 大約59%左右
但是The best range for the flat field counts using this camera is 20,000 to 29,000 counts.
所以以35000來作稍嫌高了一些 應該是1/3至1/2之間

不過原作者也說 15000-50000都差不多

•At 5,000 & 10,000 ADU there was an increase in noise in the final calibrated image.

•At 55,000 and 60,000 ADU there were artifacts created in the calibrated image due to saturation that occurred in the Flat Field image.

•However, from around 15,000 to 50,000 ADU there was very little difference created by the change in Flat Field ADU.

•Currently I take my flats around 35,000 ADU as this seemed the best compromise for quality and time to complete my flats.

http://darkhorseobservatory.org/index.php?CategoryID=62

[cuteip]

再另外一篇文章看到這段話

The saturation value of a CCD camera is a function of the full-well capacity and the gain.  It is therefore not necessarily the same as the maximum pixel value.  For example, an ST-237A has a max pixel value of around 65,000 e-.  However, the saturation level is only 9,000 e-, or a pixel value of about 3800 ADU.  The recommended flat value would be between 1250 and 1900.  Note that for some cameras, the calculated saturation value is greater than the maximum pixel value (65,535 for a 16-bit camera).  For such a camera the recommended flat field value would be 1/3 to 1/2 the max pixel value, or 21,845 to 32,767.
以上文章來源:
http://starizona.com/acb/ccd/advimcal.aspx
翻譯:一台 CCD 照相機飽和度的值是全井能力和增益的函數。故不一定最大圖元的值相同。例如，ST 237A 的最大圖元值約為 65000 e-。飽和水準卻只有 9,000 e-或約 3800 ADU的圖元值。推薦單位值將 1250年至 1900 年之間。請注意，對於某些相機，飽和度計算所得的值大於最大圖元值 （16 位相機 65,535)。這種相機推薦單位欄位值將 1/3 到 1/2 的最大圖元的值或 21,845 到 32,767。

以ST8300來看
最大圖元值Full Well Capacity    ~25,500 e-
A/D gain: 0.38e-/ADU (prototype).
飽和水準算法
25500/0.38=67105
因為67105>65535
所以飽和水準還是得採65535
因此推薦單位欄位值將 1/3 到 1/2 的最大圖元的值或 21,845 到 32,767。


所以像ST-10XME CCD
以下有算法
The current wisdom in collecting flat field frames is to expose them long enough to collect sufficient light to expose the chip to 35% to 50% of its full saturation level. The saturation level is determined by dividing the gain of the camera into the chip's full well capacity. For example, my ST-10XME has a full well capacity of 77,000 electrons. To find the saturation level, I divide the gain, 1.3 electrons/ADU, into the full well capacity. The result is 59,213. Thus, pixel saturation point of the ST-10 is 59,213 counts. The best range for the flat field counts using this camera is 20,000 to 29,000 counts.
以上算法資料來源:
http://www.astrophoto.net/calibration.htm

[genome]

天呀 看完這一大串
彷彿看見當年拿著黑白底片到濃度計下去校正顯影時間的噩夢  :