Focusing using Bahtinov mask

行星、深空天體、星野攝影等
回覆文章
anguslau
中子星
文章: 1157
註冊時間: 週五 15 4月, 2005 20:18

Focusing using Bahtinov mask

文章 anguslau » 週三 09 9月, 2009 23:02

I have become very excited about the Bahtinov mask. Just want to share with you some of my testing results and thoughts about it. Hope it will be of use to some of you... [茶]

I only made aware of the Bahtinov mask very recently. :oops: After reading all about it, I was very excited and am quite convinced that it will be my solution for focusing both at home and when traveling! [遠目]

I have been using FocusMax for a while at home and found it quite effective. But at times, I found it a bit not very consistent and I was always not sure whether I have really hit the best focus. I suspect some star elongation problems I have were at least partly due to imperfect focus. [壓力]

The problem with most focusing methods or tools is that there is no absolute infocus metric one can observe. Using the Bahtinov mask seems to have much better potential in this regard... [推眼鏡]

Since I am quite confident that it will be a tool I use often going forward, I made the investment and purchased some Bahtinov masks from Spike-a instead of trying to make my own. I performed some testings over the last few days and so far I wasn't disappointed! [微笑] In a subsequent post (I'll post it separately because it is very wide) is an image of the Cocoon nebula in Ha made a couple of days under quite challenging conditions (moon, hazy etc). Despite the relatively uninteresting image, I am quite happy with the focusing accuracy using the mask. For each 20m subs and the 4Hr stack, I am quite happy about the roundness and tightness of the stars corner to corner. So I've included the full size image for anyone interested to examine. You should magnify it 400-500% for close examination... [推眼鏡]

I have also done some testings to compare it with FocusMax. I found that for my settings, the FocusMax always end up with ~-20-30um reading when checked using the Bahtinov mask. Which is almost 2-3 critical focusing zone for the f2.8 scope! Not sure if it is because my V-curve is not accurate or other reasons. And of course I am not sure whether there are some inaccuracies with the mask design or the making. [熱] Bringing the focus to within a few um is not too difficult with the mask. Though trying to get a clean diffraction pattern at my rooftop shed is not always easy given the several dozen air-conditioners surrounding our building all pumping hot air furiously. The seeing at my rooftop is always horrifying particularly during summer periods. [吐]

One problem with using the Bahtinov mask is that sometimes it is difficult to judge perfect focus through unassisted visual examination. Fortunately a very generous amateur has developed an ingenious screen grabber to assist. This is the best companion to the Bahtinov mask! It automatically detects the three lines of the diffraction pattern and determine how far you are off perfect focus. You can even enter your imaging parameters for it to translate to um focus error! [遠目] So all you need to do is to grab your star pattern continuously using say MaxIm and let the grabber tell you how far you are off. Then its just a matter of adjust focus until you hear some beeps from the tool or a bulls eye pattern appearing on the screen. Thats it!!! [愛的轟炸]

Back to the comparison with FocusMax. I've examined some test shots to see whether one is more accurately focused than the other. Despite the metrics telling me that I should be quite a bit off with FocusMax, the differences are a bit subtle. May be partly because my seeing was very poor to start with and partly because tracking was quite poor during my testings because of the very strong haze (I can hardly see the summer triangle!) and clouds. But I do see that the images are better focused with the mask. Particularly closer to the corners, the stars are obviously tighter and more rounded...

In the past, I always feel uncomfortable with focusing, particularly in the field. But now with this mask and the tool, I am much more confident and will be willing to use it in the field rather than sticking with AF with my DSLR! Though I haven't tried it, I believe this method should work well with DSLR as well. In stead of examining the pattern on the camera LCD, I should be able to display it on the PC screen through MaxIm. This will then allow me to use the screen grabber tool! [遠目]

So far, I felt that the Bahtinov mask (together with the screen grabber tool) is one of the greatest invention for amateur astronomy for a long time. I hope you'll find it useful too... [茶]
附加檔案
090908_182551_5D01.jpg
My E-180ED fitted with a Bahtinov mask
090908_182551_5D01.jpg (100.22 KiB) 已瀏覽 30080 次
Focused By BM.jpg
Hitting near critical focus!
Focused By BM.jpg (99.75 KiB) 已瀏覽 30080 次
Focused By FM.jpg
Focus error after running FocusMax
Focused By FM.jpg (90.08 KiB) 已瀏覽 30080 次

頭像
昇仔
夸克星
文章: 2377
註冊時間: 週二 13 11月, 2007 17:40
來自: 香港

文章 昇仔 » 週三 09 9月, 2009 23:48

A very long and good sharing. I must try the mask!!

[完美計劃] [完美計劃] [完美計劃]

頭像
noodle
夸克星
文章: 2833
註冊時間: 週五 04 7月, 2003 16:34
來自: 地球
聯繫:

文章 noodle » 週四 10 9月, 2009 04:56

very informative :)
thank you [Good Job] [Good Job] [Good Job] [Good Job]

頭像
noodle
夸克星
文章: 2833
註冊時間: 週五 04 7月, 2003 16:34
來自: 地球
聯繫:

文章 noodle » 週四 10 9月, 2009 05:07

after reading your post I search the web for a while and find some "free template" for DIY BMT :)


http://www.focus-mask.com/Free_Templates.html

頭像
Wah!!
夸克星
文章: 13264
註冊時間: 週一 15 3月, 2004 21:28
聯繫:

文章 Wah!! » 週四 10 9月, 2009 09:17

剛剛下載來試, 似乎軟件需要先安裝 ASCOM 才能運作, 不能獨立地執行...

anyone
夸克星
文章: 13853
註冊時間: 週六 07 2月, 2009 12:13
來自: hong kong

文章 anyone » 週四 10 9月, 2009 10:53

謝謝分享! :)

頭像
bingsze
夸克星
文章: 3939
註冊時間: 週一 15 1月, 2007 18:13
來自: hong kong

文章 bingsze » 週四 10 9月, 2009 16:01

好想買返個玩,不過,都要等有時間先得!唉! :(

頭像
鄧登凳
夸克星
文章: 9497
註冊時間: 週一 03 8月, 2009 17:15
來自: 3rd planet of solar system

文章 鄧登凳 » 週四 10 9月, 2009 17:33

noodle 寫:after reading your post I search the web for a while and find some "free template" for DIY BMT :)


http://www.focus-mask.com/Free_Templates.html
另一說明的網頁http://astrojargon.net/MaskGen.aspx?Asp ... eSupport=1

另一可產生的網頁http://astrojargon.net/MaskGenerator.aspx 這個可以自己控制鏡的大小及焦距等 :)

產生的svg圖可用http://www.fileformat.info/convert/image/svg2raster.htm轉為jpeg.

我會用高影膠片印一個試試 :roll:

smartmars
原恆星
文章: 54
註冊時間: 週二 25 10月, 2011 13:02

文章 smartmars » 週二 17 1月, 2012 11:10

已下, 问个问题, MAXIMDL 抓取的图像是如何传给这个软件的?

頭像
鄧登凳
夸克星
文章: 9497
註冊時間: 週一 03 8月, 2009 17:15
來自: 3rd planet of solar system

文章 鄧登凳 » 週二 17 1月, 2012 12:31

smartmars 寫:已下, 问个问题, MAXIMDL 抓取的图像是如何传给这个软件的?
不需要"傳送", Bahtinov Grabber是分析電腦顯示器上的畫面的, 那只要電腦屏幕上顯示了即時影像(用任可拍攝軟件及檔案格式), Bahtinov Grabber軟件就可以分析畫面上的米字星芒了。

頭像
arsenal_hk
主序星
文章: 154
註冊時間: 週六 29 1月, 2011 19:05

文章 arsenal_hk » 週四 02 2月, 2012 15:04

各位師兄,

小弟用緊 Nikon AF 80-200mm f2.8 ,

想造個 Bahtinov mask ,

出來的結果是下圖, 請問對嗎?

D線密成咁 點 CUT ?? [昏]
附加檔案
未命名.JPG
未命名.JPG (15.47 KiB) 已瀏覽 23585 次

頭像
鄧登凳
夸克星
文章: 9497
註冊時間: 週一 03 8月, 2009 17:15
來自: 3rd planet of solar system

文章 鄧登凳 » 週四 02 2月, 2012 15:39

arsenal_hk 寫:各位師兄,

小弟用緊 Nikon AF 80-200mm f2.8 ,

想造個 Bahtinov mask ,

出來的結果是下圖, 請問對嗎?

D線密成咁 點 CUT ?? [昏]
以下的會好一點, 給200mm用, 一個是用來印的(做法見: http://www.hkastroforum.net/viewtopic.php?t=20803http://www.hkastroforum.net/viewtopic.php?t=19334), 另一個強制用1.5mm寛度, 應可用刀"界"出來。
附加檔案
Masks.PNG
請自調好大小!!
Masks.PNG (105.94 KiB) 已瀏覽 23571 次

頭像
arsenal_hk
主序星
文章: 154
註冊時間: 週六 29 1月, 2011 19:05

文章 arsenal_hk » 週二 07 2月, 2012 22:08

鄧 sir

[跪拜禮] thank you 晒


Bahtinov mask 的完成圖 及 實拍測試
附加檔案
2012-02-07 21.50.07.jpg
Bahtinov mask
2012-02-07 21.50.07.jpg (183.65 KiB) 已瀏覽 23388 次
Mark (3).jpg
安裝在小黑三上
Mark (3).jpg (322.05 KiB) 已瀏覽 23388 次
Mark (6).JPG
實拍測試
Mark (6).JPG (141.89 KiB) 已瀏覽 23388 次

回覆文章

回到「天文攝影 Astrophotography」

誰在線上

正在瀏覽這個版面的使用者:沒有註冊會員 和 2 位訪客