Focusing using Bahtinov mask
Focusing using Bahtinov mask
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. 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.
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.
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]
I only made aware of the Bahtinov mask very recently. 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.
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.
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]
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- My E-180ED fitted with a Bahtinov mask
- 090908_182551_5D01.jpg (100.22 KiB) 已瀏覽 37265 次
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- Hitting near critical focus!
- Focused By BM.jpg (99.75 KiB) 已瀏覽 37265 次
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- Focus error after running FocusMax
- Focused By FM.jpg (90.08 KiB) 已瀏覽 37265 次
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
http://www.focus-mask.com/Free_Templates.html
另一說明的網頁http://astrojargon.net/MaskGen.aspx?Asp ... eSupport=1noodle 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
- arsenal_hk
- 主序星
- 文章: 157
- 註冊時間: 週六 29 1月, 2011 19:05
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=20803 及 http://www.hkastroforum.net/viewtopic.php?t=19334), 另一個強制用1.5mm寛度, 應可用刀"界"出來。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]
- 附加檔案
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- 請自調好大小!!
- Masks.PNG (105.94 KiB) 已瀏覽 30756 次
- arsenal_hk
- 主序星
- 文章: 157
- 註冊時間: 週六 29 1月, 2011 19:05
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