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So I'm currently shooting with a Nikon D810 and a Nikon 200-500mm f/5.6 at 500mm. I've found focusing to be the hardest part of the process so far. I've tried a bahtinov mask with limited success. I've used APT in conjunction with and without the bahtinov mask. Even just turning on live view and trying to minimize the area of the star under magnification in APT yields poor results. I find the best results from using live view on the back of the camera and minimizing the star points / area. I have a long leverage knob on my focus ring for finer adjustments. I just cannot get my images to look at pinpoint sharp as many of the images I see on here. Tonight for example, I shot M13, and the stars look bloated compared other's images.  M13 Here is a cropped single, unedited light frame for you to look at if you want to see what I was working with pre-stack... https://imgur.com/a/60ax5Nm The only thing I can think of besides I'm just out of focus is pixel scale. I ran my image through Astrometry.net and it spit out a pixel scale of 2.07 arcsec/pixel (on the original finished image, this image I uploaded I doubled the resolution in Lightroom but it looked exactly the same but Astrometry.net showed a smaller pixel scale of 1.04 arcsec/pixel). I've noticed a lot of the M13 images I drool over have a pixel scale of .7 and below. Am I just pushing my 500mm "scope" too far and simply cannot resolve the stars any smaller? If not, then how do I properly focus my rig? I really thought I got it right tonight, the stars were looking nice in Lightroom as I previewed the image. I cannot figure out how to zoom and pan in APT so I load the images into Lightroom to pixel peep at in the field. I'm obviously new and may not be providing the right information, let me know if you need something else from me to answer my questions... Tips? Tricks? Suggestions? Thanks! Benjamin |
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Hi Benjamin, you have some nice images, some of them are well focused others not. What is your problem with using a bathinov mask? Should work well with your focal length. Your camera has a pixel size of 4.87 micrometers, so with 500mm focal length you're operating at 2.0 arcsec/pixel. Check here: astronomy.tools Depending of your conditions this may be slight undersampling but certainly isn't an issue in your images. Ony advice I can give you is to try to get the bathinov mask to work. Judging minimum size of the stars is not as accurate from my own experience. A zoom lens is always a compromise. You can't expect a 200-500mm zoom to perform as well as a dedicated scope. However as some of your images show once you nail focus you're able to get quite good results, so that's the main issue you have to adress. Only advice I can give is to try to get the bathinov mask to work and to check for focus shift during the imaging session. If you use a lever for focusing (a good idea in general) make sure the lever doesn't move your focus during the session. A quick check of your lights whether there is any focus shift should provide an indication. Clear skies Wolfgang |
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Hi Ben, perhaps a slight focus shift during the session? To evaluate this you can compare the first and the last single frame of the session to check out if there is any visible difference in star size. Cs Harald |
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I have nearly the same setup. a D610 and the same lens, which I shoot at full aperture (f/5.6). I run a field test for this lens+camera combo and you can see the results here: https://www.astrobin.com/full/dqkaw7/0/ Since you're shooting at f/7.1 you should get a better field correction and thus sharper stars. Not that zoom lens are to be recommended for AP work but here we are. Now about focusing: ditch the bahtinov mask. In APT shoot at around ISO3200 for 5 sec, preview mode 1:1 with panning. Check the stars and see if they are tight. Move the focus knob very very slightly (think fraction of mm) one way (clockwise or anticlockwise) and see if the star are getting tighter. If not, reverse and repeat until the start are no longer getting smaller but starts bulging up again. That is your inflection point. Take a piece of red electrical tape and stick it between the lens barrel and the focusing knob, draw a line in the middle with an ink pen and then use a sharp knife to split it so that you can rotate the focus knob. Now you have a reference tag for your subsequent forays into AP. One thing that is vital is to block the zooming knob for moving at all. Again, electrical tape would do the job and can be removed with no permanent damage to the lens. This because I experienced a very slight creeping movement of the inner lens group when pointing upward, thus changing the focal length. For all the shooting use ISO200 to improve your dynamic range and limit tracking errors affecting your results. Expect to spend hours on a subject as M13 with your setup. Depending on local conditions, such as temperature drop during the night you might have to go back and make little adjustments to the focusing of the lens. Within 10F (or 5C) temperature drop you should be fine. |
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Hi Benjamin, Wolfgang, a bahtinov mask isn't gonna help the guy. |
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andrea tasselli:Hi Benjamin, Andrea, I just don't understand why. I used it to good effect down to 135mm (270mm effective) focal length. I find it way easier, faster and more accurate than the "reduce star size" method. May be a personal preference though. What do I overlook for his setup that makes the bathinov mask useless? Didn't think about the zoom ring. Blocking it - as well as the focus ring - from moving is certainly a good hint. Clear skies Wolfgang |
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Wolfgang, Because it is hard to tell whether they (the stars) are in focus or just so slightly out of it. On top of that you would need to look at the field correction as an overall to minimize the aberrations. A little change (very very little in truth) in focus might help you there. Zooms a finicky beasts and so are modern tele lens. |
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andrea tasselli: The electrical tape is a great idea but I think there is a problem with it. You mentioned that the precise position between the 2 pieces will show you the perfect focusing point in the future. Sadly that is not true in all conditions, though I'm not completly sure: temperature affects the glass, metal and plastic in the lens and therefore a precise position of the focus ring won't show you the perfect focusing position. Vintage lenses had infinity focus lock which sometimes wouldn't make you focus to exactly infinity (I repaired 3 lenses with this problem). Nowadays they leave a little space to focus 'beyond' infinity exactly for this reason: temperature and atmospheric conditions. Having said that I don't know if this has huge effects on the image, but being a telephoto lens I think it has importance. |
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The electrical tape is a great idea but I think there is a problem with it. You mentioned that the precise position between the 2 pieces will show you the perfect focusing point in the future. Yes, I get your point. But the main reason for a reference line is to have something you want to get back to easily when you tamper with the lens' focus (e.g. use in daytime). Moreover it gives you an idea on how far you move when doing manual focusing, when typically you need to move the focus knob by a fraction of a mm. Where you to improve it with scale-marked adhesive tape instead then it would give you precise positioning information. Surprisingly there isn't a lot of thermal contraction in these lenses. |
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| Thank you for all the discussion. Good to know my setup is capable of more then I am getting and I will keep working to nail the focus. I also re-edited the M13 image with fresh eyes and I think some of my star bloating was due to processing errors. Still not top tier but I am happier with it! |
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When you want to reduce FWHM of your stars it is also guiding/tracking accuracy that influences the result, not only focusing. So you should also check your guiding or if you are not guiding, then tracking accuracy. The whole point about guiding is getting better results, so one essential question is if you are guiding and how well. CS Gernot |
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Hi Benjamin, I had the same issues using Live View on the back of my Canon DSLR and a could not reliably benefit from a traditional Bahtinov mask , especially with wide angle lens (14mm) when shooting milky way or aurora. I purchased a laser-etched plastic device that affixes to the front of the camera held by a separately purchased square filter holder; you can read about the one I am using at https://focusonstars.com/ and it has been quite helpful in refining the alignment, even with stars that aren't terribly bright. I max out Live View to 10x and am able to fine tune the focus with a lot more precision. Good Luck! Rob |
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I'm using this one to good effect: Kase Night Focus Tool (Bahtinov mask) 100x100mm - Kase Filters UK Clear skies Wolfgang |
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| I can't see them working on very fast optics, especially with small aperture. Besides, it does not help with focus creep (unless you have a bright star in the FOV). |
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andrea tasselli: I'm using the KASE filter with a Samyang 135mm f2 and a Canon 200m f2.8 without any issues. Didn't try it out for shorter focal lengths - for wider fields some people even try to make the stars bigger Kase Wolverine Starglow 100mm - Alyn Wallace Special Edition - Kase Filters UK. To me the focusonstars filter looks pretty much the same as the KASE filter just with a bit of more marketing blurb. Of course your comment about focus creep is correct. Not that much of an issue with my short focal lengths but certainly a topic for Benjamin at 500mm. Clear skies Wolfgang |
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Hi Wolfgang, I stand by my comment although in a broader sense, that involving Bahtinov masks with fast optics. I've found I could not achieve perfect focus on my Nikkor f/2.8 (which is a in way very tricky) by just looking at the pattern on the camera's display. And I think there is also an intrinsic limit from the theoretical point of view (I seems to remember having read it somewhere) as far as such apodization masks are concerned. Otherwise, as they say, YMMV. |
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Hi Andrea, I have to admit that you need a camera which allows to enlarge the image on the lcd screen quite a bit. Obviously the stars and the corresponding bahtinov pattern are fairly small with wide field. All I know about Bahtinov masks is that the grid has to somehow fit to your focal length and focal ratio. So not all masks work with all scopes / lenses. Since even the well respected Gerd Neumann sells Bahtinov masks for lenses (Bahtinov Masks (gerdneumann.net)) I assume they work in general. However I respect your personal experience with your Nikkor, so there seem to be conditions when the stars don't align ... Clear skies Wolfgang |
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Hi all, What are your views about focusing aid available in some softwares like Backyard EOS? I point towards a bright star and try to reduce the FWHM as much as I can. I am shooting with Samyang 135 F/2 lens with Canon APS-C sensor (old EOS 40D). CS R. |
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| Not a fan. I used the one in APT and it was tricky to get a firm result with the 40D. In the end I just eyeballed it taking short exposures (5s). The way I see it, it does integrate seeing and thus offer a more robust indicator of where your best focus is. |
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| Good question. I use the back of my lcd magnified to 30x, on the Canon Ra. I think I find focus okay, but also easy to knock out of focus when adjusting the setup. I usually aim to find find the point between having a red or green fringe around the star, rather than star size per se. It does require very fine, almost most imperceptible movements, if the focus ring. I also use Backyard EoS when using my Canon rebel, which has a nice adjustment mechanism. Have also used a WO Bhatinov mask as well but it seemed just to confirm my position using the lcd, and wasn’t helpful when using an 12nm Ha filet roll. When using an Ha filter it’s harder, but still possible using the LCD screen. I do spend at least 10-20 mins trying to nail the focus. Not sure how that compares to other camera or scope users? I don’t use a zoom because I feel that dealing with the zoom movement as well as focus movement would be a challenge. Cheers Peter. |
