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So, I've built a barn door tracker:

enter image description here

and I was astonished that pictures just got better:

enter image description here

That's Alcor and Mizar, and in the top right part, the fuzzy thing is the Pinwheel Galaxy (M101). It's a single exposure (30 s, +1.5 EV --> ISO 6400), shot with my Sony Alpha 6000 and a Varexon 135 mm f=2.8 lens.

Though I'm now able to reduce motion blur, the pictures still look... interesting. I made the pictures on a parking lot though, with some street lamps some 150 ft away.

And here are three exposures stacked (same 30 s, same ISO 6400), with a very interesting motion blur.

enter image description here

Are there any suggestions for my next steps? Is there a way to further improve the sharpness of the images? (I guess I have to weigh down the tripod a bit more, even when I'm careful not to shake the contraption.)

My other lenses are:

  • Speedmaster 35 mm f=0.95 (truly great but gets blurry at the edges at wide apertures)
  • Minolta 50 mm f=1.7
  • Osawa MC 28 mm f=2.8
  • a 500 mirror lens, f=8 (talk about dark images...)
  • 1
    Did you build your barn door to compensate for tangent error (i.e., with a curved drive rod, or micro controller control of the drive motor)? What is your polar alignment technique/process? – scottbb Apr 19 '18 at 18:42
  • No, it's all manual and I use a straight drive rod - it's the first barn door tracker I've ever built. The camera doesn't allow for more than 30 seconds of exposure, anyway. I just make sure to rewind the drive rod back to avoid that (small) tangent error. – user258532 Apr 19 '18 at 23:27
  • Alignment technique? Uh, I just align the hinge towards Polaris. Manually. – user258532 Apr 19 '18 at 23:51
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Also to reduce blur further dont take images in the wind and use a remote shutter release preferably a wireless one and one other point is if you dont have mirror lock then get a black card place over the lens open the shutter and count to 5 slowly then take the card away. It compensates for the camera movement.

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We can see that the shift between the exposures is only visible at the very left side and in the top right corner, while the rest seems to be ok. This might indicate that something with the alignment of the images before the stacking went wrong and that it probably is not motion blur:

With this setup of a barn door tracker you can probably improve the tracking within a single exposure but not across multiple exposures (such that the same star hits the very same pixel of your sensor in every exposure), so I assume there was a slight shift between the exposures. This means that the distortion due to the wide angle lens was slightly shifted across the exposures.

So my guess is that you did not do any (or not a good enough) correction of the lens distortion before stacking. This is especially important for wide angle lenses as their distortion (usually) varies more across the image.

This is just my guess from my (limited) experience, so maybe it is something else. But in any case, keep trying:)

  • Mh, does the 135 mm lens create such a distortion? Anyway, I've got to make a lot more pictures to get the hang out of astrophotography. That I managed to capture M101 from a parking lot is already a success in my book. – user258532 Apr 19 '18 at 23:29
  • One way to see how much distortion there might be is printing a sheet with a rectangular grid of lines or dots. (Or use an appropriate cutting mat or whatever you can get your hands on.) If you then place your camera on a tripod and take multiple shots with the very same settings but panning slightly from the left to the right (or up and down). Then take them into your favourite image editing software and try to align them with respect to the center of the sheet. By observing how much the lines from the different shots diverge from the center, you can estimate how much distortion you get. – flawr Apr 20 '18 at 17:00
  • @user258532 Wide aperture 135mm lenses can have a surprising amount of geometric distortion. Sometimes it can be pincushion, rather than barrel distortion like WA lenses, but distortion is distortion. It's usually the result of uncorrected/undercorrected field curvature that is left that way because it helps provide the creamy bokeh such (wide aperture, medium telephoto) lenses are known for when used at wide apertures for taking portraits. Lenses heavily corrected for field curvature also tend to demonstrate 'busy' or even 'harsh' bokeh which makes them less attractive to use for portraits. – Michael C Sep 11 '18 at 19:04

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