Making steady progress in astrophotography

Over the past year I have written a few posts on my progress in nightscape and astrophotography. As I’ve said before, I think it is meaningful to record the learning process from the beginner’s standpoint. I’ve read a lot of expert blogs that deliver fantastic results but are difficult for beginners to understand. In that spirit, here is another post to describe my recent gradual progress in taking good astrophotographs.

Back in March, I decided it was time to grow up and get an updated camera and a star-tracking mount (the resulting setup is described in this post). At that time, I took a picture of the Milky Way and North America nebula, and I was pretty excited about it. Now, a few months later, I am getting vastly better results using the same setup. Compare these two images of the same Milky Way region:

Both of these images were captured using a Canon 60Da with a 28mm f/1.8 lens, using the iOptron SkyTracker in similar light-pollution conditions. Now I’ll get straight to the point and explain what went wrong with the earlier image, and how it was fixed.

Problems with the earlier attempt:

  1. Coma. The stars are elongated around the corners of the earlier image, but in the more recent image they are less bad (you can still see a small “birds-beak” effect in the new image at the corners). The improvement was achieved by stepping down the aperture to f/4, which seems perfect for my lens. There is a longer discussion about coma at the Into the Night Photography blog, and some good lens recommendations are made in Part II of that discussion. Some coma test results and f/stop recommendations for common lenses are listed at Catching the Light.
  2. Focus and Stability. I used LiveView focusing in both images. I found that focus was improved by using MagicLantern to increase the exposure gain on the LCD display. This allows you to find stars and achieve focus more easily, and also reveals landscape features so that you can more easily compose a picture (although in this case there’s no landscape shown). Additionally, I tried to improve tripod stability in the second image by widening the base and mounting the camera lower to the ground, so that has less wind exposure. At Into the Night they also suggest adding ballast to improve tripod stability, but I have not tried that here.
  3. Processing. In the earlier image, I manually stacked the frames in the GIMP (similar to Photoshop). In the second image, I used Luminance HDR with Hugin’s “align_image_stack” tool to correct small shifts between the frames. The HDR tool also provides for more sophisticated tone mapping, so that you can bring out better colors and contrast in nebular features. I have found that the “Fattal” tone mapping algorithm produces excellent results with all parameters set close to 1.0. For both image, I completed final processing by using the Unsharp Mask filter in GIMP, and then by editing the tonality curves to repress the background haze (lowest light levels) and to de-saturate bright stars (highest levels).

Those are my key observations from my second summer of night photography. My approach is evolving somewhere in between Catching the Light (a site focused on astronomical features) and Into the Night (focused on starry landscapes). I’m also finding that my post-processing workflow is atypical for two reasons: I do not use Windows, and I prefer to use open-source software. Fortunately MagicLantern, LuminanceHDR, Hugin and the GIMP are all open source and provide a complete workflow and produce satisfying images — at least until I realize there’s something better.


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