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Comet Neowise in northwest Oregon on July 19, 2020. ISO 25600, 70 mm focal length, 3 s exposures, F2.8. 127 light frames, 180 dark, 89 flat. |
Many, many photographers went out last month to capture comet Neowise and I too tried my luck. If you are looking for a hobby where you can spend long hours standing around in the dark and then even more hours back at home in your study processing the pictures you took, then astrophotography might be for you! (You can see the full-size images of this post on Flickr.)
The first thing to do is find a dark site. Using the Light Pollution Map shows that Oregon has quite a few dark places to observe the night sky, once you get away from the larger towns and cities. But there are two other problems. The places where light pollution is low are covered with trees. They make Oregon beautiful and the air pleasant to breathe, but they are in the way when you are trying to see things far away. Especially things, like comet Neowise, near the horizon. The second problem is haze from the humidity, although in the summer months it is not too bad. Thanks to Covid, air pollution is down, which also helps.
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Earlier the same evening. ISO 800, 60 mm focal length, 2 s exposures, F2.8. 60 light frames, 60 dark frames, and 30 flat frames. |
First Night
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Second shot of the first evening. ISO 2000, focal length 46 mm, 3 s exposures, F2.8. 55 light frames, 90 dark, and 60 flat frames. |
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Third shot of the night. ISO 6400, focal length 24 mm, 5 s exposures, F2,8. 89 light frames, 47 dark, and 30 flat. |
I am quite new to astrophotography and in the past I have used Deep Sky Stacker (DSS) to stack my images. It is a free Windows program, but it works under Wine in Linux. It is a sophisticated but complex program and more often than not, I could not get the results I wanted. Looking for an alternative, I discovered Sequator. It is also free and works in Wine very well. It has a lot less knobs to experiment with than DSS, but it is a lot easier to use. And, it has a horizon freezing feature that I really like.
When you take a lot of pictures with an exposure of several seconds, the sky moves but the horizon stays put in relation to your tripod. (Yeah, I know it is actually the Earth that moves.) When a stacking program aligns the stars, it rotates and shifts each frame until everything matches. That means the horizon becomes blurry. Sequator lets you paint the sky in one of the frames to help it distinguish between sky and horizon. The brush's smallest diameter is not very small, but the marking doesn't have to be very precise for Sequator to do a good job. The process took less than a minute or so for my Neowise pictures. Sequator had the most trouble with the top-of-post picture. I spent hours in post-processing to fix that horizon.
Since I'm fairly new to this, I also wanted to experiment. Manual focusing at night is difficult, and I already knew that I wanted to keep my exposures to five seconds or less. I wanted sharp, pinpoint stars, not star trails. But I also knew that five seconds is too short get some of the fainter objects in the sky, or the wispy ionization trail of Neowise. The bluish, straight tail of Neowise are ions that get knocked loose, while the bent, white/yellow tail is water vapor that has been heated up by the sun. I read why one is bent and the other is straight, but I still don't understand it.
The high ISO picture on top is interesting. A single shot from that set is really grainy as you can see in the zoomed in portion below. Stacking 127 of them and combining them with 180 dark frames and another 89 flat frames, produced a fairly acceptable result. I like how the blue ion trail came out and am quite happy that the trees are not completely black. I shone a flashlight at them, but they were quite far away. I did the same with the dozer in the third set, but it was almost too bright. The light on the dozer in the first set of the night (ISO 800) is from one of the few trucks that drove by.
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Zoom of an unprocessed single frame that I used to create the image at the top of this post. The high ISO creates a lot of noise. Stacking 127 of these kinds of frames removes most of it. |
None of these pictures have stars quite like I wanted them. Especially the high ISO picture on the top of the post has highly distorted stars. That is better visible in the full-size images on Flickr. By the time I was taking the last set of frames, it had become quite windy. I was also on soft ground: recently deforested ground. So, maybe it was not just the focus.
After stacking these frames, more work was needed. I use the gimp for layering the stacked images with the light painted ones and to fix blemishes such as stacking artifacts on the horizon. For general picture manipulation, I use rawtherapee.
I mentioned earlier that the sky moves in relation to the ground we stand on. Just to show how much, I created the GIF below. It is made from the same 127 unprocessed light frames I used for the top picture. Note not only can you see how much the sky moves in 127 * 3 s = 6.35 minutes, but also how many satellites fly by (and have to be filtered out by the stacking software.) Watch closely and you can see them streaking by in all directions.
Second Night
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My equatorial mount setup |
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ISO 1600, focal length 200 mm, 30 s exposures, F5.6, 13/25 light frames, 27 dark, and 36 flat. |
I fixed the focusing after that set, but now the comet was so low to the horizon that tree branches kept getting into the way. I wish I could have had more time and do a whole set and stack it. When Neowise comes around again in more than 6,000 years, I will probably not be around anymore.
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ISO 6400, focal length 200 mm, 30 s exposures, F5.6, single frame. |