Neowise

 

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.

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.

Since we already live quite a bit away from Portland, I had to drive only about another fifteen minutes to a Class 4 Bortle dark sky site. Not perfect, but it worked, especially since I was going to look northwest, away from the light pollution. I found a spot on top of a hill where they were conveniently clearing the trees for my photographic adventure. Hence the bulldozer in the picture above. The spot is near the tiny town of Timber, OR, and I am guessing the glow along the horizon is leftover from sunset and maybe some of the lights of Timber.

First Night

On July 19th I went out with big plans: I would take pictures of the comet but also setup an equatorial mount and take pictures of deep space objects in the Milky Way. I thought I could do this in parallel, but was too busy and had to serialize. Comet pictures until it dropped below the horizon, then another two hours of deep-sky shots. I'm not too happy about the latter, but blog about them later. In total, I spent five hours in that clearing, took almost 1,500 frames, was visited by a couple of deer, tons of bugs, and saw four pick-up trucks drive by during that time.

None of the comet pictures of the first night were tracked. Just a tri-pod, a fairly wide angle lens, and short exposures. I was there before sunset to set up and you can see the progression from civil to nautical to astronomical twilight to night in these pictures. The one with the dozer above was first. The one on top of the page was last. As the night went on, the comet and that part of the sky dropped closer and closer to the horizon and disappeared after midnight. Late in July, Neowise was circumpolar, but my observation spot was below another hill.

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.

My camera is fairly light sensitive, but in order to take these kind of pictures, you need to take lots of them. Tens and hundreds. Later they all get stacked together to filter out the noise and enhance the light. There are several free and commercial programs available that make this stacking possible. You can assist these programs by taking additional type of pictures. The ones with the subject in them are called light frames. Dark frames use the same ISO and exposure settings, but you cover the front of your lens so that these pictures come out black. This has to be done at the same ambient temperature as the light frames. The camera sensor behaves differently at different temperatures. Settings like ISO and exposure time also have an influence. The collection of these dark frames contain signal variation from the sensor that is also present in the light frames. With both of these sets available, the stacking programs can remove some of the sensor noise.

Flat frames are taken with a white light source, same ISO as the light frames. I used an empty pdf page on a tablet covered with a sheet of paper as my light source. I held it in front of the lens and took pictures in aperture priority mode. The stacking software uses these frames to subtract dust on the lens or sensor, as well as hot and dead pixels on the sensor.

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.

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

A week later, I went out again. Same spot, but this time with a longer lens and an equatorial mount: I wanted a close-up! It was quite windy that night, the ground was still soft and squishy, and my setup is heavy and wobbly. The lens and camera alone are 8 lbs. The counter weights and arm add another 9.5 lbs. A heavier duty tripod could help, especially for locations that are reachable by car. Add a larger EQ mount and now it is way too heavy for me to carry up Saddle Mountain.

My equatorial mount setup

Despite the difficulties, I got a decent picture. I took longer exposures, 30 seconds, and had to eliminate about half of my light frames that did not turn out. On my second night out, the comet had become too faint to see with my (bad) naked eyes. Once I found it and knew where exactly to look, I could kind of see/guess it with averted vision. My camera, by collecting photons for 30 seconds at a time, was able to get the shot below.

ISO 1600, focal length 200 mm, 30 s exposures, F5.6. 14 usable of 27 light frames, 27 dark, and 36 flat.

Because the comet tail is so long, I thought a portrait shot would be good. When I reoriented the camera, I must have touched the focus ring in the dark. Unfortunately, I did not check and the picture below has little disks for stars, instead of the sharp , tiny dots I wanted.

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.

ISO 6400, focal length 200 mm, 30 s exposures, F5.6, single frame.

That was a  lot of work. I learned a few things and did get a few decent pictures. Now I have to decide which one to print and hang onto my wall. I'm thinking maybe the first or third one at the top of this post. Or both! Please, head over to Flickr and have a look at the full resolution versions.