We are in the midst of a once-in-a-lifetime celestial event. Comet 2022 E3 (ZTF) is approaching Earth for the first time in 50,000 years. Astronomers discovered it in March 2022 and it will be closest to Earth (26 million miles) on February 1, 2023. We are witnessing an event that was last seen when Neanderthals shared the planet with homo sapiens. It blows my mind to think about it. Trying to view and photograph the comet has been a subject of discussion on many of the astrophotography forums I participate in and I wanted to give it a try myself. Comets are really difficult to photograph, more difficult than anything I’ve photographed so far. Why is that? The first reason is that they are small and kind of dim and typically quite far away. But that describes a lot of deep space objects. The main reason comets are so challenging to photograph is that they move at different speeds and in different directions compared to the rest of the night sky. What does that mean? First, comets are small and kind of dim, especially if you are trying to capture the tail. Here’s a single 2 minute exposure of Comet 2022 E3 (ZTF):
To try to get detail out of these images, we have to “stretch” the image. Doing so will make details (which we call “signal”) more visible but it will also make noise more visible. So we have to have a way of increasing the signal without increasing the noise at the same rate. Taking multiple exposures and stacking them one on top of the other allows us to increase the signal to noise ratio during processing. As we take these multiple exposures, however, the objects in the night sky appear to move because the Earth is rotating. They typically all appear to move in the same direction at the same speed. The comet, however, is moving in its own direction at its own speed (pretty fast!) in a way that is not related to the rotation of the Earth. When we put multiple exposures together, we might ensure that the stars are always in the same place. When we do that, we get a blurred image of the comet because the comet has moved between exposures. Here’s what that looks like:
Instead, we might make sure that the comet is not smudged by stacking exposures in such a way that the comet doesn’t move from image to image. When we do that, we get star trails. Here’s what that looks like. I kind of like the looks of this but the comet is not particularly visible:
The solution is to stack the exposures twice, once focused on the stars and once focused on the comet. Then we can make a composite of those two images. This by itself is harder than it sounds because you have to remove things in each stacked image before you combine them into a composite. In one, you have to remove the blurred comet but keep the stars. In the other, you have to remove the star trails but keep the conet. This is challenging. In my final image below, for example, you can kind of see the remnants of the star trails because I couldn’t remove them completely from my stack focused on the comet.
But I’m pretty happy with this result. And I learned a ton creating it. For example, I had never made a composite image before. I had never used pixel math. I had never used a tool called PixInsight before. Because it was my first time doing each of these things, I was definitely not perfect in my practice but I was happy to be learning new things and experiencing the wonder of this rare celestial event. An interesting phenomenon has been happening in astrophotography forums that makes me sad and worried about what we focus on in education when we teach people how to do things. As I said, comets are difficult to photograph. Increasingly, I’ve been seeing comments like this: “I attempted that stinking comet C/2022 E3 (ZTF) again last night. I was not able to get the tail. I am officially off this target, I have wasted so many hours on it and have very little to show for it.” This is a pretty common comment on these forums as people have goals for their astrophotography and this comet makes it very difficult to achieve your goals. I feel like our society focuses so much on goals, products, results, that we forget that learning is about practice and process and learning from failure. We forget that failure is not wasted effort. This is something we talk a lot about when we talk about the cluster learning model at Plymouth State University but how many of us design experiences in our classes where it really is ok for students not to be perfect, where it is ok, or even expected, for students to fail to achieve their goals and to learn incredibly important lessons from those failures? Once again, I am learning so much in this hobby that I inspires me to think about my teaching in new ways. I love all of it!
The featured image was created by me on January 30, 2023. I took 25 2-minute exposures and processed them in PixInsight.