Green Comet (C/2022 E3 ZTF)

Discovered by the Zwicky Transient Facility in 2022, C/2022 E3 was found to have an orbital period of 50,033 years. This means this green comet only comes around Earth about every 50,000 years. It reached closest to Earth on February 1, 2023 with a distance of 0.28 AU (a little over 1/4 the distance from Earth to the Sun). At this point, it was measured to have an apparent magnitude of about 5. For comparison, the Moon has an apparent magnitude of -12.74, whereas Uranus has a brightness of 5.68, much closer to C/2022 E3. This once in a lifetime (over 500 lifetimes, actually) event happened to take place during a time where we have the technology to image it.

Observing and Processing

I was lucky enough to have a classmate and group member who was able to put in an observation through Skynet's system of robotic telescopes for this spectacular green comet on February 1. Andreas Buzan used Mars Desert Research Station's MDRS-14 telescope to take 10 20 second exposures with the B filter, 10 15 second exposures with the V filter, and 10 10 second exposures with the R filter. Since telescopes take images in black and white, these filters allow us to make a full colored image.

Once returned, I was able to use Andreas' observation to process and edit the final image using Afterglow. I first used the Source Catalog tool to identify the comet in each of the thirty pictures, and merge the sources. I then used the Aligner tool and selected the merged sources to manually align each image to allow for an aligned stack. Next, I used the Stacker tool to create three separate stacks, one for each filter: R, V, and B. I also set the rejection to "chauvenet" with a low and high value of 1 in order to hide the scattered stars in the image (since the comet is a fast moving object!) Without this step, the final image would look something like this:

The final stacks (with rejection) are shown below. Left to right, the images are: V stack, R stack, B stack.

The last step to create the image was to group all three images together. This will make a colored image! Result below:

Let's Clear Things Up

While this image isn't terrible, it is fuzzy and some of the star scatter is still easily visible, and could be much better. Afterglow has a set of built in display settings that can drastically improve the look of the image. Using the midtone stretch mode and adjusting background, midtone, and saturation levels, I get something more like these:

(Note: In these images, the Midtone Stretch Mode is used in percentile mode.)

Much better! Now it's time for the final touches. I exported and downloaded the processed and edited images and used a generic editing program to make slight modifications to bring out more colors and features.

Although these are all very similar, each has a slightly different amount of green glow around the comet. Balancing background darkness and color of the object is always difficult, and it isn't always easy to not sacrifice too much of one for the other.