I was finally able to shoot some pictures of the bright comet C/2022 E3 (ZTF), that has bombarded my social feed. You might have seen this comet in popular media, where they refer to it as the “Green Comet“. This statement is true, as you can see the green color very clearly in the many photos of this comet, and also odd because almost all known comets have a green coma and it’s not a specific feature. See for example my shots of C/2019 Y4 (ATLAS) and C/2020 F3 (NEOWISE).

Dutch astronomer Gideon Vanbuitenen has made an amazing simulated timelapse of the comet’s appearance as it passes around the sun. The comet’s nearest approach to the Earth occurred on 1 Feb 2023, and it’s appearance changed dramatically during that time. It was around magnitude 6 when I captured it on the morning and evening of 8 Feb 2023. That’s invisible for the naked eye in light pollution, but easy to spot with a telescope and camera.

We had a full Moon during the two clear nights this week. All that moonlight is definitely not ideal for shooting broadband, but based on the terrible weather forecast it was probably going to be my only chance to capture this comet. The cold Winter weather made it extra hard to setup and adjust the telescope. It was still setup for planetary imaging and I stupidly forgot that my deepsky camera setup requires a different focus position! After a bunch of adjustments I could reach focus again. However, my frozen fingers were so numb that I could no longer adjust the fine screws on the tilt adjustment plate and I almost gave up. Luckily my setup is now fully automated and once inside the warm house, I could control the telescope remotely.

Shooting a moving object with a mono camera and multiple filters required some extra post-processing work. You need separate stacks for the stars, and the comet. An average comet stack showed star trails because the comet moves relative to the stars. These star trails need to be removed in order to create an image with normal stars. An easy way to do that, is to stack with pixel rejection. Typical sigma values for normal deepsky images are 2 or 3, which rejects respectively 2.5% and 0.1% of the brightest pixels from the dataset. Normal deepsky images have very little abnormal pixels, and only very strong outliers/hot pixels need to be remove in order to clean up the stack. But for this comet image I used a very hard bright threshold of 1.2 sigma in order to get rid of the star remnants. Choosing a lower sigma also throws away more of the faint comet signal (about 12% of all my data in this case), and the final choice is a balance between rejecting most stars while keeping the comet tail intact.

Revisions: Comet stack without pixel rejection (Rev. B), and with pixel rejection (Rev. C)
Exposure: 30 x 60 sec per RGB filter (90 minutes total)

The brown dust tail of the comet is obvious in my shot, while the ion tail of the comet is very faint. It goes almost straight up and is best seen in the comet stack without any pixel rejection. The stars were extracted from my normal stack with Starnet V2, and added to the comet-only image. Noise reduction was done with the excellent DeepSNR module.

This comet moves very fast! It crossed a substantial part of my telescope’s field of view (1.5 degrees in diameter) in the course of almost 2 hours. I calculated the comet speed from it’s RA/Dec position in the first and the last frame: it moved a whopping 0.179 degrees per hour.



Movement of C/2022 E3 (ZTF) over 30 frames (green filter)