Comet C/2022 E3 (ZTF)

Comet C/2022 E3 (ZTF), often called the “green comet,” became one of the most noticeable astronomical events of early 2023. It was discovered on March 2, 2022, using the wide-field camera of the Zwicky Transient Facility (ZTF) at Palomar Observatory, California. This is a long-period comet from the Oort Cloud: the last time it visited the inner Solar System was roughly 50,000 years ago. That’s precisely why its return generated so much excitement among amateur and professional astronomers worldwide. The comet owes its nickname to the characteristic green glow of its coma — caused by fluorescence of diatomic carbon C₂ and dicyanogen C₂N₂ molecules.

Perihelion — the closest point to the Sun — was reached on January 12, 2023, at a distance of 1.11 au (about 166 million km). When it became clear that the comet had survived its solar approach well and was within observing range, I naturally decided to try to capture it. Forecasts suggested that by late January it should reach an apparent magnitude of around +5–6^m. That’s already at the limit of naked-eye visibility under dark rural skies, although in city conditions it remained barely detectable. Unlike the very bright comet C/2020 F3 (NEOWISE) in the summer of 2020, C/2022 E3 (ZTF) appeared more like a fuzzy greenish patch rather than a brilliant “star with a tail.” Comets generally differ from point-like stars: their light is spread over a large area, making them much harder to spot without optics or at least binoculars.

Despite its decent brightness, detailed observation and photography of this comet still required a telescope. For this purpose I used my SkyWatcher BKP 150/750 Newtonian reflector on a motorized EQ3-2 mount paired with a Canon EOS 550D DSLR. I also decided to photograph the comet with regular lenses — the Canon EF-S 18–55 mm and a 50 mm f/1.8 “nifty fifty” — to build a more complete picture of how it looks at different scales.

Observations and imaging took place on January 25–26, 2023, in Dnipro. The weather unexpectedly gave me two clear evenings in a row — a rarity for January. The Moon was in the waxing crescent phase (~20%) and by imaging time had already set below the horizon, causing no interference at all. The comet’s position was also very favorable: it was in the north, high enough above the horizon and visible all night. Its proximity to the north celestial pole had a positive effect on the EQ3-2 mount’s tracking performance — in near-polar regions of the sky, periodic errors are noticeably smaller, allowing longer exposures and a lower percentage of rejected frames.

The first photo was taken right in the yard, with the 18–55 mm lens on a tripod. It was interesting to capture the comet against the starry sky above the forest. It appeared as a small greenish smudge among countless stars. The comet was located in the constellation Ursa Minor, to the right of the Little Dipper. But it’s not easy to spot without help — so click the icon to see the markers.

Here’s the frame taken with the 50 mm lens. Now the Little Dipper is clearly visible close-up in the center of the frame, with the comet positioned to its right. On this shot a small tail is already discernible, pointing away from the dipper. The difference from the wide-angle frame isn’t huge, but the tail is noticeably more evident.

And finally — the main telescope shot. Here the comet reveals itself in full glory: a bright green coma and several distinct tails. As it approaches the Sun, ice in the nucleus sublimates into gas, releasing gases and dust that form the coma (the envelope around the nucleus) and the tails. The most prominent is the broad dust tail; farther behind stretches a thin, long ion tail, while in the lower part one can barely make out yet another feature — the so-called anti-tail, an interesting optical effect that was especially well visible on the days when Earth crossed the plane of the comet’s orbit.

However, imaging the comet through a telescope turned out to be quite challenging — and the hardest part wasn’t even the shooting itself (which took place in freezing temperatures for quite a while), but processing the resulting material. To get a decent result I had to capture a series of 60 frames at 30 seconds each — almost half an hour of continuous work. During that time the comet noticeably drifted among the stars, moving quite quickly — roughly 3–7 arcminutes per hour. If the frames are simply stacked on stars as usual, the comet turns into a smeared streak. So processing was done in two stages: first stacking on stars, then stacking on the comet (to preserve sharpness of its details), and finally the two intermediate results were carefully aligned and combined.

Despite all the difficulties, the final image turned out well — especially considering this was my first serious attempt at photographing a comet through a telescope. It’s particularly gratifying to realize that I managed to capture this rare visitor. In general, it’s worth keeping a closer eye on comet forecasts: such comets appear unexpectedly and, surprisingly, not as rarely as one might think.

Comet C/2022 E3 (ZTF)

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