Orwell Astronomical Society (Ipswich)
Comet C/2022 E3 (ZTF), 11 January - 14 February 2023
Comet C/2022 E3 (ZTF) was discovered by astronomers using the wide-field survey camera at the Zwicky Transient Facility (ZTF), at Mount Palomar, in March 2022. The long-period comet came to perihelion, its closest to the Sun, on 12 January 2023, and its closest to the Earth on 01 February. Having brightened substantially since discovery, in early 2023 the comet swept across the pre-dawn sky and became visible in the evenings. By mid-February, it was conveniently placed for evening observing but was fading fast.
On the morning of 11 January I imaged the comet in two telescopes, under moonlit suburban skies. The view through a Celestron 200 mm Celestron EdgeHD showed the comet with an ion tail that extended out of the field-of-view. A 90 mm Megrez refractor, with a wider FOV, showed the ion tail was well over a degree in length.
In the coming weeks, as the comet gets higher and closer to Earth in a Moon-free sky, we should get a good view!
On the morning of 15 January, I had another session observing C/2022 E3 (ZTF). The Moon was at third quarter, but close to the comet in the sky.
In the Celestron EdgeHD, the comet continued to show a pencil-like ion tail stretching beyond the field of view. The wider field of view of the colour camera showed more of the ion tail - see first image below. Getting the colour balance right proved challenging - see second image below. Splitting apart the three colour channels (red, green and blue) shows an artefact in the red channel that is not present in the blue or green: possibly some form of circular reflection or shadow. In any case, the tail is far less prominent in the red channel.
While comets are dynamic objects that move across the sky from day to day and show an ever changing display of their tail(s), I have never thought of them as "alive". However, when a comet is close to the Sun, the nucleus is heated, releasing both and gas. The dust drifts away from the nucleus at just a few meters per second and then is propelled away by the force of sunlight. Meanwhile, the gas can be ionised and quickly interact with the magnetic field of the solar wind, which moves at several hundred kilometres per second.
If the production of dust and gas is constant and the solar magnetic field is quiet, there is usually no change in the appearance of a comet's tail(s). However, C/2022 E3 has a prominent gas tail and in recent sessions at the telescope I have noticed changes between images. Nick James (UK) and Michael Jaeger (Austria) and others recorded a tail disconnection event.
The still frame below, taken on 20 January, records both the thin ion tail to the right and the more diffuse dust tail above. The image is formed by averaging many sub-exposures, a process which can lose fine detail.
The following video shows in the LH pane the direct view, and in the RH pane the difference between successive frames. Compared to the still image, the frames in the video are necessarily shorter and noisier. The direct view shows structure in the ion tail, and the difference frame shows changes in the tail.
Below, videos taken on the next two morning also show changes in the tail.
As the comet approachs its closest to Earth on 01 February, unfortunately the Moon is waxing and moonlight is becoming a more serious problem.
On 27 January, Comet C/2022 E3 was at an altitude of approximately 37° to the North, which allowed me to observe it from the bottom of the garden looking over the top of my house. I found the comet quickly using the go-to function of the HEQ5 mount. Using a x55 magnification eyepiece, the comet was clearly visible as a small fuzzy blob, without a tail.
I attached a camera and using an android tablet together with the DSLR controller app was able to focus the image and take several photos at different shutter / ISO settings. There is an interesting circle of stars below and to the left of the comet.
The images below were taken with a Skywatcher 200PDS 200 mm, f/5 Newtonian on HEQ5 mount with Canon EOS 850d camera. The wide-field image was taken at 20:01:04 UT, ISO 400, 93 s exposure. I was surprised how rapidly the comet appeared to move, and the two cropped images illustrate this: they were taken at ISO 1000, 10 s exposure, 90 s apart.
The below image was taken between clouds on 30 January 2023. Light, high cloud appeared in nearly all images, and I had to deal wiht this in processing. Equipment: William Optics RedCat51 Apochromatic astrograph, ZWO ASI183mc pro camera, ZWO ASI120mm camera, ASIAIR+ wi-fi, ZWO mini-guide, Sky-Watcher Az-gti mount, ZWO Optolong L-Pro nebula filter. Images: 60 x 60 s exposures, 12 x darks, 30 x dark flats, 30 x flats. Camera parameters: gain 111, temp -10°C. Images processed with DSS using comet mode, then Photoshop, then starxterminator.
At last, no more getting up at stupid times of the night to see the comet! On 30 January it was circumpolar and at its most northerly declination of approximately 79°, with upper culmination just after midnight. There were two downsides: a nine-day old Moon at a distance of only 60° and the comet was moving at nearly 16 arcseconds a minute.
With a long focal length telescope, exposure times capable of revealing detail in the comet are very short. I elected therefore to use a relatively fast instrument with a focal length of 450 mm. Even with a pixel resolution of about 3 arcseconds, exposures were still limited to some 10 s. To counteract the short exposures, I took 1400 of them!
Stacking was necessary to show the motion of the comet relative to the stars. Each exposure showed little detail and trial and error was necessary to balance the conflicting requirements of stacking sufficiently many to reveal some detail versus not so many as to blur out short term changes. I created the below video as follows:
Comparing the two panes, it is clear that short term variations are visible in the RH pane that are not in the LH pane. In the LH pane there is a curious crater-like impression in the background; this appears to be a reflection of the Moon within the telescope, as it fades as the Moon is hidden by my house.
On 01 Feb, the comet will be at its closest to Earth, at a distance of 0.28 AU. After 07 February, a period will open up where the Moon will be less intrusive and eventually absent from evening observations as the comet moves through Auriga and Taurus. On 11 February, the comet appears to pass close to Mars.
On 06 February 2023 there was a brief opportunity to image The Kids, a naked eye asterism in Auriga formed by the stars Almaaz, Heodus I and Heodus II, with C/2022 E3 in its midst. By the time it became dark, the Moon had already risen and clouds were forecast to roll in soon. Moonlight made the sky too bright to make it worthwhile trying to record the ion tail of the comet, so instead I decided to use a DSLR with telephoto lens to capture a wider view.
The comet appeared as a fuzzy blob among the stars, but with an odd colouration nearby. A superb animation by Nick James led me to realise that there were two comets in the field of view! The other comet is C/2022 U2 (ATLAS), some six magnitudes fainter than C/2022 E3.
So what wcould I salvage from my imaging? Usually I stack a series of images so as to keep the stars fixed, then so as to keep the comet fixed. On this occasion, I had a choice of comets to keep fixed during stacking. I also made a movie (not a patch on Nick's), but at least it gives an impression of the two comets moving.
Frames for the following video were captured using a Skywatcher 200p and Canon EOS 850. On 09 February, I took a series of 10 s exposures of the comet over a period of one hour. I stacked the frames into 60 images, representing the comet at each minute through the hour, and formed a video using Microsoft Photos.
One week after my last observation and the comet is now moving at only about 6 arcseconds/minute, allowing longer exposures. But it is also fading, as it leaves the neighbourhood of the Sun, and activity near the coma is much less obvious. I captured the following video shortly before the sky vanished in haze.
In the below wide field image of Taurus, taken under a suburban sky, there are many transient interlopers, including Mars, C/2022 E3 and several asteroids.
Nigel Evans, Martin Cook, Alan Buttivant