Winter Storms Versus Comet

2022 departed with bad weather, and 2023 roared in with even worse weather, bringing waves of storms marching across North America. Sometimes, bad weather brings opportunities to photograph interesting atmospheric sights, but on balance, as an astrophotographer, I’d rather see clear skies!

C/2022 E3 (ZTF)

One of the astronomical treats that early 2023 is bringing us is designated comet C/2022 E3 (ZTF). Barring any surprises, I would rank this one to be in the “pretty good” category, contrary to the hyperbolic headlines declaring this comet as a “Rare Green Comet not seen in 50,000 Years.” 

It’s true that this comet has a green halo, but that is not unusual for comets which travel near the sun where frozen gasses are vaporized and ionized. Compounds of carbon, nitrogen, and oxygen are responsible for this.

It’s also not unusual for comets to have such elongated orbits that they approach the sun only after thousands of years. Nor is it a rare comet in terms of its predicted maximum brightness (magnitude 4.7 by some optimistic predictions). I’d rate it as uncommon, as many comets do not brighten to the point of being visible to the naked eye, but not rare. This comet won’t be visible in urban skies, and even in the countryside, you would probably have to be intentionally looking for it.

Comet Photo Op

C/2022 E3 (ZTF), however, should be a good photo op challenge for Northern Hemisphere observers as it will be well-placed, high in the northeastern skies after passing the Sun, and growing closer to Earth. Besides the green coma, recent photos show a distinct blue ionization tail as well as a white or yellow dust tail. 

The dust tail consists of uncharged dust — particles freed as the comet ices melt. As the comet travels in its orbit, some of the comet’s shedding ball of dust (think of the character Pigpen in the Peanuts cartoons) is left behind the comet. This dust tends to delineate the orbital path of the comet until the solar wind blows the smaller particles away from the sun, giving it a rooster tail appearance. 

The blue ionization tail consists of the comet’s frozen gasses not only melting but also being ionized. Once ionized, the charged particles are strongly influenced by the charged particles in the solar wind, and so, these tend to blow directly away from the comet head, away from the sun.

A fortunate circumstance for this comet is that it will be visible for many weeks from the Northern Hemisphere, even past its closest approach to us, which will happen on February 2. At its closest, it will be less than a third of the Earth-Sun distance. The comet will probably be most photogenic as it approaches us, but you should be aware that it will be moving about half a lunar diameter per hour against the stars, so short exposures will be required if you are tracking the stars. If you have a computerized mount capable of tracking the comet itself, longer exposures can be done, but with trailed stars, of course. 

If you opt for shorter exposures, modern astronomical stacking software such as Deep Sky Stacker has the capability of stacking on the stars and creating a second stack on the comet head, resulting in a composited shot with both a sharp comet and sharp stars. Personally, I prefer shots with trailed stars showing the motion of the comet if only stars are visible, but if other objects such as galaxies and nebulae are visible, a composite works for me.

Even after the closest approach, it’s worth watching and photographing as it recedes from us. If it’s a “normal” comet, as it recedes, it will gradually dim, losing its tails, and disappearing into obscurity. However, comets have been known to have outbursts (perhaps a gas pocket belching dust), causing a sudden brightening. This was the case for Comet 17P/Holmes back in 2007, when this nondescript, periodically reappearing comet (orbital period of ~6.9 years) went from a fading 17th magnitude to 2.8, easily visible to the naked eye. This brightening occurred within just 42 hours (see Wikipedia article)! Luckily, this happened while it was receding from us and high in the northern sky, providing some great photo ops for many weeks as it passed by deep sky objects.

In the set of photos above, taken over the course of a month, multiple 20-minute shots could be stacked because the comet was receding from us and not moving much relative to the stars. These shots were done at 350mm focal length on medium format (6x9) Ektachrome 200 film, pushed to ISO 640.

Tracking the Comet

To keep track of Comet C/2022 E3 (ZTF), a good tool is Stellarium. The current position of the comet is already marked in the free and paid version of the Android app. But to get the comet to show on the (free) desktop tool takes a little work! Here’s the procedure:

• From the left-edge pop-up menu, select Configuration
• In the Configuration menu, select the Plugins tab.
• On the left side, scroll down to Solar System Editor, then click on Configure.
• Select the Solar System tab and click on Import Orbital Elements in MPC format…
• From the next popup window select the Lists tab and click on:
  • Comets
  • Download a list of objects from the internet
  • Select the source: MPC’s list of observable comets
  • Click Get Orbital Elements
• The list of comets should be downloaded, and the Import Data window should open. Click Add new and update existing objects, then press Add Objects.

Now you’re done! All of the configuration windows can be closed, and now, if you type the comet designation into the search tool, the comet should appear on the sky map (if it’s above the horizon). From my location in southern California, here’s the view at midnight on successive nights from mid-January to early February.

Incidentally, MPC refers to the Minor Planet Center, run by Harvard University, which maintains a database of orbital parameters for minor planets (asteroids) and comets. The designation ZTF in the comet name refers to the automated search project (Zwicky Transient Facility) which discovered the comet in 2022.

Equipment?

With the unpredictability of sky conditions and development of the comet, it’s difficult to say what equipment to use. Generally, a fast telephoto lens on any standard DSLR or mirrorless camera (preferably full frame) is desirable. But having said that, wide angle (e.g. 24mm lens) shots have shown that the comet is large and bright enough to show up in relatively short single frame landscape shots. Recent photos show a faint ion tail several degrees long. Because a long, faint tail is a possibility, in tight shots, be sure to offset the comet head in your frame. You may not be able to see the full extent of the tail in your framing shots.

 A telescope will be needed as the comet recedes if you wish to follow the comet over the course of several weeks. A star tracker should be used to at least track the stars, since exposures are likely to be needed in the range of a few minutes long. The nice thing about shooting a receding comet is that it should be moving less in relation to the background stars.

Good comet hunting! May the clouds part for all of us.