3 Transient Targets for Astrophotographers (Winter 2021)

My preferred targets for astrophotography are what we might call transient targets. In this article, I will identify three targets I will be aiming for this winter.

Conventional deep-sky targets are the traditional ones listed in the Messier, NGC (New General Catalog), or other astronomical catalogs. These objects are static in the sense that over a long period (even our lifetimes), they do not change. This makes them good targets for us to shoot and re-shoot in our quest to hone our astrophotography skills to a fine edge. But transient targets such as meteors, comets, and eclipses come and go, making them much more of a challenge to our base skill as well as our ability to adapt to unique shooting situations.

The next few months present several of these transient opportunities:

• Meteor showers:Taurids and Leonids (November), Geminids (December)
• Eclipses: Partial Lunar eclipse (November)
• Comets: Comet Leonard (peak in December)

Meteor Showers

The Taurids are actually two separate meteor showers (Northern and Southern Taurid showers) happening in the month of November with sparse activity and no distinct peak time of activity. As with most meteor showers, persistence and luck are the keys to capturing a nice meteor.

The technical challenge is not so bad — all that is required is to have a camera with a wide-angle lens on a fixed tripod. The main difficulty is to assemble a setup with a big enough storage card and provide a large battery that will last all night. My standard setup is to use a Sigma 15mm fisheye lens on a Nikon body (D600 or D850) with an AC battery eliminator. Both cameras have internal intervalometers which will cover a night of continuous shooting. Note: the D600 will shoot a maximum of “only” 999 shots while the D850 will shoot up to 9,999 shots!

The Leonid (peaking on the evening of Nov. 16-17) and Geminid (peaking on the evening of December 13-14) meteor showers present more of a challenge in 2021 because they are happening close to the full-moon phase. Having the Moon in the sky will increase the sky brightness, making the window of darkness for capturing a good meteor shot narrow. You can still capture bright meteors (for which both showers are well-known), by reducing your camera’s exposure time if necessary and increasing the number of frames you shoot when the Moon is lighting up the sky.

Lunar Eclipse

A partial lunar eclipse will take place on the night of November 18-19, best visible from North and South America and the eastern Pacific Ocean. It is important to note that the Moon just misses being completely inside the deepest part of the Earth’s shadow (the sun isn’t completely blocked by the Earth from the Moon’s point of view). Consequently, a sliver of the moon (about 1%) will still be relatively bright compared to the eclipsed part of the Moon, making it potentially a challenging target in terms of dynamic range. You may wish to shoot bracketed shots and try using HDR techniques to combine the shots in post-processing.

For equipment, a setup as simple as a standard camera with telephoto lens on a tripod is adequate, but a tracking mount would be much more convenient as lunar eclipses span several hours and North Americans (especially those on the west coast) will have a prime viewing location for the entire eclipse. As with total lunar eclipses, if you want to have a chance of capturing the darkened part of the Moon in your photos, you need to be at a reasonably dark site.

Comets

Although it may seem as though seeing a comet in the sky is a rare event, at any given time, there are a surprising number of comets in our skies. The problem is that most of them are so small and far away that it takes a large telescope to photograph them, and because the comets are so far away from the sun, they do not have discernable tails. Instead, they may look like slightly fuzzy stars.

Every few years, however, we do get a treat when a comet’s path brings it close enough to us to become visible to the naked eye. 

This December, Comet Leonard, officially designated C/2021 A1 (Leonard), may reach magnitude 4, which would make it visible to the naked eye in a dark sky and the brightest comet for 2021. But this should be taken with a grain of salt, as there is considerable uncertainty when it comes to predicting a comet’s appearance (see Sky & Telescope magazine's predictions for Comet Leonard).

For astrophotographers, the technical challenges include:

• Comet brightness and size
• Subtle tail structure
• Rapid motion against the background stars.
• Interfering light (moonlight and light pollution)

The comet’s core brightness will depend on its distance from the sun as well as its distance from us. The tail structure will depend on the comet’s chemical composition and its distance from the sun. If the comet consists of icy volatile compounds instead of rocky particles, we’re more likely to see a nice tail structure. Two tails may be visible if some compounds are ionized by the Sun. In this case, a blue-green-tinted ionized tail will stream away from the Sun, while a white dust tail of non-ionized particles will follow the comet in its orbital path as an expanding dust cloud. The visibility of these tails will also depend on our viewing angle to the comet.

Because comet tails are most prominent when the comet is close to the Sun, photographing the comet becomes challenging as the comet will be low on the horizon in the glow of twilight, light pollution, and a thick atmosphere. The glow of twilight is especially challenging as the background lighting is continuously changing as you are shooting.

When the comet is close to the Earth, then the problem of its rapid movement against the stars becomes another technical challenge. The usual astronomical mount which tracks the stars is not adequate for tracking a comet unless it is precise, has been polar aligned well, and is sophisticated enough to be programmed to follow the comet’s path instead of the stars. Generally, an easier way to get around this problem is to shoot a large number of shorter frames and later combine them in post-processing.

For northern hemisphere viewers of Comet Leonard, here are some viewing details:

• 11 Dec.: closest approach to Earth, but very close to the eastern horizon at sunrise
• 15 Dec. (approximate): transition from morning to evening (sunset) visibility
• 25 Dec. (approximate): best visibility for evening views
• 3 Jan. 2022: closest approach to Sun

After December, southern hemisphere observers are favored and the comet is essentially out of reach of northern hemisphere viewers.

I doubt that this comet will turn out to be as nice as last year’s Comet Neowise, but if you want to challenge yourself, comet photography is the way to go!