Shooting Perseids

Meteoroid, meteor, meteorite

Space is not empty. Specks of dust, some tiny, some as large as sand grains or pebbles float around in that seemingly endless void. Every day an estimated 100 tons of this material hits the Earth’s atmosphere at staggering speeds, exceeding 20 kilometers per second. Most of it burns up completely. This happens at altitudes up to 100 kilometers. Some of it makes it through our dense atmosphere.
We see only the brightest ones, we call them shooting stars and we make a wish.

Let’s take a single speck of dust, floating around in space on a collision course with our planet and its atmosphere. This speck, in space, is a meteoroid. Not until it hits our atmosphere does is become a meteor. If it is big enough to survive the atmospheric entry and make it to the surface, it becomes a meteorite.

Most of what our eyes perceive, the shooting star, is actually not the dust particle burning up high in our atmosphere. Although the particle may be very small, it packs quite a punch at the speed it is moving. This kinetic energy is absorbed by our atmosphere and, by doing so, ionizes atoms in our sky. This ionization creates a glow, producing visible light. That is what we see. Larger particles burning up actually do produce sufficient light of their own. These are the really bright ones we call fireballs or bolides. Still, their physical size is not huge.

Wikipedia: meteoroid, meteor, meteorite

The Perseid meteor shower

This time of year, Earth is moving through a stream of particles left in the wake of comet 109P/Swift–Tuttle. Tiny dust grains from this stream hit our planet’s atmosphere and provide for what is arguably the best meteor shower of the year.

The Perseid meteor shower peaks in the early morning hours of August 13, meaning Earth is passing through the densest part of Swift-Tuttle’s stream around this date. Days, even weeks before and after this peak, plenty of Perseid meteors can be spotted.

All meteors belonging to a specific meteor shower can be traced to one point of origin in the sky we call the radiant. When observing Perseids meteors, draw an imaginary line across the sky, opposite the meteor’s direction of travel. Do so for all meteors observed. You will find most if not all of these lines to converge at one point, high in the constellation of Perseus: the Perseid meteor radiant. A point from where all Perseid meteors appear to originate. As meteors in meteor showers are named after the constellation that contains their radiant, the meteors in the Perseid meteor shower are referred to as the Perseids. Often, a meteor will be spotted that is not traceable to a known radiant. These meteors are called sporadics. Ever so often, radiants for “new” meteor showers are determined.

Meteors can be seen all over the sky. The higher the radiant is in the sky at a given location, the more meteors will impact the atmosphere at a point visible from that location. This means fewer Perseids will be observed before midnight when the radiant is at a lower elevation. As Earth rotates and the radiant climbs, generally more Perseids will be seen. The exception would be when our planet is passing though a particularly dense part of Swift-Tuttle’s stream before the radiant is high in the sky, but this can not be accurately predicted.
The number of meteors observable is expressed as zenithal hourly rate (ZHR). The higher the number, the more meteors you can expect to see.

Wikipedia: Perseids, radiant, ZHR

Photographing meteors – Equipment

Sophisticated equipment is not required to photograph meteors. Of course, a better camera-lens combination will produce a better image, but any camera that can take an exposure of a few seconds, can photograph a shooting star. Due to the erratic appearance of meteors, wide-angle lenses covering more sky have a higher chance of capturing one. Narrow fields of view can produce good images, but the chance of a meteor streaking through a smaller piece of sky is simply not as great.

The light of a meteor passes in the blink of an eye. Literally in a flash. Therefor, contrary to deepsky photography, longer exposures will not result in brighter meteors: in that (very) short amount of time as much light as possible needs to be captured. No point in trying to capture photons of something that already past. A fast lens (low f-value), a low f-stop setting and a relatively high ISO setting produce the best results. Of course, local conditions need to be factored in. Light pollution or interfering moonlight will brighten the overall image.

Mount the camera on a tripod. Preferably, use a remote shutter for continuous exposures. Do not spend a fortune, remotes for many cameras are available for relatively little money on sites such as Ebay or Dealextreme. Make sure to use a memory card with sufficient storage for lots of images, or bring extras.

If humidity is an issue, use dew protection as you would on a telescope. This will require a (portable) power source. A short dew heater and a simple controller will be all you need.

Camera settings

Let me emphasize this again: when it comes to meteors, as much light as possible needs to be captured in a short amount of time. All exposures are a combination of time, f-stop and ISO setting. Choose the manual setting (M) on your camera. Set the f-stop to a low value. If you have an f/2.8 lens, consider using it “wide open” at f/2.8, not stopping it down: setting an f/2.8 lens to f/4 will cut the amount of light in half. Select a relatively high ISO setting but don’t go to extremes as this will produce grainy images. Set the white balance quite “cool”: 3200K if your camera allows for a custom white balance setting. If not, select the setting for artificial light.

The maximum exposure time is determined by local conditions. When using a fast lens, low f-stop and high ISO setting, generally a five second exposure will do. If you are at a very dark site and the Moon is not interfering, the exposure time can be increased. Take a few test shots to see what exposure time creates a nice image, without overexposing. If the image is too bright, decrease the exposure time, preferably. You could also select a higher f-stop and/or lower ISO setting, but this will result in less light being captured of a passing meteor. Remember, it is the light of a meteor we are after. Do not set the camera to very low exposure times, e.g. <3 seconds. Shorter exposure times will increase the chance of “cutting a meteor in half” in an image. Even though meteors move very fast, a small amount of time does pass. There’s always the risk of an exposure ending during a meteor’s time of flight, but sufficiently long exposures will decrease the likelihood of an exposure ending in mid flight.

Take as many images as possible. You never know when the next meteor will appear. Really bright ones can leave a lingering glow of ionized atmosphere or even a smoke trail that can also be photographed. Using a remote shutter, you can simply let the camera click away. If you have a lot of sequential images you could even construct a timelapse.

Planning the shoot

When do you start shooting and where do you point the camera? Well, you can point it anywhere and you can start shooting as soon as it is dark. Everything else is personal preference. This is what I do:

I prefer a wide field image with something earthly in the frame. This can be just the horizon, or perhaps a tree in a corner of the field of view. Do make sure there is plenty of sky in the image. I strive to not point the camera too far from the radiant, but I don’t center it either. With the Perseid radiant rising in the northeast, I point the camera eastwards or to the north. By doing so the radiant will not be in the center of the image. Keep it in the left or right third of the frame, but no closer to center. Perseids near the radiant will not cover as much angular distance across the sky as they are, relative to us, coming straight at us. This is a matter of perspective. The farther away from the radiant a meteor appears, the longer its streak of light will appear. Local conditions, cities or other sources of light pollution will also be a factor in the choice of direction to shoot in.

To determine where the radiant will rise at your location, use a planetarium app. Simply look where the constellation of Perseus appears and note the azimuth. Google Earth can help pinpoint where that exact position is at your location. You can determine a landmark or and/or use a compass to point you in the right direction. This little planning ahead will take away a lot of guesswork. By knowing where to point your camera you can easily set it up before darkness falls.

The higher the radiant is in the sky, the more meteors it seems to produce. For Perseids, that is well after midnight. You may want to plan for an all-nighter.

Perseid meteors

Not meteors

Shooting the sky, you will capture quite a few bright streaks crossing the heavens. Not all are meteors. Artificial satellites orbiting our planet reflect sunlight and thereby reveal themselves. Space junk will appear as a moving, flashing light, varying in brightness as it tumbles through space. Airplanes will inevitably cross the field of view of your camera. Remember, meteors do not have flashing lights, nor do they tend to be visible for minutes and they definitely do not alter their direction of flight.

Browsing the web, you will find plenty of images of supposed meteors. Look closely. Many are not. In 2013, a renowned Dutch newspaper published an article about the Perseids. Regrettably, there is not a single meteor in the ten images they included. Please see for yourself.

A meteor will not leave a long, faint trail of light in an image before it brightens. It appears and then it is gone. Perseids are known for quite a lot of green color, especially in (what photographically appears to be…) their tails.

Personal care

To enjoy a meteor shower, just find a nice spot, recline and gaze skywards. Your Mk1 eyeball will be all you need to observe. Sit back, relax and enjoy the show. A reclining chair or stretcher makes it all the more comfortable. Be sure to bring some warm clothing or a sleeping bag. Even warm summer night can get chilly, especially when you are not moving.

Other meteor showers

The Perseids may be the best-known meteor shower of the year, but there are more. The Leonids, visible in November can produce quite a lot of bright meteors and so can the Geminids in December. Browse the web to find out when the next shower is, what constellation the meteors seem to appear from and where the radiant is. Take the Moon’s phase, rise and/or set time into account and plan accordingly.

Final thoughts

It can payoff to always have a camera clicking when you are under clear, starry skies. You never know when that really bright one appears, and it can produce quite an image. If you catch a bright meteor in a photograph, please share it. Sites like Spaceweather.com post a lot of images taken all over the world. Be sure to submit your own.