Star trails

Learning goal

You will be able to make visible in one frame how the sky turns through the night, and build a trail image in which the stars draw nested rings around the pole star or slanted lines toward the horizon. You will know how the shape of the trails changes depending on the direction you point the camera, and how many degrees of arc an hour-long series draws in the sky. Instead of a single very long exposure, you will shoot a long series made of short frames, understand why the trails turn into broken lines if gaps remain between the frames, and set the interval accordingly. You will turn the series into a single trail image with a combination that keeps only the brightest value at each point. In a shoot that lasts hours, you will take into account the lens fogging up, the battery running out and passing planes. If a person will enter the frame, you will plan for them to be lit in only one frame and for their skin to stay in its real tone in that frame.

The question

At the end of the Milky Way night, your friend had pointed the camera at the northern horizon and left it working by itself for an hour. At home, opening the hundred and twenty frames one by one, they all look alike: a handful of stars, a hill. But when someone at the club lays these frames on top of each other in a program, a surprising image appears on the screen: the stars draw thin, nested rings around a single point in the north, and in the middle of the rings a single bright point that barely moves. How does the sky turn?

Context

In the previous lessons you saw that, with the earth's rotation, the sky slides roughly a quarter of a degree a minute and that this slide happens around the celestial pole (Lesson G.5). In the multiple exposure lesson you met the blending mode that keeps, at each point, only the lighter of the frames (Lesson 22.8). In the remote release lesson you also learned to have the camera shoot frames by itself at set intervals (Lesson 13.4).

In the previous lessons you shortened the time so that a star wouldn't leave a trail; star trails are the opposite: showing the sky's rotation instead of hiding it. The sky's slide over an hour, two hours, sometimes a whole night, is gathered into a single image, and the eye sees at once a movement it could never see. This lesson takes up building this image with a long series, combining the series and the practical problems of a night shoot that will last for hours.

Simple explanation

What shape do the trails take?

A star trail is the line left by a star's slide across the sky in a long exposure or in a combined series. Because the dome of the sky advances about fifteen degrees in an hour, an hour-long series makes each star draw a fifteen-degree arc around the celestial pole; a four-hour series gives sixty-degree, that is, longer and more distinct, arcs. Stars near the pole draw small arcs, those far away wide ones.

The shape of the trails depends on which direction the camera is facing. Looking north, at the pole star, the trails become nested rings and in the middle the pole star barely moves. Looking east or west, the trails turn into parallel lines stretching slantwise toward the horizon. Looking south, wide, gently curving arcs appear. Which shape to choose is a decision made according to the frame's foreground and what it wants to tell.

How is the series shot, and how is it combined?

Instead of a single hour-long exposure, it is better to shoot a long series of short frames, each lasting roughly twenty to thirty seconds. Short frames don't overheat the sensor, they limit noise, and when a plane or a torch passes they let you throw away only that frame; the same frames can also be turned into a time-lapse video. The camera's interval shooting feature, or a remote with a timer, keeps the series going without you touching it.

The interval gap is the time that passes in a series between the end of one frame and the start of the next, leaving broken lines in the trails. If this gap reaches a few seconds, in the combined image each trail turns into a dotted line. Keeping the interval around one second makes the trails unbroken. Long exposure noise reduction is also turned off, because after every frame it makes the camera wait for the same length of time. The series is turned into a single image with a lighten blend that keeps, at each point, only the brightest value of the frames; there are also special programs that do this job.

What are the problems of the long night?

A dew heater is a thin band, usually run by a power source, wrapped around the lens that keeps it a little warmer than the air and prevents dew from forming on the front glass. On clear, humid nights the front glass of the lens can fog up as the temperature drops; in a series lasting hours, this makes the trails suddenly turn into blurred smudges at some point. A heater often prevents this problem. Spare batteries are a must in a long series, because batteries run out faster in the cold.

Passing planes and satellites leave straight, dashed lines among the trails; they can be removed from the individual frames, or those frames can be taken out of the series. If you want to add a person to the frame, because they can't stand still through a long series, you can light them in only one frame, for example the first frame, with a palm-sized dim light, and add this frame to the combination separately; in the remaining frames the person leaves the frame. The light in that single frame is adjusted without blowing out light skin or darkening dark skin; in the combination the face is taken from that frame as it is and keeps its real tone.

Visual evidence

On the next new moon night your friend goes back to the plateau. They point the camera north, above an old stone sheepfold standing below the pole star. They wrap a dew heater around the lens and put two spare batteries beside it. They set up interval shooting with thirty-second frames and a one-second interval, with long exposure noise reduction turned off.

In the first frame Selin stands at the door of the sheepfold; your friend holds a dimmed, warm lamp to her face for two seconds. Then Selin leaves the frame and the series runs for two hours. Toward midnight the humidity rises; thanks to the heater the front glass stays dry.

At home they combine the two hundred and forty frames with a lighten blend. Nested thirty-degree arcs around the pole star, a sky turning above the sheepfold. They take the plane trails in three frames out of the series. They take Selin's face from the first frame; on her olive-brown face her cheekbones and eyes can be picked out, the sky turning above her.

Common mistakes

Summary

The dome of the sky advances about fifteen degrees every hour around the polar point; this rotation appears as star trails when a long series of short frames is turned into a single image with a lighten blend that keeps the brightest value at each point: nested rings in the north, slanted lines in the east and west, wide arcs in the south. The interval gap is kept short so the trails are unbroken; a dew heater and spare batteries carry the long night, plane trails are weeded out; the person in the frame is lit in only one frame and taken from that frame, showing their skin in its real tone.

Mini assignment

In this assignment you will build your first star trail image.

1. On a dark night, point the camera at the pole star or to the east and frame it with an interesting foreground.
2. Set up interval shooting with frames of twenty to thirty seconds and an interval of around one second, with long exposure noise reduction turned off; shoot for at least an hour, and protect the lens against fogging.
3. Combine the frames with a lighten blend and weed out the plane trails; if there will be a person in the frame, light them in only one frame and take their face from that frame.

Check yourself

• Are the trails unbroken, or dotted?
• How did the camera's direction decide the shape of the trails?
• Is the face in the first frame sharp, its color as it is?

Glossary

Mini quiz

1. An hour-long series makes each star draw an arc of roughly how many degrees around the celestial pole?

• A) One
• B) Fifteen
• C) Ninety
• D) Three hundred and sixty

2. How do the trails look when the camera is pointed at the pole star?

• A) Slanted parallel lines
• B) Points
• C) Nested rings
• D) Wavy lines

3. Why do the trails turn into dotted lines?

• A) Because gaps of a few seconds remain between the frames
• B) Because the aperture is stopped down too far
• C) Because the ISO is low
• D) Because the moon has risen

4. How is a person in the frame shown in a long series?

• A) By standing still throughout the whole series
• B) They aren't shown at all
• C) With a different light in every frame
• D) By being lit in only one frame and having their face taken from that frame

Answers

1. B — Because the rotation speed is around fifteen degrees an hour, an hour-long series leaves an arc of that width.
2. C — Stars turning around the celestial pole form rings around the pole star.
3. A — The slide not recorded during the gap stays as a break in the trail.
4. D — A person moving through a long series leaves a ghost, while a single frame keeps the face sharp and in the right tone.

Summary card

• Fifteen degrees an hour: trails make the rotation visible.
• Rings in the north, slanted lines in the east and west, arcs in the south.
• Short frames, a one-second interval, long exposure noise reduction off.
• Lighten blend; weed out the plane trails.
• Dew heater, spare battery; light the person in one frame, take the face from that frame.

Bridge to the next lesson

After the trail image your friend wants the exact opposite: gathering the faint details of the galactic core with exposures of several minutes while keeping the stars like points. But in every exposure longer than fifteen seconds the stars slide. Someone at the club shows them a small device that fits on top of the tripod and turns together with the sky. How are longer, cleaner exposures possible? The next lesson takes up the star tracker that compensates for the earth's rotation.