Glance at the summary firstThe summary from the end of the lesson, as a road map before you start reading.
A star tracker keeps stars like points for minutes by turning the camera against the earth's rotation at the sidereal rate and around an axis aligned to the celestial pole; this makes it possible to lengthen the time, lower the ISO and shoot a less noisy, more detailed sky frame. Polar alignment becomes more critical with long lenses; the ground and people that slide in the tracked frame are shot in a separate untracked frame, the two frames are combined and the combination is stated, and the person in the frame is lit in that frame with a softened, dimmed light suited to their skin.
Learning goal
You will be able to explain how it is possible for a star tracker to keep stars like points for minutes by turning the camera together with the sky. You will know that the device's axis must be parallel to the earth's axis of rotation, and how to align it to the celestial pole near the pole star for this. Thanks to tracking, by lengthening the time and lowering the ISO, you will get a less noisy, more detailed sky frame. Seeing why everything on the ground, trees, hills and people, slides in the tracked frame, you will shoot the foreground in a separate frame with tracking off and tell the viewer about the combination. You will take into account the weight the device can carry, the balance and the wind. You will photograph a person in the frame in the untracked frame, with a small light and keeping their skin in its real tone.
The question
After the trail frame your friend is after something else this time: letting the stars stay as specks but gathering the faint dust lanes of the Milky Way with more detail. Even at a wide angle, every exposure longer than twenty seconds stretches the stars and they have to raise the ISO to 6400; the frame is noisy. A member at the club has fitted a palm-sized device on top of their tripod, puts their camera on it and starts a two-minute exposure. On the screen the stars are still small points. How are longer, cleaner exposures possible?
Context
In the lesson on shooting stars as points you calculated that the sky slides with the earth's rotation, that this slide happens around the celestial pole near the pole star, and the time stars stay points (Lesson G.5). In the Milky Way lesson you saw shooting the ground that stays jet black in the dark separately from the same place and combining it with the sky frame (Lesson G.6). In the tripod lesson you also learned the weight a tripod or a head can carry without slipping (Lesson 13.1).
Until now you have shortened the time according to the sky's slide and made up for the missing light by widening the opening and raising the sensitivity. A star tracker changes this equation: you can't stop the sky, but you can turn the camera together with it. So the time goes up to minutes, the ISO drops and the sky appears with less noise and richer colors. This lesson takes up how a tracker works, how it is set up and its costs.
Simple explanation
What does a tracker do?
A star tracker is a small motorized mount fitted on top of a tripod that slowly turns the camera on it against the earth's rotation, at the same speed as the sky and around the same axis. As the sky slides because of the earth's rotation, the device turns the camera together with it; so the stars stay in the same place on the sensor and look like points even in a long exposure.
The sidereal rate is the speed at which the sky slides because of the earth's rotation; one full turn takes about twenty-three hours and fifty-six minutes. Trackers turn at this speed; some also offer other speeds for following the moon or the sun. When the device turns at the right speed and on the right axis, exposures of a few minutes at a wide angle of 14 mm, and of a minute or two with longer lenses, keep the stars like points. This long time makes it possible to lower the ISO to 800 or 1600 and get a cleaner, more detailed sky.
How is the device aligned?
Polar alignment is pointing the tracker's axis of rotation at the celestial pole so that it is parallel to the earth's axis of rotation. In the northern hemisphere the celestial pole is right next to the pole star, less than a degree away from it. Many devices have, for making this alignment, a small scope you look through inside the axis, a polar scope; the marks in this scope help place the pole star at the point where it should be at that hour. An app can also show the alignment.
The more accurate the alignment, the longer the stars stay points. With wide-angle lenses a small error is easily tolerated; with long lenses the same error turns the stars into lines within a few minutes. Not exceeding the weight the device can carry, placing the camera and lens in balance, and shortening the tripod's legs on a windy night also keep the tracking smooth. After the alignment, checking with a short test shot by magnifying the stars shows the error before you start a long exposure.
What happens to the ground and people?
Because the tracker turns the camera together with the sky, everything on the ground, hills, trees and people, slides during the long exposure and blurs. The sky is sharp, the ground blurred. That is why the sky frame shot with tracking is combined with a foreground frame shot from the same place with tracking off, or with the device turned back to its start. Even if the two frames are from the same night and the same place, the combined image is a combination and is told to the viewer.
If a person will enter the frame, they too need to be shot in the untracked frame; in the tracked frame the person slides and turns into a ghost. In the untracked frame, lighting the person for a few seconds with a dim light the size of a palm keeps their face sharp. Softening the light with a piece of paper prevents shine on light skin; on dark skin, bringing the light a little closer or lengthening the time a little brings the detail back. In the combination the person is taken from this frame and their skin keeps its own tone.
Visual evidence
Your friend fits a tracker borrowed from the club on top of their tripod. Looking through the polar scope, they bring the pole star to the point the ring in the scope shows for that hour. They shoot a short test shot at a wide angle; when they magnify the screen, every star is a tiny speck.
They turn on tracking and make a two-minute exposure at f/2.8 and ISO 1600. The galactic core's dust lanes are much more detailed and less noisy than in the twenty-second frames at ISO 6400. But the stone sheepfold and the tree in the foreground have softened, slid slightly.
They turn off tracking and shoot a foreground frame of the sheepfold and the tree. Selin stands at the sheepfold's door; your friend holds the dim torch, its mouth covered with paper, to her face for two seconds; the lines of her olive-brown face are clear and its color warm. At home they combine the two frames and write under it: "sky with tracking, foreground and Selin with tracking off, the same night from the same place."
Common mistakes
Common mistakeUsing the tracker without aligning it.
The stars slide again within a few minutes. The fix: align to the celestial pole with the polar scope or an app, and check with a short test shot.
Common mistakeThinking the foreground will also stay sharp in the tracked frame.
The ground and people slide. The fix: shoot the foreground and the person in a separate frame with tracking off.
Common mistakeLoading a camera and lens heavier than the device can carry.
The tracking becomes irregular. The fix: keep within the weight limit and place the load in balance.
Summary
A star tracker keeps stars like points for minutes by turning the camera against the earth's rotation at the sidereal rate and around an axis aligned to the celestial pole; this makes it possible to lengthen the time, lower the ISO and shoot a less noisy, more detailed sky frame. Polar alignment becomes more critical with long lenses; the ground and people that slide in the tracked frame are shot in a separate untracked frame, the two frames are combined and the combination is stated, and the person in the frame is lit in that frame with a softened, dimmed light suited to their skin.
Mini assignment
In this assignment you will work with a star tracker, borrowing one if you can.
- Fit the tracker to the tripod, align it to the celestial pole with the polar scope or an app, and check by magnifying the stars with a short test shot.
- Shoot the same framing first with tracking off, high sensitivity and a short time, then with tracking on, a low ISO and a two-minute time; compare the sky's noise and detail.
- Shoot the foreground and the person in the frame, if any, in a separate frame with tracking off, open the face with a dim light and adjust the amount of light to their skin, and if you combine, write so.
Check yourself
- In the tracked frame, did the stars stay points for two minutes?
- How different is the noise in the tracked and untracked sky frames?
- In the combination, is the person's face sharp and in its own tone?
Glossary
| Term | Short definition |
|---|---|
| Star tracker | A motorized mount fitted to the tripod that keeps stars like points in a long exposure by turning the camera on it at the same speed and on the same axis as the sky. |
| Polar alignment | Pointing the tracker's axis of rotation at the celestial pole so that it is parallel to the earth's axis of rotation. |
| Sidereal rate | The speed at which the sky slides because of the earth's rotation; one full turn takes about twenty-three hours and fifty-six minutes. |
Mini quiz
1. How does a star tracker keep stars like points?
- A) By stopping down the aperture by itself
- B) By turning the camera at the same speed and on the same axis as the sky
- C) By raising the ISO
- D) By vibrating the shutter
2. In polar alignment, where is the tracker's axis pointed?
- A) At the celestial pole, next to the pole star in the northern hemisphere
- B) At the galactic core
- C) At the moon
- D) South, at the horizon
3. Why does the foreground blur in the tracked frame?
- A) Because the lens is too wide-angle
- B) Because the aperture is too open
- C) Because the ground slides as the camera turns with the sky
- D) Because the ISO is low
4. How is a person in the frame shown in tracked shooting?
- A) Without moving at all for two minutes with tracking on
- B) Without being taken into the frame
- C) With a strong flash in the tracked frame
- D) By being lit in a separate untracked frame and combined
Answers
- B — The device counters the sky's slide by turning the camera on the same axis.
- A — The axis of rotation must be parallel to the earth's axis, and this axis points at the celestial pole.
- C — As the camera follows the sky, the ground slides relative to it and blurs in a long exposure.
- D — The person stays sharp in the untracked frame and is taken from there in the combination.
Summary card
- A tracker turns the camera together with the sky; stars are points for minutes.
- Align to the celestial pole with a polar scope or an app, check with a test shot.
- Long time, low ISO: a cleaner, more detailed sky.
- In the tracked frame the ground slides; shoot the foreground separately, say it is combined.
- Weight limit and balance; shoot the person in the still frame with a dim light suited to their skin.
Bridge to the next lesson
After the Milky Way frame shot with tracking, your friend turns to another subject of the sky: the full moon is rising over the plateau. In every frame they shoot with a long lens, the moon is a snow-white, detail-less disc in a jet-black sky. Yet looking with the eye, the dark patches on its surface can be picked out. Why does the moon always come out as a white blob? The next lesson takes up photographing the moon and the planets, and how to expose at night a surface lit by the sun.