How to Plan a Star Trail Sequence

Ninety minutes of frames stacked into one image, and the trails are dashed — a neat row of dots and gaps arcing across the sky.

The camera was writing files between exposures. Every gap in the sequence is a gap in the trail, and no amount of stacking fills them.

The sky moves 15 degrees an hour

The Earth rotates once in roughly 24 hours, so the sky appears to rotate 15 degrees per hour.

Trail length in your frame depends on that rate, the total duration, and how far your target is from the celestial pole:

arc = 15° per hour × duration × cos(declination)

A star on the celestial equator traces the full 15 degrees an hour. A star near Polaris traces a tiny circle in the same time, because it is close to the axis of rotation.

That is a compositional decision rather than a technical one:

Pointing at the pole gives concentric circles. Dramatic, and the trails are short in angular terms, so long durations are needed for a strong effect.

Pointing east or west gives long sweeping arcs, close to straight lines near the celestial equator.

Pointing north or south at mid-declinations gives curved arcs, which many find the most attractive compromise.

Why stack rather than one long exposure

A single three-hour exposure is possible and rarely the right approach.

Sensor noise accumulates with exposure time, and long exposures generate heat which generates more noise.

One ruined frame ruins everything. A passing car’s headlights, an aircraft, a cloud, someone with a torch — in a single exposure that is the whole image. In a stack it is one frame you discard.

Highlights clip. Any bright object in frame saturates over three hours.

Battery and buffer. Practical limits on very long single exposures.

Stacking gives all the benefits and one requirement: the frames must be continuous.

Gaps are the main failure

Each frame ends, the camera writes the file, and the next begins. If the interval between exposures exceeds the write time, there is a gap, and gaps appear as breaks in every trail.

The causes are consistent:

Long exposure noise reduction. The camera takes a second dark frame of equal length after every exposure, doubling the cycle time and producing a gap as long as the exposure itself. Turning this off is the single most important setting.

Intervalometer interval too long. Setting a 32-second interval for 30-second exposures leaves a two-second gap in every trail. The interval should be as close to the exposure length as the camera can sustain.

Slow card. A card that cannot keep up with the write rate stalls the sequence.

Shooting in JPEG rather than raw reduces write time considerably, and for star trails — where the frames are combined rather than individually processed — that is a defensible trade.

Frame length and count

Thirty seconds per frame is the common choice, and the reasoning is a balance.

Longer frames mean fewer files and less write overhead, and more sensor heat and more lost if one is ruined.

Shorter frames mean more files, more write cycles and more opportunity for gaps, and finer control over which frames to discard.

At 30 seconds, ninety minutes is 180 frames. That is a manageable number to review and stack, and it keeps individual frame noise low.

Battery life is the practical constraint on total duration. An external power supply or a battery grip is what makes multi-hour sequences possible.

Common mistakes to avoid

• Leaving long exposure noise reduction on, which halves the duty cycle.
• An intervalometer interval longer than the exposure plus write time.
• Shooting raw when JPEG would avoid write-time gaps.
• Not checking the first few frames before leaving the camera for two hours.
• Forgetting that focus and framing cannot be adjusted mid-sequence without a visible jump.

How to do it with Star Trail Exposure Calculator

The Star Trail Exposure Calculator plans the sequence.

1. Set the total duration for the arc length you want.
2. Choose a frame length — 30 seconds is a common compromise.
3. Read the frame count and check your battery and card can sustain it.
4. Turn off long exposure noise reduction and set the interval as tight as the camera allows.

Other photography tools are in the tools directory.

Frequently asked questions

Why stack frames instead of one long exposure?

Because a long exposure accumulates sensor noise and heat, and any single interruption — a car, a plane, a cloud — ruins the whole frame. Stacking lets you discard the bad ones.

Why are my trails dashed?

Gaps between frames. Long exposure noise reduction is the usual cause, since it takes a dark frame of equal length after every exposure. Turn it off and tighten the interval.

Which direction should I point?

Toward the celestial pole for concentric circles, or east or west for long sweeping arcs. Near the pole the trails are short even over hours, which is a compositional choice.

Final thought

Turn off long exposure noise reduction before anything else. It is one setting, and leaving it on guarantees dashed trails no matter how good the rest of the plan is.

Try the free Star Trail Exposure Calculator