How to Shoot Star Trails: Frame Counts and Shooting Time for Lighten Compositing

What lighten compositing is

Lighten compositing stacks multiple photos and keeps only the brightest value at each pixel. Stars may be points or short lines in any single frame, but when frames shot in sequence are stacked, their paths connect into long trails.

• Each exposure is short, so the sky is less likely to wash out
• If a plane or car lights cross the frame, removing that one shot removes them
• It holds down the thermal noise that builds up in long exposures
• If clouds roll in partway, you can still make a picture from the frames you have
• The same material can also make a time-lapse video

There’s also the traditional method of opening the shutter on bulb for 30 minutes to several hours, but noise and sky brightness are problems with digital cameras, so lighten compositing has become the norm.

Shooting time and arc angle

Stars make one full turn around the north celestial pole in about 23 hours 56 minutes. That’s about 15.04° per hour, so the arc angle is almost entirely determined by shooting time. Focal length and direction change how long the trails look on screen, but not the angle.

A full circle (360°) takes about 24 hours, so one session never makes a complete circle. Even 10 hours on a long winter night gives only about 150°. For a stronger concentric look, place Polaris toward the center of the frame and aim for at least 2 hours.

Frames needed for each exposure time

Frame count = shooting time ÷ (exposure time + gap). The gap is the time between the end of one exposure and the start of the next, which allows for writing to the card and leaves margin in the interval setting.

If trails are all you want, 30-second exposures take the fewest frames. Since star movement is the whole point here, there’s no exposure limit like the NPF rule. But the longer each exposure, the brighter the sky, so near city lights shorten it to 15–20 seconds and lower the ISO too. At F2.8 and 30 seconds, aim for about ISO 2000 under very dark skies, ISO 1000 in the suburbs, and ISO 500 in moonlight.

Broken trails come from the length of the gap

Composited trails look like dotted lines because the shutter is closed between one frame and the next. At a 30-second exposure, here’s how the gap length changes the share of time spent not exposing.

Viewed at 100%, breaks stay inconspicuous when the gap is within a few percent of the exposure time. For 30-second exposures, keep it around 1 second. Too short a gap can outrun card writing and skip frames, so shoot about 10 test frames first to check the spacing.

Trail shapes by direction

In the northern sky, trails are shorter for the same time, and the circular shape is easy to see within the frame. In the eastern and southern skies, stars move faster, so the same hour gives longer lines. When facing north with a wide-angle lens, Polaris’s altitude is about the same as your latitude (about 35° in Tokyo), a useful guide for composition. These directions assume the Northern Hemisphere.

Estimating card capacity and battery

A RAW file from a 24-megapixel camera is about 26 MB. At a 30-second exposure and 1-second gap, 1 hour is 116 frames, about 2.9 GB; 2 hours is about 5.9 GB; and 3 hours is about 8.8 GB. Recording JPEG as well adds to that, so bring a card with at least 32 GB free.

1. 1

   Work out the frame count from the shooting time

   In the Astrophotography Calculator’s “Trails & time-lapse” tab, enter the exposure time, gap, and shooting time to see the frame count, arc angle, and card capacity.

2. 2

   Enter the interval settings

   Use your camera’s interval shooting function to set the interval (exposure + gap) and frame count. Cameras without one can shoot in continuous mode with a locked remote release.

3. 3

   Prepare power and prevent lens fogging

   Batteries drain fast on cold nights, so USB power or a battery grip keeps it from stopping midway. A lens heater is also essential for sessions of several hours.

The same material can make a time-lapse video: 116 frames at 30 fps is about 3.9 seconds. For how to think about frame counts when video is the main goal, see Star Time-Lapse Settings.

FAQ

How many frames do I need for star trails with lighten compositing?

With a 30-second exposure and 1-second gap, 116 frames per hour and 232 in 2 hours. With 15-second exposures, it’s 225 per hour. Shooting time decides the result more than frame count: about a 15° arc in 1 hour and about 30° in 2 hours.

Why do my star trails look like dotted lines?

The usual causes are too long a gap between frames or long exposure noise reduction being on. For 30-second exposures, keep the gap around 1 second and always turn long exposure noise reduction off.

How many hours does it take to make a circle of star trails?

Stars make one full turn in about 23 hours 56 minutes, so a complete circle takes a full day and can’t be shot in one session. You get about a 15° arc in 1 hour and about 60° in 4 hours. Including Polaris at the center gives a concentric feel even in a short time.

What ISO and F-number should I use for star trails?

At F2.8 and a 30-second exposure, aim for about ISO 2000 under very dark skies, about ISO 1000 in the suburbs, and about ISO 500 in moonlight. You don’t need to capture faint light like the Milky Way, so a lower ISO works.

Which is better: one long exposure or lighten compositing?

With digital cameras, lighten compositing is recommended. A long exposure tends to wash out the sky and adds thermal noise, and lights from planes or cars that enter partway can’t be removed. With lighten compositing, you can simply drop the problem frames.