Star Trail Photography

Taking star trails is one of the easiest forms of nighttime photography. it requires less equipment than full out astrophotography, only a camera that can take a long exposure and a tripod. In a pinch you can do without the tripod.

Three Lasers
Three lasers in operation, Subaru, Keck 1 and Keck 2, 23 x 4min with a Canon 60D
In star trail photography a long exposure is used to reveal the scene. With illumination provided by starlight the needed exposure will be minutes long, during which time the rotation of the Earth will cause the stars to trail. Each star will trace a short streak on the camera detector as it moves through the field of view.

For a number of reasons taking one very long exposure is a problem with digital cameras. Without getting into a technical discussion of noise, dark current and hot pixels we will simply advise taking short exposures. You can always try a twenty minute or half hour exposure and see for yourself. Thus the technique is to take a series of short exposures, usually one to five minutes long, and add these together in processing. By taking a series of short exposures, the final exposure length is limited only by the camera battery or the arrival of dawn.

If the camera is sensitive enough, and you have a fast lens, you might try starscape photography, where the stars are not trailed by the motion of the Earth. In contrast, star trail photography can be done by almost any camera that can take a long exposure. The difference is in the length of the exposures, long versus short, star trail or starscape.

The Camera

To take the needed long exposures a camera with a bulb mode is needed. All DSLR cameras have this, many upper end compacts do as well. In addition some sort of remote shutter release is needed, or a way to program the camera to take a series of long exposures.

The usual solution is an accessory called an intervalometer. This device can trigger the camera at a programmed interval, allowing a series of exposures to be taken. The intervalometer must be programmed with several parameters… The length of each exposure, the pause between exposures, and the number of exposures desired. More on these settings will be discussed later in this article.

Mauna Kea Star Trails
Star trail image taken from Hale Pohaku looking towards the mountain.
The setup is now quite simple. Aim the camera, connect and program the intervalometer, focus and shoot. What if you do not have a tripod? I have used stone walls, a piled jacket, the top of a camera bag, or simply the hood of a car. Anything that allows the camera to be held in position for the series of exposures.

Focus

Focusing in the dark is not easy. Poor focus has ruined more nighttime shots than all of the other causes put together. Auto-focus is not going to save you here, there is simply not enough light. The camera may indicate it managed to achieve focus, but did it really? You will usually need to focus manually. There are many tricks to getting a decent focus. You can find some of them in focusing on another photography basics post here on Darker View.

Often you will not have any special equipment available. Thus the best approach will be to use a magnified live view to focus on the brightest star or planet in the sky, or simply a distant streetlight.

Some prime lenses will have an upper stop that occurs right at infinity focus. This makes focus trivial, simply rotate the focus ring until it stops. The Rokinon 14mm f/2.8 used for many of the shots here on Darker View exhibits this feature. If the stop is not set correctly it is possible to adjust, under one of the rubber grips are a set of screws that allow adjustment of the infinity stop.

Exposure

In other forms of astrophotography the goal is to push the camera to the limit, using the highest gain (ISO) possible. Star trail is an exception to this, often it is necessary to cut back on the gain or use a smaller f/stop to limit the light into the camera. This allows a longer exposure to capture the stars as they trail across the sky.

This is not to say you can take a single exposure to capture a few hours of star trails. This worked back in the days of film, but will not work now. Other factors limit the maximum exposure in a digital camera. Modern CCD and CMOS detectors suffer from an effect called dark current, sort of a leakage signal into each pixel even when no light is present. This can be seen by taking a long exposure with the lens cap on, a grainy pattern in the image. There are some pixels in the detector that are far worse than others, creating bright pinpoints that will be red, green, or blue. These bright pixels are generally referred to as hot pixels.

Dark current is always present and will slowly overwhelm the image in any exposure longer than a few minutes. Temperature is a big factor here, dark current doubles for every 6°C (11°F). Thus the problem will be far more apparent when taking photos on a warm summer evening versus aurora photos during a cold Alaskan winter’s night.

In general you will be limited to five to ten minutes per exposure.

Processing

The chore in assembling a star trail photo is processing the dozens of photographs into a single image. This will require a photo editing package with the necessary features to add photos together. Photoshop supports this function, but so do a few less expensive options. There are even free software packages that allow stacking of multiple images…

  • StarStaX – A freeware package (donations accepted) available for MacOS, WinOS and Linux specifically designed for star trail photography
  • DeepSkyStacker – A free WinOS software package designed for processing astrophotos with full alignment functions, can also be used for star trail photos.
  • Startrails – A free (donations accepted) WinOS application for assembling star trail photographs.

My usual technique is to import each image into Photoshop as a new layer. The layer blending mode is set to “lighten”, this takes the brighter parts of each image. The image is then flattened to merge the image into the result. This is done for each image in the sequence. The process is aided by setting up the steps needed as a script, called an “action” in photoshop. It is then possible to simple run a batch process on the directory to merge all of the images without the tediousness.

Once merged the images can be processed. A little unsharp masking will clean up the stars, a curves adjustment will lighten the landscape under the stars or darken the sky between the stars. A color balance will also be required.

One thing you learn quickly with nighttime photography… The sky is green.

Mauna Kea Observing
Polar star trails above our telescopes and Mauna Kea, 14 x 5min exposures stacked for 70min of exposure
The green cast comes from softly glowing nitrogen. Sometimes brighter bands of glow slowly cross the sky, a phenomena called, simply enough, airglow. A good digital camera will faithfully record this glow, resulting in an unattractive sky color. Well? To be honest, unattractive is being charitable, it can be pretty bad, appearing a lovely shade of baby-puke brown-green between the stars.

The solution to this green sky cast to adjust the color balance of the resulting image. My favorite method of doing this is to use the histogram or curves tool to separately adjust the red-green and blue channels for a neutral or slightly bluish cast to the background. By adjusting the black point of each channel, not the highlights, this should not effect the star colors. the color of the background sky can now be adjusted to suit the photograph and create an attractive result.

Just Shoot

A little knowledge is useful, it allows you to make a good attempt the first time out in the dark. Even with the hints found above you will make mistakes, something will go wrong. There is no substitute for experience. You simply need to get out into the dark and shoot. Find yourself a dark site and try it!

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on Mauna Kea, Hawai'i.

2 thoughts on “Star Trail Photography”

  1. It’s as if you wrote this one just for me. I had forgotten the names of the free processing programs you had mentioned when we had bumped into each other at the Hakalau Forest Refuge last weekend, and had it in mind to get in touch with you. People talk about Lightroom 5 and I am thinking that I should replace Aperture with it although they say it is similar. WinOS…does that mean it will be compatible with both windows and mac? I will wait for your reply before downloading any software.

    I am thinking that I should take a drive to the Vis tonight while the forecast is for clear skies. Hope to see you there!

    1. Coming down from a long day at the summit. maybe, if I still have any energy, I might set up in my driveway.

      If you are using a Mac you should try StarStaX, the only option I have seen for MacOS.

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