Stars & Trails

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A star trail photo is a long-exposure image that shows the apparent motion of the stars across the night sky, producing a pattern of light trails stretching across the image. A good star trail can be mesmerising, the circular pattern offering up a new perspective on the rhythms of the night sky. The reason star trails are circular is is caused by the rotation of the Earth.

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For any given exposure time these trails appear longer and less curved nearer the celestial equator, while nearer the celestial pole the paths are shorter and more strongly curved. Star trails are all perfectly centred on the celestial pole, and are the arcs of full circles whose diameters increase with distance from the pole.

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Careful measurement of a star trail image may reveal that the curves aren’t perfect arcs of a circle. This is usually because the image was taken with a wide-angle or fish-eye lens, which wasn’t perfectly centred on the north celestial pole. Although some will gaze at star trail images photos with awe, producing an image that shows the apparent movement of the night sky over an hour or so is a relatively simple technique, and is as much about planning and commitment as it is about skill.

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As with any night-sky photography, too much light will wash out the stars.To maximise the number of stars in the final image, you need to take your astrophoto from somewhere dark, away from artificial light. However, this doesn’t mean you can’t create a star trail from a back garden in the middle of the city. Indeed it is possible to do astrophotography from a light-polluted city. You can, and the finished image can still be impressive, but it won’t have as many stars as one taken in the countryside. If you want to capture the classic ‘rotating sky’ effect like in the image above, in the northern hemisphere you will have to shoot north at Polaris. If you're shooting under a southern hemisphere sky, point south at the Southern Cross (from March to September) or near bright stars Achernar or Canopus (from October to February). Take a compass with you in daylight and you can plan your shot before it even gets dark. Remember to follow current travel restrictions wherever you are, and stay safe at all times.

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Just as important as avoiding light pollution is the position of the Moon. A lot of astro imagers have three weeks off every month and only venture outside during the week of a new Moon, when the slim lunar crescent rises late and sets early. However, while a bright Moon does wash out a lot of stars, it’s perfectly possible to capture good-looking star trails even near the full phase. Moonlight causes a bluish night sky, but it can illuminate the foreground nicely – it all depends on your shot.

 

There are two ways to produce a star trail; the first is to open the shutter on your camera for a long period, and the second is to take multiple short exposures and stack them into one image.The ‘old’ way of opening the shutter for over an hour is now best avoided. It was fine when using film cameras, but digital cameras’ sensors just get too hot, so it’s best to take lots of 30-second exposures, about 15 seconds apart, and stack them together. However, you can also choose between shooting about 10-20 three-minute exposures or 50-100 exposures of about 30 seconds.The latter is easier.  If you’re shooting multiple 30-second images to stack later – which you should if you’re using a digital camera – then the settings are the same as for any night-sky shot. Use a tripod, a wide-angle lens on the lowest aperture setting possible, and shoot at around IS0 800 to avoid too much noise. Always shoot in RAW format...that way you can use Photoshop to reduce the noise and any light pollution.

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Set the lens to manual focus, then either pre-focus it to infinity (some cameras have a ∞ symbol on the lens), or shine a torch on something 20m or so away and focus on it using autofocus (then switch back to manual). Tungsten white balance can help reduce the effects of any light pollution.

You can experiment with light, too. Once you have finished capturing your star trail images, consider taking a couple more using one-second bursts of torchlight on your subject to create different lighting effects. You can then either use them or discard them when you come to stack your images, thus creating a few different versions of your final shot.

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Few of us want to spend hours editing photos, but it’s crucial for producing a good, clean image. Photographers all have their favourite photo-editing suite, but it’s hard to beat Adobe Photoshop (paid) for night-sky images. It’s possible to batch-process the 80-200 images you might have from a star trail shoot relatively quickly, though a computer with 16GB RAM helps enormously, so an average laptop will be slow. Another capable software option is StarStaX (free). This is a small application that lets you drag in multiple images that it then merges into a single star-trail image before your very eyes.

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