What are these strange lines that arc across the sky? These are star trails! Each and every line is a star. The duration of a typical star trail photo varies from a few minutes to many hours. This means that in order to capture the photo we have to leave the camera shutter open for an extended period of time. The stars appear like curved lines instead of light dots due to the Earth’s rotation. As the Earth spins around its axis the stars seem to ‘move’. This is actually an apparent motion. Stars are stationary, therefore it is the Earth’s motion that you photograph. Star trail photography really is a mind game, especially if you consider the fact that our distance from the stars is measured in light years. So, every time you take a photo of these bright dots, or bright star trails you actually take a photo of the past! For the sake of accuracy, stars seem stationary for an observer during a whole lifetime. This practically means that Orion, or the Big Dipper or any other constellation will look the same to us, our children and grandchildren. In the course of thousands of years though, they visually change, forming new cosmic patterns on our night sky.
So, let’s get started. Back on the old days of film photography, the most common way to shoot star trails was to leave the shutter open and take one really long exposure. You still can do this with a digital SLR camera but there is another recommended approach. Instead of one long exposure, we shoot many consequent exposures and then stack them at post processing in order to produce the equivalent of a single long exposure. Stacking has a number of advantages:
- The signal to noise ratio is improved. The more you stack the better noise-free image you get. In case of a single long exposure, there will be a significant amount of digital noise in your picture.
- You can shoot in light polluted areas without worrying about overexposing the foreground.
- Many things can go wrong during a long shooting. Clouds, unexpected car lights or an accidental kick in the camera tripod are only a few. Only one big exposure means that you lose all of your work. With multiple exposures you may just reject a number of shots and keep the ‘good’ ones to produce the final image.
- You can use the photo sequence to create a timelapse.
Star trail photography equipment and some suggestions:
1 For your basic equipment you will need:
- A sturdy tripod.
- A DSLR camera with a wide field lens. Zoom lenses are more flexible but prime lenses perform better.
- A high capacity, fast memory card.
- Fresh charged batteries.
- An intervalometer. I can hear you say: ‘An inter-what?’. The intervalometer is a device that allows you to program your shooting. A typical scenario will be: 300 shots, 30 seconds each, at 1 second intervals. Some cameras have this function built in and for some others you have to purchase a separate device. Alternatively you may use a simple (and cheaper) wired shutter release or even a rubber band to lock the shutter and allow your camera to take continuous shots, but using an intervalometer will give you much more flexibility.
2 One second intervals? Yes! So the camera takes a shot, then at least one second is required to be stored in the memory card and the camera continues with the next shot. Some photographers suggest that you should leave longer intervals between the shots in order to give more time to the camera sensor to cool down, producing images with lower noise. I don’t agree with that. Unless your main goal is to produce a timelapse -so in this case you really need low noise at your individual shots- in the case of the star trail photo go for the 1 second intervals. If you leave longer interval time, let’s say 4 or 5 seconds you will notice these annoying gaps on your star trails. The gap is there even at 1 second intervals but the lower the interval time the less visible it is.
3 Extra equipment. The following are not absolutely necessary but certainly recommended:
- A battery grip that holds two batteries will double your shooting time. It is not recommended to interrupt a star trail shooting to change a battery or for any other reason because even the slightest re-position of the camera will result to a blurry image after the photo stacking.
- A lens hood and a dew heater to prevent humidity from blurring your lens.
- The longest night of the year is more than 12 hours during the Winter Solstice. If you want to create such a long star trail photo you need a different power source. If you are taking the photos from your back yard, you may use an AC power adapter. If you are on the top of a mountain, most probably you will not have a plug to connect your adapter. There are some alternatives. I use a 12V rechargeable battery and this adapter. If you are lucky enough to find a cloudless night in the middle of the winter your star trails will look like the following photo. Notice Polaris, the Northern Star. After 12 hours of continuous shooting it looks like a semi-circle.
- Warm clothes. In many areas especially in high altitude it’s cold at night any time of the year.
- Mosquito repellent, food, water, peanuts, a cell phone, a GPS unit, a set of binoculars to admire the heavens, a book etc…
- A friend! Your chances to survive if you get lost in the wild are greater if you are not alone!
4 Camera settings:
- Exposure. In case you use a wired shutter release, set your camera exposure to the maximum available (typically 30 seconds). If you have an intervalometer set the exposure to Bulb. The Bulb setting allows you to set the exposure time to whatever you want at your intervalometer. Dark places typically require longer exposures. I rarely shoot though more than one minute exposures. If you shoot to a light polluted area or with the Moon on the sky you probably will not exceed the 30 second exposures.
- ISO. I mostly use 400 or 800. When I shoot in a really dark place and I want to capture vivid star trails and colors I may use ISO 1600. Again, don’t worry much for the digital noise, resulting from the high ISO. If you stack even a small number of shots, noise is greatly reduced and you can virtually eliminate the remaining noise at post processing. High end, low noise full frame cameras allow you to use higher, noise free ISOs.
- Aperture. Set it wide open if you want prominent and vivid stars or one stop closed for sharper results. Closing the aperture will give you more depth of field which is good if you have an Earthy subject like a tree or a building in your frame. In practice, if you use a wide field lens even with the aperture wide open stars and objects only a few feet away will all be nice and focused.
- RAW vs JPG. Shooting in RAW is better. You can alter your white balance and have much more flexibility at post processing. There are a few drawbacks though when it come to star trails. A RAW image requires more storage space; therefore you need a high capacity fast memory card for your camera and more storage space in your computer. Additionally, the process time is increased as you will need to convert your images to jpg or tiff format before you stack them.
- White Balance. If you shoot in RAW it doesn’t really matter. If you shoot in jpg use Daylight as it gives in my opinion the most realistic results in most cases of night photography.
- Noise reduction. Set it to OFF. If you omit to do so you will have 1 minute intervals between the shots.
- Image stabilizer: OFF.
5 Weather. Star trail photos may require a significant amount of time. Two or three hours or even more is typical. Clouds, humidity and strong winds can ruin your photo. On the other hand a partially cloudy sky may result to a really nice image. The gaps on the clouds to the following ‘cloud trail’ image is the result of the one second interval between the shots.
Sometimes a great deal of persistence and luck may result to a star trail – lightning photo. The image below is the result of a distant storm cloud combined with star trails. You can find more info about capturing lightning at this tutorial.
6 Illuminating the foreground. If you include an earthy foreground to your photo you have a number of options for your light source. In some cases you may just leave it dark, just like the following photo where a dark silhouette of a tree adds more drama to your synthesis. Some light sources you can use:
- The Moon. Stars are dim in the moonlit sky but the foreground is perfectly illuminated. A 30 second exposure with the full Moon in the sky will result to a day like scene.
- The stars. If you rely only on the star light as a light source for your foreground you will need to take really long -several minute- exposures. I rarely choose this option.
- A flash light. Either point your flash light directly on the foreground:
…or diffuse the light like in the following example:
- Light pollution. You can even use the ambient light from buildings, street lights, cars and everything else included on a populated area to illuminate your foreground. The foreground lighting to the photo below comes from some street lights:
7 Location! This is perhaps the most important factor that determines the difference between an OK photo and a breathtaking photo. Choose interesting places, preferably away from the city lights.
8 Taking the photo.
- Orientation. If you turn your camera to the North (assuming that you live in the northern hemisphere), your star trails will look like spinning wheel with Polaris, the Northern Star near the center. If you turn the camera elsewhere your star trails will be curved lines.
- Focusing. Set the camera to auto focus (if available) focus on the Moon or a distant light. If none of the above is available leave a flash light open far from your spot and focus on that. You can even focus manually on a bright star. Strangely, on digital SLRs the focus to infinity is not at the end of your lens focus ring; it is just a little bit before the end. Take some test shots on a bright star. If you have live view on your camera, use it. Inspect the results until you have pin point sharp stars. Don’t forget to set the focus on your lens to ‘Manual‘ after you done focusing, or else your camera will continue trying to focus during the shooting, resulting to blurry images.
- Frame your subject. Keep in mind that most of the action takes place on the sky but do not overlook the foreground as well. Experiment with every shooting angle available.
- Take test shots and carefully inspect them. The equivalent of a 32 second exposure with ISO 400 is a 4 second exposure with ISO 3200.
- Add perspective to your photo including foreground objects at various distances. Astrophotography is just another kind of photography, so all rules and suggestions that apply to daylight photography apply to star trail photos as well.
- Place the camera on the desired spot, ensure that it is firmly locked on the tripod and the tripod is stable (remember it must be stay completely still for hours).
- Set the camera to Manual mode, adjust the ISO, aperture and speed, set it to Bulb and program the intervalometer and you are ready to go.
9 Break the rules. Some of my best star trail photos are the result of a less conventional approach. Some of my favorite rules to break:
- (Don’t shoot star trails when there is a full Moon around). Well, probably my best star trail so far was taken with the full Moon light bathing the landscape.
- (Don’t point your camera towards the Moon when shooting star trails). Why not? The very bright trail in the photo below is the Moon.
- (Use low ISO). The image above was taken with ISO 6400 (f/4, 30 sec exposures) because I wanted to create a really bright scene.
- (Prefer dark places, away from light pollution). Light pollution is still light! If used correctly you can achieve the desirable result:
10 Photo processing. Some guidelines:
- If you shoot in RAW format, you have to convert the images to jpg or tiff. Use the software that comes with your camera or a better software that supports batch processing like Lightroom or Photoshop. You can alter/improve the brightness-contrast, white balance, curves etc before converting from RAW.
- If you want to remove unwanted elements like airplane trails you have to do separately it in every image. I do this in Photoshop with the ‘spot healing’ and the ‘clone stamp’ tools.
- Stack the images. I use startrails program. It is freeware and very easy to use. You may use a few ‘dark frames’ for better, noise-free results. A dark frame is a shot taken with the lens cap on. Take 5 to 10 dark frames with the exact settings as your normal shots before or after the shooting.
- The final touch can be done in Photoshop or any other image processing software you prefer. I like to do only minor changes, so I usually adjust just a little bit the contrast, saturation and sharpness of the image.
11 … and the most aspect of star trail photography: Have fun! You are more than welcome to add your comments, corrections and suggestions!