Star Trail Photography

Would you like to take some interesting photos that will wow your viewers, and you get to enjoy some time in solitude? Why not try some star trail photography!?

As our planet Earth rotates around its axis every 24 hours, the star field that we see in our night sky rotates around the polar axis in a complete circle. Of course, in reality the star field is relatively stationary with respect to the Earth (over eons it is not, but in the context of our lifetime it is); it is the Earth that rotates about its own axis. Photographing this phenomenon is exciting and challenging.

In this article learn:

• All the information you need to take successful star trail photos.
• How to take these types of shots in a single exposure.
• Techniques where one takes a series of short exposures rather than one long one.
• How to assemble the resulting images in Photoshop or some other application capable of automating this assembly process.

Tufa Star Trail – Mono Lake California, 45 minutes, f/4, ISO 200 (Polar aligned image – facing north)

Equipment

In order to take successful star trail photos, the photographer needs a few items besides the obvious. The photographic equipment required includes the following:

• Camera capable of bulb mode and long exposure noise correction (also called dark frame subtraction).
• Freshly charged warm battery or an external power source – keep the battery in a warm pocket if the temperature is below 50 degrees Fahrenheit to maximize its power. The battery should not be a battery that is old or nearing its service life (500 charge cycles). If your battery charger has a deep discharge cycle, use it prior to charging the battery.
• A fast wide to normal zoom or prime lens (16-35mm f/2.8, 24-70mm f/2.8L, 24mm f/1.4, 35mm f/1.4, 50mm f/1.4).
• Locking cable release. Most cameras will only allow up to a 30-second exposure without either holding down the shutter button or locking the shutter via a cable release.
• Sturdy tripod and tripod head.
• Bubble level.

You may ask why is such a fast lens needed? It isn’t for the exposure itself since we will typically make those around f/4, but you need it to be able to see your subject and compose the shot. The more light the lens transmits to your eyes, the easier it is to set up the photo.

Digital cameras can have stuck and false color pixels when exposed for such a long time. Dark frame or long exposure noise reduction eliminates this by taking a second exposure where no light hits the sensor for the same amount of time. It then maps any pixels that aren’t black and subtracts those from the photo, thereby eliminating these falsely rendered pixels.

Other equipment you will need to simplify the process and keep yourself healthy is as follows:

• Headlamp to see where you are going and help operate the camera.
• A strong flashlight to illuminate foreground subject so that you can compose the shot and/or focus properly.
• Hat/gloves except in summer – the temperature can drop as much as 20 degrees during a single exposure.
• Something to keep you occupied in the dark with absolutely no light.

Preparations

Preparation can make the difference between a successful shot and a lot of wasted time and loss of sleep. Picking the spot for a star trail shot is best done during the day. The best star trail shots have an interesting foreground element and a view of the sky from the east towards the north or west in the northern hemisphere and from the east towards the south or west in the southern hemisphere.

The location should also be away from artificial light.

High elevation areas will reveal more stars than low elevation areas. In general I recommend 150 miles from a major metropolitan area of 1 million plus, 125 miles from cities of 100,000 to 1,000,000, 75 miles from cities in the 50,000 range, 50 miles from a 25,000 person area, and 25 miles from a smaller town. Nothing can ruin a star trail image faster than light pollution, so this also means staying well away from any roadways or any other source of light. Humanmade light is your enemy. Use your lens hood as an additional precaution.

Check the weather forecast – you are looking for a totally clear night. While checking this, also check sunrise and sunset times as well as moonrise and moonset times. Check the moon phase. This will be important for exposure (to be covered later). If it’s a full moon, you might want to wait a few days as the full moon provides too much illumination in most cases. The new moon phase is also not very conducive but is better than a full moon – it is easier to add light to your foreground than it is dealing with too much light.

Make sure you dress appropriately for the nighttime conditions. As temperatures can drop dramatically, especially in the early morning hours after midnight, be prepared for the worst. Plan to shoot after 11:00PM to avoid airplanes, start at least 2 hours after sunset and finish 2 hours before sunrise.

Here are a few more preparation items that are important:

• Locate hyperfocal setting of lens at f/4 and note the setting on the lens’ distance scale or mark on the lens if the foreground object is relatively close. It is almost impossible to accurately determine the hyperfocal setting in the dark.
• Turn off image stabilization/vibration reduction; not only will it drain the battery faster but it will cause some image drift if left on for really long exposures.
• Turn on long exposure noise reduction – this will eliminate most or all pixel problems due to the very long exposures.
• Remove camera straps or anything that can blow around or vibrate the camera/tripod set-up should a wind be present or come up.

Star Trails over Halfdome, 25 minutes, f/4, ISO 200 (Non-polar aligned image – facing east)

Time

Picking a time to take your shots is important. In general, the darker it is, the brighter your star trails will be. Lack of pollution or haze can also be a big factor in clear sharp star trails.

While it is very dark in most places by about 1 ½ to 2 hours after sunset, shots taken in the evening hours often have the problem of airplanes flying through the frame. The strobes can cause a repeating light streak through the photo that isn’t difficult to eliminate but certainly very tedious, especially if there are several airplane trails. Most air traffic dies down by 11:00PM or so and in the hours between 2:00AM and 5:00AM there are virtually no overland flights. I typically prefer to photograph star trails in the early morning hours around 2:00 to 5:00AM. In the winter where the sun rises in the southeast in the northern hemisphere, letting the end of a star trail shot end around 1 hour before sunrise can sometimes add a little illumination to your foreground subject while not destroying the star trail. This does not work as well in the summer when the sun rises in the northeast as it can be very easy to have an overexposed area near the horizon.

Locating Polaris

There are two types of star trail shots. The first is the polar-aligned shot which results in a circular star trail pattern around the North Star, Polaris. The second is a non-polar aligned shot which results in nice arcs but not complete circles. For non-polar shots, any alignment north of the east-west plane will give you these arcs in the northern hemisphere (or south of the east-west plane in the southern hemisphere). I generally avoid southerly shots in the northern hemisphere and northerly shots in the southern hemisphere as the trails straighten out too much and often do not look like star trails and just look like a scratched negative.

For the polar-aligned shot, one must learn to locate Polaris in the northern hemisphere. Polaris is quite easy to locate as you can extend a line from the outer edge of the pan on the Big Dipper (Ursa Major) to the next bright star as illustrated below with a picture of the Alaska State Flag. For a polar aligned shot in the northern hemisphere, making sure Polaris is at or near the horizontal center of the frame will yield circular star trails. The farther south you are in the northern hemisphere, the lower in the northern sky the circles will appear since Polaris will be closer to the north horizon. In the southern hemisphere, locating the Southern Cross will yield similar results, but you will not get the single stationary point that Polaris provides in the northern hemisphere.

Alaska State Flag – align the outer edge of the Big Dipper to the next bright star (note that the Big Dipper will rotate around Polaris)

Taking the Shot

Finally, all of the preparations are complete. You have your equipment properly prepared, you have your locations selected, it is a clear night, the planes have stopped flying, and you are ready to shoot.

Set up your tripod, camera/lens, and cable release. Make sure the camera is in bulb mode. Set the lens to f/4. Use your flashlight if needed to illuminate the foreground and make sure the foreground object’s distance is within the depth of field of the lens at the hyperfocal setting. If the foreground object is effectively at infinity for your lens, you can autofocus on the moon and then turn off autofocus. Just be sure not to bump the focus setting. Use your headlamp to check your camera set-up and your bubble level to make sure that the shot is level. If you are planning on a polar-aligned shot, make sure that the center of rotation for the star trail (North Star Polaris in the northern hemisphere and Southern Cross in the southern hemisphere) is in the right place in the frame. I usually shoot these types of shots in vertical mode; however, you can let your creativity run wild here.

At this point you are ready to take the shot. There is just one thing left to be decided. What exposure time should I use? After doing many of these types of shots I have come up with values that almost always work. Note that photography during a full moon is

not recommended as exposure times are too short for effective star trails. Lowering the ISO to lengthen the shot generally makes the star trails a bit too faint unless at high altitude and an exceptionally clear night.

Fortunately there is a lot of margin for exposure error. Let’s say you are taking a 45-minute shot and fall asleep for an hour. The extra 15 minutes is only 1/3 stop more than the 45 minutes you had planned which will rarely cause a problem. As photographers we tend to get very nervous as the exposure times get long. We fear over-exposure more than anything. This leads to the very strong temptation to get impatient and cut the exposure short. Even doubling the exposure times above will not overexpose the shot to the point of it being unusable, so rest easy and resist the urge to stop the exposure too soon. It is necessary to point out again that you generally will not want the moon in your shot. The moon is a light source for the shot but not part of the subject in the vast majority of cases.

If you are taking a star trail photo under a new moon or before the moon has risen, you have a lot of latitude for shutter speed. If you do not want to wait a very long time, you can take a 15 or 20 minute shot and paint your foreground subject with light. Either a flash burst of about half the normal power for the subject distance or painting the subject with a flashlight can add light to the foreground when lacking the moon as a light source. When doing this, be careful to avoid unnatural shadows.

Once you trip the shutter with the cable release, lock the cable release and walk away from your set-up. You do not want to risk accidentally bumping the set-up. At this point, resist the temptation to use a light source unless it is intentional such as when painting with light. Even the light from lighters can be enough to cause problems in the shot.

A healthy, fully charged EOS 1D Mark II or 1Ds Mark II battery can take two 45-minute exposures at 32 degrees Fahrenheit and complete the dark frame subtraction with no problem if the camera is shut off and brought into a warmer environment such as your car.

Post Exposure Processing

Post exposure processing of the shot starts with the in-camera dark frame subtraction/noise reduction. The camera takes a dark frame of equal time and then analyzes this for hot or stuck pixels to know what to zero out in the photo you just took. Some cameras allow you to keep shooting while the dark frame subtraction is happening, others don’t. Turn off the camera as soon as you are done shooting. The dark frame subtraction will continue to be processed without any other parts of the camera drawing power from your battery. If you can get your camera into a warm environment, this will also prolong the battery’s life. Do not remove the flash card until the camera is done.

Once the image is in the computer, you will need to select a white balance setting. The moon’s illumination is about 5500 degrees Kelvin; however, sometimes for effect, a lower temperature will render a better result.

When adjusting your levels, it is OK to clip the highlights, as these will be the star trails. Usually its OK if some of these are totally white. This will then help the contrast of the foreground as well. Set the black point as you would normally.

Besides these considerations, post processing is like any other photo.

Star trail photography can be an extremely rewarding and inspiring addition to your photography repertoire. It can open up new ways of expressing the beauty of a subject and excite the viewers of your pictures. While not difficult, per se, the logistics and preparations to get a great shot do require time, pre-thought, and patience.