Taking pictures of Star Trails

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Believe it or not, there are people who live in areas where they can only see the brightest stars, and only directly overhead at that. For those more celestially-blessed, you can walk out and see a plethora of twinkling stars outside your back door. Well, just about anyone can achieve those fantastic \"star trail\" photos you have seen in magazines and websites.

The first thing you may notice about star trail photos the rotation about an axis. That axis is the pole star Polaris (or the Southern Cross for you southern hemispherians). That star is located directly above the Earth's north pole, so as the planet rotates, that star stays nearly immobile. So why is the north star important for taking star trail pictures? Well, the most dramatic of these photos show the pole star as a draw to the eye. When properly positioned in the FOV, a more dramatic photograph can be taken.

Locating the Pole star(s)

In the Northern hemisphere, locating the North star (Polaris) is extremely easy. You have two huge guides to help you: The Big Dipper and the Little Dipper (the constellations Ursa Major and Ursa Minor, respectively). The Big Dipper leads to it and the Little Dipper's tail is formed by it. The Big Dipper should be easy to find in almost any location, though depending on the month of the year, it could be directly overhead or low to the northern horizon, but it will always be visible. Use its two end stars that make up the \"dipper\" and follow them up to a bright star, which forms the tail of the Little Dipper. That should be Polaris. If you're unable to find either, a simple compass should get you oriented to the north.

Getting Set up

While technically simple, to ensure good results there are a few minimum requirements for taking these photos:

• Sturdy tripod
• Shutter release cable or remote
• Fully-charged battery(s)
• Wide-angle lens
• Mirror Lock-up (MLU)

The obvious key component here is a stable tripod. Something that will hold your camera stable as long as you wish to make your exposure. The shutter release cable or remote is a great tool that will eliminate any sort of vibration created when you release the shutter button with your hand. Additional vibration can be eliminated by using a firmware feature called \"Mirror Lock-up\" or MLU. While standard in the XT, this feature is only enabled on the 300d with the use of a modified firmware. What MLU does is locks up the mirror, waits a predefined number of seconds, then opens and closes the shutter. A subtle source of in-camera vibration is the \"mirror slap\" cause by the movement of the mirror during each shot taken. (The mirror must move out of the way of the shutter). If you don't have a shutter release remote or cable, MLU is your next best way to eliminate any sort of vibration.

Unlike mechanical film SLRs, modern film and DSLRs require battery current to hold the shutter open: These cameras use electronic mechanisms rather than springs and mechanical locks to keep the shutter open, so a full battery (or batteries with a vertical grip) should be fully charged before you start.

To capture the widest field (the FOV is really up to you) the wider lens you use, the better. I recommend using a distant street light or porch light to focus as close to infinity as possible, then once you're satisfied with the focus, set the lens to manual focus to ensure that it doesn't try to refocus when you activate the shutter. If its not possible to use the AF or your viewfinder to achieve sharp focus, you will have to use trial-and-error with the viewfinder to check your focus. Set your camera to the \"M\" mode. Use the f-stop that you know is your lens' particular \"sweet spot\" where its sharpest edge to edge and set it, then set the exposure time to \"B\" for \"bulb mode\" (the term originates from the old bulb-style shutter releases of yore). Double check that the camera has MLU set (its under the Custom Functions menu). Set it for at least 3 seconds, optimally at 10 seconds.

Composing the Photo

As I discussed above, the central focus of the eye will naturally be the pole star(s); the concentric arcs lead the eye there. I'm not going to get too far into ideal composition here, but it would be a good idea if the pole star was not centered in the FOV, but in the top or bottom thirds of the frame, slightly off center left-to-right. If you have any interesting foreground objects, then you're very lucky and should find a way to incorporate it: windmills, barns, interesting trees or even natural landforms make an interesting foreground for your photo.

Taking the Shot

There is no bible on how long to make the exposure, but there are some things to think about. If you're using a narrow FoV and you're field includes the pole star, you will need a longer exposure to show longer trails. stars closer to the pole appear to move slower in relation to it as it rotates. stars farther away from the pole star will leave longer streaks in the same amount of time. See the photo to the left.

If you have a significant amount of light pollution in your area, a longer exposure may result in \"sky fog\". You may have to step down your aperture to compensate, but don't worry, the brighter stars will still leave their trails. For extremely long exposures a low ISO will counteract this better than ISO1600. Again, experiment with your settings for your particular sky conditions.

Each star will take 24 hours to fully rotate around the pole star, but with dawn and dusk, you'll never achieve a complete circle (unless you actually live AT the pole!). Experiment with exposure lengths. Try 10 minutes, 15 minutes or 45 minutes. Longer streaks don't necessarily mean they'll be more interesting than shorter ones! In fact, if you have dark skies with a lot of stars visible to the naked eye, a long exposure of so many stars might make for an overwhelming star trail photo, but a less dark sky with only brighter stars visible might make for a better long exposure.

Some other tips

Black Hat Trick If you're using a stock 300d and you don't have MLU or a shutter release cable available to you, you can still manage to take vibration-free, long exposure photographs. One old method is the Black Hat Trick. It involves blocking the front of the lens with a dark hat or something similar, opening the shutter and counting to 10, then quickly moving the hat away from the lens letting the light through. The vibration is allowed to dampen out before the light hits the camera sensor or film.

Lines and dot

An interesting effect I've seen a few times is a long exposure as normal, then before the shutter is closed, a black hat or other light blocking object is placed in front of the lens blocking all starlight for a few minutes, then removed for a minute or two before closing the shutter. This leads to a trail accentuated with a dot.

star Colors

This is another effect I've seen a few people do and the most interesting result is the true colors of the stars is revealed. This requires some patience and a stop watch. Basically what happens is at specific timed increments, the focus is adjusted away from infinity. The trick here is to do it smoothly and evenly. Define your increment, it could be something like 5 minutes and use a stopwatch to time it. At each interval, block the lens with a dark hat or other object, then lightly adjust the focus, let the settle (5-10 seconds) then removing the light block. Photo credit: Andrew Chatman.

In Conclusion As you can see, there are a lot of steps involved in taking what seems to be a simple photograph of star trails! In addition, there are more ways than one to capture this mystical effect. Patience and trial-and-error will eventually lead to fantastic results with the Digital Rebel SLR.