The key to a good star trail photograph is clarity. A long exposure will render stars as a mass of densely packed lines, as the Earth rotates on its axis. Having a static feature in the foreground helps in the composition with the movement of the stars in the background. Branches and leaves are usually not a good choice unless the wind is calm.
To make a nice photo with stat trails expose for the foreground then take several exposures with the same settings. Use some software such as StarTrails.exe or StarStax to stack all your images creating the final result. This is better than a single very long exposure for several reasons: you avoid thermal noise due to sensor heating and if something like an aeroplane, car or flashlight bearing human crosses the frame you only lose one frame and not the whole picture. If the battery dies you can change it and continue.
An intervalometer is a must-have piece of equipment for star trail photos. Program it to take as many exposures as you want with a given exposure time and then just sit and relax.
Equipment for Star Trail Photography
The mighty trio: tripod, head and camera. A solid sturdy tripod with a good head is a must for this type of photography. You will generally be taking exposures from four seconds to one or two minutes, and you need the camera to remain stable during the whole exposure. A little wind can seem like a major hurricane if you use a flimsy tripod. Choose your tripod head carefully. Geared heads, like the Manfrotto 405 or 410, which can be adjusted in any direction step by step are really useful for nightscapes.
An Intervalometer is also an essential piece of equipment. A remote controller, such as the Canon TC-80N3, functions as an intervalometer and can be set to trigger exposures at set intervals.
If you want to shoot long exposure times without star trails, you might want to consider a portable tracking mount, such as the AstroTrac or Vixen Polarie. These go between two tripod heads and once aligned will follow the Earths rotation making the stars stay round for even longer exposures. With a tracking mount exposures of one to five minutes are common with a wide angle lens. A disadvantage is that the landscape will blur, so you have to find the maximum exposure that will make your landscape look good. Having the land features as far as you can from the camera helps.
Dealing with Noise
The mighty enemy of your attempts at astrophotography is noise, and to deal with this nothing beasts sensor size. If you can use a full-frame camera, by all means, do. Modern APS-C sized cameras are becoming better and better, so you can also use them and get great results. Using a fast lens also helps.
Star Trail Photography Tips
- The longer the focal length, the faster the stars will trail; with a wide angle lens you need several minutes, sometimes even hours for the effect to be nice.
- Point the camera east or west if you want the stars to move faster. The movement is slower when you aim the camera close to the celestial pole.
- If you want nice concentric circles, use a compass and point the camera to the north (if you're in the Northern hemisphere); the point of rotation known as the celestial pole will be as high in the sky as your degrees latitude. If you are shooting at 66° north, the celestial pole is 66° up towards the north.
- Noise isn't just a problem at high ISO's, it can also be a problem when using long exposures. This is why it is better to merge multiple shorter exposures.
- If your camera has built-in long exposure noise reduction, turn it off. When activated the camera tries to subtract ugly noise by taking a second dark frame. This process will double the exposure time and leaves long gaps in the star trails.
Star Trail Calculator
Without tracking or a motor driven mount, long photographic exposures will create star trails as the Earth rotates. This gives the appearance of stars streaking across the sky, which can make interesting photos. Sometimes, however, you may want to take a photo of the stars without trails. How long exposure can I use before stars start to trail? or in reverse How long do an exposure do I need before star trails are shown?.
This simple calculator will tell you how many seconds before stars start to trail. Any exposure above this will have star trails, below should have no trails.
This calculator will only give a rough estimate based on the focal length of the camera and the declination of the target in degrees. Actual results will vary between lenses and cameras, so a little experimentation will be required. This tool will give a ballpark figure.