Star Trails

One of the best experiences you can have outdoors is to bivouac under the stars. For my ramblings on this topic see my page on bivouacs. If you have chosen your time well then one of the memorable things about these nights is the stars. There are so many, and they are so bright, that it is possible to read by starlight. Being photographers it is a small jump from admiring a sky full of stars to wanting to capture them on film. It’s not difficult technically but it does require special circumstances to come together.

Equipment

There are two bits of equipment you will need to take star trails and other night sky shots. The first is a tripod. The second is a fast lens. The focal length of the lens is not important but the size of its objective is. The larger the objective the more light will go in and the more faint stars you will record. The focal length is therefore a compromise between the angle of view and the light capturing ability of the lens. A 28mm f2 will have a larger field of view but will let in substantially less light than a 85mm f2, but the 85mm will have a field of view that is a bit too narrow. A good compromise is that old veteran, the 50mm. For the objective it’s a case of size that counts. An f2 is a minimum, but if you can stretch the budget a bit then f1.4 is ideal. True, f1.2 is a bit faster but they are generally much more expensive than f1.4 and the gain over f1.4 is not that great. The good news is that with everyone dumping their old kit for new, there are some amazing bargains to be had. I picked up my 50mm f1.4 on eBay for US$25 – the outside is worn but the glass is in 100% original condition. Be careful to check that your camera can take older lenses before buying. Not all can and some of them can even damage the camera.
This is one subject where a film camera thrashes a digital hands down. For star trails the exposures are long, and they can be very long. With digital you will flatten the battery and the sensor will heat during the exposure leading to coloured points of noise that you will have to clean up later. Film is much better. I use my old FM10 totally mechanical body and it works superbly. If you do use film then stick with speeds of 200ASA or less. If you go higher you will record more stars but the noise will become too obvious.

Shooting
To take star trails you need two circumstances to come together. The night, or at least part of it, must be moonless. If even a crescent moon is out the number of visible stars will drop off rapidly. Also you must be out in a place that has no artificial light. In exposures measured in hours even a level of light that is invisible to the eye will be recorded. You should be many miles from any towns and major cities should be even further. You should be able to see stars clearly right on the horizon. A rough guide is to be at least 10 miles from small towns and at least 30 miles from big cities. Once you have met these conditions then the rest is easy.
Set the camera on its tripod and aim it to the part of the sky you want to record. You can aim anywhere in the sky but these photos really work at their best when there is something else in the frame. Another difficulty is that the other object you want in the frame has to be quite far off or it will be blurred as the depth of field with a 50mm f1.4 lens is quite small. For this reason wood or forest clearings are poor places for these images, whereas mountain and open country camps are perfect. Trees are fabulous if you can get one far enough away, as are mountains or rock features on the horizon.
Ensure that the camera is on its B setting, the lens is wide open and focussed on infinity. You will need a cable release to hold the shutter open. Attach this, fire the shutter, lock the cable release and retire. Try exposures of at least half an hour but if you are in a really dark location then you can record for three or four hours. If you want curved trails then you must aim the camera toward the pole. if you have the pole star in the frame then you will get round trails centered on the pole star. If you aim toward the equator, ie ninety degrees from the pole star then you will get almost straight trails.