My first experience with startrails was for an Earth Science project in 8th grade. Back then it was fairly straightforward. Growing up in a rural part of New York, I simply waited for a chilly, clear night to set up the tripod in the backyard with my Pentax K-1000, set the focus at infinity, opened the aperture all the way, put the shutter on Bulb mode and tightened down the shutter release for an hour or so. Repeat a few times and then develop the film and see what you get!
The procedure is not quite as straightforward with digital cameras. The obstacle with digital is that exposures beyond a few tens of seconds will be dominated by noise from the sensor. If you have never tried a long exposure before, give it a try some clear night. You may be surprised to find that your image will include not only white stairs but also vibrant blue, green and red “stars”. This is noise and it is inherent in any digital camera. The approach for startrails is fundamentally different than with film. Rather than take a single long exposure, the idea is to take multiple shorter exposures and stack them in the processing stage. Moose Peterson has a very well written overview for star trails – including his “prescription” for camera settings. However, it is vitally important that you experiment with your equipment and find what works best. For example, Peterson suggests shooting 4-minute long exposures at f/1.4 with an ISO of 200. Since I do not own an lens that allows such a large aperture, I found what worked best for me was to shoot 2-minute long exposures at f/4 with an ISO of 100. In addition to a sturdy tripod, you will also need a remote shutter release with an interval timer. That way you can simply program it to take, for example, 50 exposures of 2-minutes each and separated by 1 second. One absolutely essential step is to take a dark frame. Simply put the lens cap on and take one more exposure (of whatever length works best). The dark frame will later be subtracted from your stacked images and will do wonders in removing noise. Now that the images have been captured, it is time to stack them. Fortunately, there is free software to do that for you. And within 30 seconds you’ll have a star trail image!
The steps are straightforward enough but getting a good image is actually quite difficult as it requires the confluence of a clear night (free from clouds or haze), no light pollution and no moon. If you can put all those elements together, then you will be rewarded with a spectacular photograph!