Star trails
Portray the movement of the sky at night
As we’ve discussed, the heavens are actually moving through the night at a fair old whack, enough to be recorded as motion blur if your exposure length exceeds the ‘500 rule’ (see page 21), resulting in stars being recorded as elongated streaks, rather than dots. But photography is a creative medium, so what if we were to deliberately allow these streaks to occur, to help convey the stars’ sense of movement in our static image? This technique is known as star trails, but for a meaningful movement to be recorded we really need an exposure time of at least an hour, stretching to several hours long for longer, more impressivelooking trails. Such lengthy exposures introduce their own set of problems. The longer the sensor is exposed, the greater the build-up of image noise, and over extremely long exposure times indiscernible ambient light will build up, fogging the image.
Combined exposures
The solution is to take several shorter exposures of a few seconds long, then merge them together in software. To do this you’ll need your Nikon’s built-in interval timer, or use a remote release that can be programmed to open the shutter at set intervals. Where you point your camera will have an effect on the trails – for concentric circles, align your lens with Polaris, though trails also work well to the northeast or northwest in the northern hemisphere. If you face south, you end up with horizontally orientated trails.
It’s important to also take foreground interest into account to give the image context. Silhouetted forms can work well, or you can light paint a final frame with a torch. You can change your camera settings for a better quality image of this final shot, setting Long Exposure Noise Reduction and a lower ISO for a cleaner foreground frame.
Then it’s a case of performing basic exposure tweaks in Lightroom to one of the images, syncing this to the others in the sequence, and merging the shots in software such as StarStaX (bit.ly/starstax) to form the trails.