Using a star tracker
Get the ultimate image quality in your astro-landscape scenes
Guy Edwardes
Based in his home county of Dorset, Guy Edwardes has been a professional landscape and nature photographer for over 25 years. His work has been widely published and he runs a series of popular photographic workshops all over the world. Visit www.guyedwardes.com.
I SHOOT mainly wide-field astro-landscape images using a wideangle lens to capture both the night sky and some form of interesting landscape element in the foreground. I use a Canon EOS 5D Mark IV with either an EF 16-35mm f/2.8L III lens or an EF 8-15mm f/4 and a star tracker. This involves mounting my camera on a device that, once aligned with the celestial pole (North Star), will turn it at the precise speed necessary to compensate for the Earth’s rotation.
There are many advantages to using a star tracker. Firstly, you are no longer limited to exposure times of 15-20 seconds, as the tracker allows the use of much longer exposure times whilst recording the stars as sharp point sources of light. Therefore, much lower ISO settings can be used, which results in virtually no digital noise in the final image. Fainter stars will become more visible and there will be more detail and colour captured in features such as the Milky Way. Basically, the quality of the image will be vastly improved. Secondly, you can continue using whatever wideangle lens you already own, as you no longer need to shoot at very wide apertures – I typically shoot at around f/5.6. When you consider that the basic star tracker I use costs far less than most high-quality fastaperture wideangle lenses, this can be a serious consideration.
One important point to bear in mind is that because any foreground elements in your composition will be blurred owing to the movement of the tracker, you will always need to shoot two images – one image for the foreground with the tracker turned off, and one for the night sky with the tracker turned on. This is all done without changing your composition. The two photographs must then be blended together in software such as Photoshop using layers and masks. This is usually a simple process that only takes a few minutes, as long as you have a fairly simple well-defined horizon.
It can be difficult to blend the images in some circumstances, such as where the beams of a lighthouse project against the night sky, and in these cases I would normally revert to shooting an un-tracked image at a high ISO.
There are some disadvantages to using a tracker. It takes time to set up and each time you move your tripod you have to align the tracker with the celestial pole all over again. You also have to spend time blending the two images together. However, the huge increase in overall image quality that a star tracker provides means I can overlook these issues for the amazing results that it makes possible!