Deep South

I thought I should post a little technical explanation as I've been asked a few times about the mechanics of a shot like this. I'm a great believer in photographic gear being the least important ingredient in a good image. But sometimes there's a minimum of gear and technique required. For star-trails the basics are a sturdy tripod, a digital SLR with a fully charged battery, a torch, a locking shutter release and a free bit of software. Even a compact digital camera with manual controls can work, but image quality with longer exposures is always going to be an issue.

While these ingredients seem simple, there's a few traps for the unwary. The first is seeing what your aiming at! Without a bright subject it can be very difficult to see through the eyepiece or even use a live-view lcd screen. The easiest answer is to take some short test shots at high ISO until it's right. Then you realise that focussing becomes an issue! Manual focus is a must and can be done through the eyepiece if there are bright lights in view, although stars usually aren't bright enough. A magnified live view is the best answer but without that, my favourite technique is to test the best focus point of a lens (for infinity) during the day and make a note. Then it's easy to set. Provided you have a torch handy. And don't forget to turn off any lens or body image stabilisation. Once focus and framing is sorted, attach a locking remote control, set the “drive mode” to continuous, select 30 seconds in manual mode, the lowest ISO, and an aperture of f4 or faster.

It might be tempting to put the camera on “bulb” mode and shoot a 30 minute exposure. But longer exposures lead to vastly more digital “noise” which is blotchy unwanted colour, especially in the darker areas of an image. So short (30 second) images are the way to go. Take, then check, one test image. Then lock the remote on and pull out a good book. Or settle back and watch the sky for a while. The stars will make a full circle in almost 24 hours so a total time of 2 hours will give you 1/12 of a circle, centred about the south celestial pole for those us down under. The end result below was from 150 30-second frames or 75 minutes worth.

One the photos are downloaded, the next step is to stack them together. My choice is the excellent and free program called StarStaX (for win, mac & linux). The end result should look good unless you've included a test shot by mistake, bumped the tripod or the lens has become covered in dew! With a bit of processing you can bring out the fainter trails and, in the example above, I reduced the brightness of the city lights by using only one frame for that area.

I hope that doesn't sound too daunting. Unlike many things in life, it's easier than the instructions would suggest!