I love to make long exposures of star trails, particularly star trails that feature Polaris – the North Star* – so you get nice circular trails. But there are some gotchas when doing this the traditional way.
Back in the film days, long exposures opened you up to reciprocity failure. And that meant if it was a cold night you were going to be colder then usual.
Reciprocity, and its failing, is not an issue with digital sensors, but noise is. If you want to do a one-hour exposure, for example, you really open yourself up to fixed-pattern noise – something we covered a few weeks ago. Now you can deal rather easily with that noise by using your camera’s long-exposure noise reduction turned on – but the trade off is that this requires an additional exposure at the same length as your first exposure. So a one-hour exposure turns into a two-hour exposure and once again you’re goinna be colder then usual.
I presented a technique just over a year ago that got into stacking – the idea of taking many shots over the same time period instead of just one. This has a number of advantages to it in terms of how long you spend in the field but means a little more work on the computer after the fact. I’m perfectly at home with that idea on a cold night actually. But to make it even better, I found a tutorial about star trails on the Interwebs this morning from Reno, NV-based photographer Dan Newton that absolutely rocks. Dan meticuously outlines his in-camera and post-processing techniques so that anyone can pull this off. It’s so good that I plan to abandon all previous techniques and just use his method. In fact I think if you make a nice “long-exposure” image with his technique you should post a link to the image in the comments of this post. The nice thing about Dan technique is the fact that it doesn’t rely on long-exposure NR at all – at least in camera. Instead of doing your one-hour exposure and then one-hour of noise reduction, imaging doing 120 30-second exposures (one hour) and then ONE 30-second exposure (30 seconds) for all of you noise-reduction. Yeah – I can dig that. Dude.
If you spend a little time on Dan’s Portfolio site, you can see plenty of examples on star trails. It’s some nice work – and some of it was just featured on the cover of the UK’s Practical Photography magazine. Also, here’s some additional background on long-exposure/star trails from photographer Dan Heller that focuses more on the traditional one-shot technique.
* – you noticed the asterik at the top of the article? Hooray. There’s a little mystery over the North Star. It is not the brightest star in the sky – in fact it’s a little on the dim side (but fortunately surrounded by even dimmer stars making it a little easier to find). The magic is – in the Northern Hemisphere – is that it is the pole star. If you stood exactly on the north pole, this star would be almost perfectly overhead. Imagine then, as the earth rotates, that the North Star would stay perfectly still while all other stars rotate around it. The bonus is you still get that effect if you put the North Star in your shot – all other stars appear to rotate around it as the earth spins beneath you – it just takes a little time to see the effect. So it would probably be helpful if you knew how to find the North Star (also called Polaris), doncha think?
UPDATE: I was reading more old posts on Dan Newton’s site, and I came across this update to his technique post. He’s started using an intervalometer to overcome the buffer limitations on his camera. Some cameras have an intervalometer built in (lucky me and my D300). Some require a cable-release with a built-in intervalometer. His original technique is still king, but requires a little higher ISO because of the 30-second limitation in shutter speed. And most cameras, on continuous, eventually fill up their buffers before very long. With the intervalometer, the file is written out after every shot and you can shoot until you fill the card. And you can shoot slightly longer at a better ISO.