They take some doing to get right, but once you do, they are fantastic.
This was taken on a very clear night on the 12th/13th of September 2009 on the Great Orme, Llandudno.
First, I’ll tell you what you need.
Scout your location in the day time, because your going to be working (ideally) in the pitch black night. You need somewhere with a clear view of the sky, and no light pollution. That means anywhere built up will not work, and even looking across to somewhere with a built up area. I use the sea because there are no cities in the sea… obviously!
Make sure it is accessible via foot or car, and plan. You need a clear night, and you need to be relatively awake.
Now for the photography part.
Get there, and use a torch to see where you are. Set your camera on a sturdy tripod, and take a few test exposures. You want to see the stars, and with as little digital noise as possible. I recommend for a completely dark area using 30″ exposure, as wide an aperture as possible, and an ISO of 200-250. Obviously adjust as necessary. The focus just before infinity (a lens with a focusing ring and dial such as the Sigma 10-20mm) is perfect for this. Nice and wide, and you have a focus guide.
Ensure nothing gets on the lens, and nothing knocks the tripod, including wind. A decent tripod is essential. I use a Manfrotto 190XPROB with a Ball head. Check the exposure, and how level it is. If you are by the sea, use a spirit level for the hotshoe – great for levelling out the camera, straightening the horizon, and of course, the rules of physics never lie…
When you are sure you have a good exposure, lock the exposure onto manual and put it on continuous mode, with the cord locked on to take photos one after another. This leaves you free to do whatever you want. Bear in mind it is a deserted, pitch black area where you will be… take someone or something along for comfort or company. Also, This is where a clear memory card comes in. Take as many photos as you want.. the one above was taken with 191 photos over the course of 105 minutes.
For the something, I put to episodes of Spooks from iTunes on my iPhone and watched them whilst it was taking pictures. Also, take a spare battery (it can get very cold), a cloth to wipe the lens in case dew forms.
When your done, pack away as normal. And go and process them using something like startrails.exe.
For more, check out this flickr group – http://www.flickr.com/groups/startrails/