Pretty much spot on as Daniel put it. You need to be looking directly at polar north/south to get the best concentric effect.


You'll notice too that the further away you go from the polar origin the longer the star trails will be as the radius of curvature increases.


To get a decent trail you need to expose the equivalent ofat least an hour of the sky (my opinion anway). The reason I say "equivalent of" is that opening the shutter for this longyou riskone (or all) of three things:


1 - the image will be super super incredibly super dupernoisy (that's a bad thing btw) because as the sensor heats up from gathering all that info for such a long time excess digital noise is introduced. So you won't get a usable photo.


2 - you will turn your camera into a molten blob of plastic resembling an under exposed dog poo. That's a slight exaggeration (slight) but there is a real risk of overheating the internals on the camera and frying circuit boards/ chips etc.


3 - you will run out of battery. Especially with long exposure noise reduction (LENR)on as it will need roughly the same time as the exposure to complete LENR.


If I take a photo of the night sky and want star trails I generally do a couple of test shots to work out the best exposure. i.e exposing the stars enough to see them brightly but not introducing too much ambient light into the sky/scenery - sometimes 10-15sec is enough depending on the moon.


Once I've found the best exposure I'll switch OFF LENR, set the camera in continuous shooting mode with a remote trigger and leave it going to an hour or two. This way if the battery dies mid shot you don't lose a whole lot.


Then you need to merge all of the photos into one using either photoshop or another stacking application. I'vegot the best results using the startrails.exe app.


Remember to keep something interesting in the foreground that you can expose using a torch, or better yet take a couple of shots as the sun goes down and you'll get some beautiful colours.


I've scoped out a nice spot and I'm going to head there over the weekend to get a shot or two... hundred. I'll post something up if I think it's worthy.


Best thing to do is have a go and experiment. Have fun!