Advice on star trail shots.

[reissigree]

I have a D3100 with a 35mm lens, an 18-55mm lens, and a 55-300mm lens. I want to create an image of star trails. Typically, what's the best way to do this? Should I break up the exposures? Or just do an hour long exposure?
I did a 3 minute long exposure here. Am I on track? Am I in focus? Just want to make sure before I hold that button for 20 minutes! Thanks!

ISO was set at 1600, f/7.1, exp. was 3 minutes. Is it too noisy?

DSC_0003 by reissigree, on Flickr

EDIT: I darkened a little in Photoshop using levels.

 

[pgriz]

If you keep the shutter open too long, you'll build up the sky fog. It seems with digital, a better choice is to do short exposures and then stitch them together to get the long trails. But there are members here who have done that very, very well - I just read their stuff...

 

[djacobox372]

Buy a cheap film slr, that way u can do extremely long exposures without worrying about sensor heat causing noise.