Long exposure shots...what am I doing wrong?

[skillz03]

Thanks guys! I think I may have narrowed down the problem after reading through this. More info on the environment I was shooting in:
1. I was already using a tripod and it wasn't windy at all (so there probably wasn't much shaking).
2. I was also using a wireless shutter release, so no shaking there.
3. There is no VR on this lens, so it couldn't have been that.
4. I don't think the blurriness was an issue with the time the picture was exposed for (though I will keep in mind all of the advice here posted about taking static pictures of stars vs. star trails).

I think this has to do with an issue of setting my focus to infinity. When I took the pictures, I thought I was setting it to infinity by switching to manual and turning the focus ring completly clockwise to the end (since I don't have an infinity marker on my lens). However, as I started reading online, it turns out infinity isn't always at the end.

So my new question is, how do I focus on infinity without the infinity marker on my lens. Is there a camera setting on the D3100 that I can turn on that tells me when I'm focused at infinity(like the green light that shows up in the viewfinder when you're in focus)?

Thanks for all the help everyone!

 

[manaheim]

Some tips on this stuff in my sig...