I have always wanted to do this kind of photography, but with most digital cameras there is simply no way you can leave the shutter open and the sensor on for a few hours. My D40 starts to get some serious noise after a few minutes, and there clearly must be an upper limit due to the sensor overheating. Luckily for me there is a relatively simple way to compose LOTS of shots into one image and achieve the same effect.
For those who only want to mile high review of it, here it is. Take a ton of pictures with a long-ish shutter speed. (for this one I used 10 seconds) and then layer them all in photoshop on top of each other and use the
“screen” blending mode to get them all visible.
The actual application of this can be difficult, because it is either REALLY time consuming, or it requires some thought and scripting. I decided to go for the latter and use my noggin a bit.
Scripting a photoshop action.