I have taken a 20 minute exposure before. I used a simple 35mm camera with a 200mm lense. Used a plunger shutter control and locked it open.
The trick was that we mounted it onto a telescope that I was looking through with my own eye... so the camera was pointed in the same general direction as my telescope was.
The eyepiece I was using had a bullseye. I found a star that was bright enough for me to track and kept it inside that bullseye with electric slow motion controls. I was able to capture an incredible picture with relatively little blur but definitely no star trails.
Of course, many people spend more money and time getting a motor for their camera mounts and polar aligning it so that it follows the sky automatically and without you having to bend over your apparatus for x amount of time...
Consider this one....
http://www.galaxyphoto.com. I've been to this guy's site
(this one)and know that he has his camera set in concrete... permanantly polar aligned!
