How can I set an exposure time over 30 seconds with a DSLR?
Most DSLRs have an exposure time limit of 30 seconds. Now I'm asking myself how is it possible to create images about over 1 hour or longer, like these two:
(I have a Canon 60D)
Yes it is possible with all DSLRs.
The 30s limit of all non-Olympus DSLRs is for timed exposures, meaning you dial in the time ahead of time and the exposure takes up to 30s (or 60s for Olympus).
All DSLRs also have a bulb mode which you press the shutter to start the exposure and let go when you are done. This can also be done with a remote control which is highly recommended to avoid shaking the camera during the exposure.
Bulb modes also have limits but manufacturers do not all publish what it is. For Olympus and Panasonic, this is known to be 30 minutes. Some cameras can take a single exposure of several hours. Regardless of the model you are always limited to the battery-life of your camera unless you use an AC-Adapter and have somewhere to plug it in.
Digitally you can simulate extremely long exposures using Exposure Stacking which basically adds up multiple exposures to make a longer one. A lot of astro photography is done that way, just make sure that your camera does not apply Dark-Frame subtraction between shots otherwise you will have gaps in your star trails. On some cameras you can disable Dark-Frame Subtraction but not on all.
Worthy of note is that some remote controls have just a simple button that you have to hold down for the duration of the exposure. Others have a locking mechanism to hold down the button (so you don't have to hold it for 30 minutes), and others are really advanced with build in electronics so you can program a specific exposure, or if you are doing time-lapse, program it to create an exposure at specific intervals.