It's a process I use which requires the camera be attached to a telescope all riding on an equatorial mount which precisely tracks objects as they appear to move across the sky. It's basically compensating for the movement of the earth as it turns underneath the celestial sphere.
For example if I want to take a 3 hour exposure of a deep space object, I would set the camera to take 1 minute exposures, 180 times which equals 3 hours.
It's done this way to avoid attempting to track object for 3 hours straight with shutter open. No way could my equipment do that as it's simply not precise enough to perfectly track that long. If I tried that it would result in images with stars and objects blurring and having big trails on them not to mention causing other issues like heating up the camera electronics etc.
I then take those 180 shots or exposures and process them using software that can combine all 180 frames into one single image. To obtain really nice quality images, this is not an easy or a simple quick process.
Here's my amateur shot below I took of the Veil Nebula which is an old supernova remnant and the remains of cataclysmic explosion of star that exploded about 7,000 years ago. This nebula is about 1,860 light-years distance from us. This was 16x150 or 16 separate pics at 150 second exposures each, then combined into one.
