Shooting flats seems difficult, but once you figure it out it’s easy and very reproducible, assuming you’re pedantic about the details. Hopefully my method for creating good flat frames will make sense to you!
When I started astrophotography I was like most people, and only shot light frames, I didn’t want to take the time to shoot darks, bias, and flats. Now I won’t process astronomical data without them. I consider it a wasted imaging session if I’m not able to shoot darks and flats to match (bias are easy and can be shot anytime). That’s also why in the interest of time saving, I consider a master dark library one of the most important things you can keep around, but that’s for a another blog post.
While it’s best to shoot flats immediately after you finish your imaging session, you can buy yourself a little time if you are able to remove the scope from the mount without detaching the camera or moving focus. I’ve also made successful flats this way in the field.
Note: This method works for telescopes and camera lenses.