I am surprised at the Harlan Ellison connection having just watched all 16 episodes back to back. As the story more correctly resembles a book called the Star Seekers from Milton Lesser in 1953 for the first part. The mix with strong elements of Robert Heinlein's Orphans of the Sky from 1963. Then add the ship design from Silent Running modified for more domes. You get the story concept of the Starlost. So if considering how much these earlier these stories predate Ellison it more likely he adapted them and the other authors were never given credit.
Oh and in case no one corrected the commented about Silent Running. It predated Starlost and it was the Silent Running footage that was used to pitch the project to US television networks.