This show is like UFO, ran for a season and is hard to find. Well, not too hard. The Starlost was a Canadian show set on a generation ship 50 miles wide and 200 miles long. The three main characters live in an isolated section of the Ark. An accident centuries earlier had isolated different sections of the ship, and they’re shocked to learn their world is much bigger than they thought. The show’s 16 episodes, all available on YouTube, depict them exploring the ship and eventually learning that it is on a collision course with a star.
Kier Dullea of 2001 fame stars. Harlan Ellison helped develop the show but quit when budget cuts “dumbed it down.” Douglas Trumbull, who did the effects for 2001 and would go on to do Star Trek: The Motion Picture and Blade Runner, handled the effects.