Up at the top of the telescope is the prime focus. In days of yore, the observing astronomer (or an assistant) would have to stand up here for an entire observing run, changing photographic plates. In modern times, the main sensor is a CCD (the same sort of device used in video and digital still cameras), so access is required much less frequently. There’s an elevator at the north end of the observing floor, so those astronomers were not usually required to carry their heavy stacks of plates up this ladder.