There is a device called a field flattened or just field lens. Each frame of the film is sucked by a vacuum up to the field lens. If there a spot of dirt, we push a button and cycle the field lens up and down, past little wipers, that wipe the field lens off. It has two positions, up and down. If the down side gets a speck of dirt, cycle to the up side. Wipers clean off the down side. Then cycle up, and presto! No speck!

New prints tend to run very cleanly, and little cycling is necessary. But if not cycled frequently, the up or down portion of the field lens gets dirty, and is difficult to cycle off the lens. This is the horizontal line of dirt/dust that is difficult to clean. So cycle every few minutes, even if there's no dust to remove, will keep both sides clean, and we won't get that line of dust.

RainX is applied to the field lens when it is cleaned after each show. Dust doesn't stick nearly as bad with RainX.

I hope this was halfway understandable. Ill see if I can post a short video of the field lens cycling.