Please don't put tape over the lenses of Oehler Skyscreen III units. While the little lens system might appear trivial, I spent almost a year on its design and did not include the effects of tape.
While I'm sure that others have encountered dusty sensors and may have solved the problems with tape, I recall only one instance where it caused a significant problem. Bruce Hodgdon reported trouble with the photoscreens in his test tunnel. When I stopped by for a visit, we removed the screen covers and found the insides coated with the firing residue from many rounds of Pyrodex and other dirty loads. A convenient air nozzle took care of the residue and operation returned to normal. Incidentally, the light slot did include a clear plastic cover; the junk found another way in.
