Reading this:
http://www.uboataces.com/periscopes.shtml
it came the idea to my mind to implement such a described fogging effect to the periscope.
Can this be done by adding some semi-transparent layer to the graphics of the peri-screen ?
It also could be related to damage on the boat in general.
So that there would be a chance (when hit by DCs) to get a slightly damaged periscope (or even two).
Any suggestions whether this can be done ?