It also stands to reason that as a U-Boat commander you could leave the scope up and dive to periscope depth without ever losing visual contact with an object...
I don't believe they were ordered to sail on the surface with retracted scopes. When the only thing blocking view is the curvature of the earth, the elevation and magnification of attack/obsevation scopes would have been quite useful to spot "smoke on the horizon" and totally avoid sun reflection giving away the U-Boat. (at least early in the war...later in war it would have increased the U-Boat's radar profile....and decreased its suviveability.)
All that being said... this was not a preferred method of use... because the magnification and field of view from scopes even in calm sea states... produced nauseating sea-sickness as even calm pitching and rolling appears magnified.
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