Lysergic,

The small one was the 'attack' periscope and the large one was the 'sky' periscope.The 'sky' periscope had optics which allowed the user to scan the sky from horizon to vertical for enemy planes prior to surfacing. The larger head of teh periscope allowed more light into the optics though the realistic chances of seeing an aircraft through this periscope were pretty slim.The small attack periscope was of the 'all height'' type and was viewed from the conning tower.'All height' means that the eyepeice stayer at the same level even as the head moved up and down...no more bending down to look through the eye-peice as on allied subs.The attack periscope had an electrical drive asn was quite a large peice of equipment in the conning tower.The u-boat commander sat on a seat and turned the periscope by hitting foot pedals to drive the periscope right or left.Incorported into the optics was a stadimetric ranging system using split optics to find the range to the target.The periscope had other optic goddies which i cannot remember at the moment but i'll check up and post some more info.

Rgrds

Sniper.