The attack periscope has a blind spot i.e. immediately above the u-boat. The observation scope largly covers this blind spot and is therefore better at spotting approaching aircraft. I gathered this information from a book I am reading at the moment 'U333 The Story of a U-boat Ace' by Peter Cremer.
Nemo
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"I'm afraid there is no disguising the fact that King's obsession with the Pacific and the Battle of Washington cost us dear in the Battle of the Atlantic".
Sir John Slessor GCB, DSO, MC, DL
AOC-in-C Coastal Command RAF
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