http://www.raf.mod.uk/history/tirpitz.html
One aspect which has remained something of a mystery is the total lack of interference from enemy fighters. The first reports of the Lancasters meeting at the rendezvous point were received on the Tirpitz at about 9 a.m., an hour and a half before the aircraft attacked, and fighter protection was requested immediately. It appears that one reason why this request was not granted may have been that the Germans believed that the fighter base at Bardufoss was the intended target for the British bombers.
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On Cajus Bekkerīs book on Kriegsmarine he mentions that the German fighters were approaching Tirpitz as they were signalled to return home immediately as bombers were assumed (according to radar)to be closing in on the air field. It seems according to Bekker that there were bombers making a decoy bomber run towards the air field. When it was too late the Germans noted more bombers and they were heading for Tirpitz.
Well, one would think the RAF would know...
Any ideas on this as this has been bit of a mystery?