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We propose STIS coronagraphic imaging of an emblematic transition disk system comma PDS 70 comma known to be hosting a transition disk with a large cavity comma a dust ring with an asymmetric brightness distribution and two forming giant proto-planets that are still in their gas accretion phase. PDS 70 is currently the only known system where giant planet formation can be witnessed and studied directly. With this program we propose to obtained the deepest images ever obtained of this system comma with the same spatial resolution as ground-based 8-m telescope in the near-infrared. With these optical images showing light scattered by submicron particles that follow gas drag in the system comma we will study 1- gas accretion by the forming protoplanets carving the cavity comma and 2- the extended structure of the disk beyond 80au that has never been resolved from the ground.