Seeing London From Long Island with a Telescope

Someone from the Flat Earth Society has countered a statement I made about Lake Pontchartrain in Louisiana.
I said that because of the curvature of the earth you cannot see New Orleans from the North Shore of the lake. This person has written to me that with a powerful enough telescope you can see downtown New Orleans from the North Shore of the lake. Therefore, he says, the Earth is flat.
I have nothing to say to this poor fool. Apparently if you have a strong enough telescope you can stand on Long Island and see London!