What optical device is the Dutch scientist, after whom the lunar crater Jansen is named, credited for? 
Jansen (crater): Jansen is a lunar crater in the north part of the Mare Tranquillitatis, named after Zacharias Janssen. Zacharias Janssen: Zacharias Janssen (also Zacharias Jansen or Sacharias Jansen) (1585 – pre-1632) was a Dutch spectacle-maker from Middelburg associated with the invention of the first optical telescope.
optical telescope