After tests with two experimental gyro gunsights which had begun in 1939, the first production gyro gunsight was the British Mark I Gyro Sight, developed at Farnborough in 1941. To save time in development the sight was based on the already existing type G prismatic sight, basically a telescopic gun sight folded into a shorter length by a series of prisms. Prototypes were tested in a Supermarine Spitfire and the turret of a Boulton Paul Defiant in the early part of that year. With the successful conclusion of these tests the sight was put into production by Ferranti, the first limited-production versions being available by the spring of 1941, with the sights being first used operationally against "Luftwaffe" raids on Britain in July the same year. The Mark I sight had a number of drawbacks, however, including a limited field of view, erratic behaviour of the reticle, and requiring the pilot/gunner to put their eye up against an eyepiece during violent manoeuvres.