The main problem you will find with a mirror lens on your FG, as has been mentioned, is the fixed aperture of f8, which will be impossible to focus using the standard split-image/microprism screen. You really need one of the Nikons that takes interchangable focussing screens. The all over plain focussing screen or the one with grid lines are the ones that work best with a 500mm mirror lens. Using a x2 converter is not a particularly good idea with a fixed aperture mirror lens if you want any knid of a quality result.