Hi My mistake the 500mm mirror lens does NOT have an aperture ring, when i put the lens on the camera all i get is F---- and cannot take any pictures

Hi i have decided to go for a digital SLR i decided on pentax because i have a pentax SF7 SLR with various lenses all work on the KM except my 500 mirror lens it fits the camera but i cant seem to set the F stop all i get in the lcd display is F ---- i have used manual mode is there a way to set the F stop that i have not yet found only had the Camera for a week