Isnt that ring screwed onto the threaded shaft that goes into the rear sight, and the purpose of the ring is to act a a lock-nut. If the iris threaded part has flats upon it, try using a small appropriate sized spanner to turn it anti clockwise. Sometimes the whole unit will then unscrew, failing that, I think I have one or two different sized c spanners that are for those rings.
Try aiming the heat from a hair dryer onto the ring, once warm it should come off, careful not to damage the sight or set fire to your hair!
Jonty