I bought a used Peak Model 1 Grain Focuser. I got it for cheaper than expected because the mirror has a lot of micro surface scratches. No problem - I bought the replacement mirror from B&H. However, in attempting to change the mirror, I have encountered a problem. I can't get the old mirror out! I'm assuming that I would just unscrew the top four screws and pop the old mirror out - but those screws just won't budge. I've tried and tried each of them and now I'm just running the risk of stripping the heads. Has anyone here ever changed the mirror before? Am I doing something incorrectly? Anyone have any advice / tricks / helpful suggestions for me? Thank you!!