If you look on the inside of the reel you will see where the black plastic tab goes thru a u shape plastic, that is the optic
To expand on that, the "black plastic tab" runs the entire circumference of the inside of the reel basket, and has cutouts in it that almost look like Morse code. The U-shaped enclosure that those cutouts spin through are the optic assembly. Beyond maybe taking a little bit of compressed air and shooting it in there, there's not a lot you can or should need to do with the reel optics.
If something is dragging on the reel basket causing it to not spin properly, that can cause a reel error. A suspect in a 42 error, since that's the middle reel and most likely to be affected, is the wiring on the top of the door for the fluorescent bulb at the top of the reel glass. Over time, those wires stretch a little, and you'll notice as you slowly swing the door shut that those wires tend to start creeping down a little -- if there's enough slack that they're rubbing on the edge of the reel basket, that could be your culprit.