How will the gap between the encoder disk and the detector side of the encoder module affect the output?
In general, US Digital encoders are not that sensitive to the encoder/disk gap. This is because the lens collimates the LED light into a parallel beam and keeps the light/dark pattern fairly uniform. However, as the encoder disk gets closer to the detector, the shadows cast by the encoder disk get cleaner and sharper. This results in better light/dark contrast and subsequently somewhat better performance.