Dust is a problem and often difficult to avoid. Dust is quite disturbing when doing darkfield microscopy, as even the smallest specks form bright spots on the image. Dust on the eyepiece can be easily blown or brushed away, dust on the optical parts inside the microscope are much more difficult to remove. You can minimize dust accumulation by:
- Using a dust cover
- Keeping the room reasonably free of dust
- Mimimizing air currents. These carry dust particles into the room, which then settle down.
- Using pressurized air from a can: be careful here, often there are additives which accumulate on the optical surfaces and the rapidly expanding air causes a local cooling of the lens elements. I do not know if the rapid cooling and warming of the lenses has a negative effect, but would be careful. Only use pure air and from a safe distance.