Use isopropyl alcohol that is 98 % pure or more. When using isopropyl alcohol to clean an optical device, do not proceed immediately to dry the surface with compressed air (except when you are cleaning very sensitive optical devices).
This is because the dust and dirt are in solution and will leave behind a filmy deposit once the alcohol has evaporated. Therefore, you should first remove alcohol and dust with a soft tissue, and then use compressed air to blow away any remaining particles.