The more light there is the less critical the focusing. Same holds true for night vision. Small F stop openings are also safer for the intensifier tube if a lot of light should happen unexpectedly. Cleaning the optics - this section will apply equally to night vision, video, film cameras, weapons optics, binoculars, eyeglasses, or anything else with quality lenses. Many people don't take the proper care of their optics.
Good lenses are expensive - sometimes comprising almost the total cost of a piece of equipment.
Don't touch anything made of glass with your fingers. Handle filters by their edges only. Not only will your fingerprints degrade the performance of the lens, but the oil from your fingerprints is corrosive. If left uncleaned, an old fingerprint literally can etch itself into the surface of a lens permanently. Try not to grunge up anything to start with, but if it happens be sure to clean things up at the first opportunity.