Protecting And Cleaning Lenses/Mirrors
Protection Beats Cleaning
The best way to protect your lenses or mirrors is to use a good fitting dust cap/s. If you own an open-tube reflector like a Newtonian, it's best to put a dust cover over the end of the tube where the main mirror is. This main mirror cover can double as a cover to block stray light from getting in around where the mirror is attached to the tube. To preserve the reflective coatings for longer and prevent rusting, it is important for telescopes to be kept indoors when not in use. Aluminium coatings generally last about 8-10 years when kept clean and dry.
Protecting telescope optics is far better than cleaning them. Cleaning optics, regardless of which type (refractor, reflector, binocular) could reduce its light gathering power by removing any enhancing coating placed on them before, or even by any accidental scratches. However, if a mirror or lens does eventually get dirty, some necessary precautions can be taken to ensure no damage is done to your scope’s optic. Dirty lens or mirrors reduces the light gathering power and reflectivity (for mirrors).
Cleaning Reflector Telescopes
Any moisture on telescopes or its cap should be wiped dry before fitting or putting away with a clean, soft cloth (the same type we use for our spectacles would be good). If there is dew on the mirrors or lenses, let them dry naturally instead of wiping them down. Any water marks which may appear on mirrors through condensation and dust can be removed. This is done by first removing the mirror, then pouring distilled water over them. Distilled water would generally not leave any water marks.
Cleaning Refractor Telescopes and Binoculars
Refractor and binocular lenses are more forgiving than mirrored surfaces. However it is quite easy to remove or scratch the enhanced coatings on binoculars and refractors just by wiping with a soft cloth too hard. Most optics manual will give suggestions to remove any unusual marks or dirt without removing the enhanced coatings. Otherwise a rub with a soft cloth is the only cure, but try using a soft brush first (camel hair) or a "Lens Pen". A Lens Pen has a soft brush at one end and at the other end a special device for getting into corners and removing dirt or sand.