How do I clean eyepiece lenses safely?
Very carefully — and not often. If you rub fine optics, you risk rubbing microscopic bits of gritty dust against them. These leave tiny scratches (“sleeks”) that will scatter light and degrade contrast. A few sleeks won’t matter, but many of them eventually will, so your goal should be to prevent them completely.
First blow off or gently brush away loose dust using your breath or a soft lens brush. Dampen a bit of clean, soft cloth with alcohol from a pharmacy. Swirl it gently on the lens, applying no pressure. Be sure not to let any liquid seep around the edge of the lens to stain internal surfaces. Gently dry with the cloth. If necessary, rinse by breathing on the lens to leave a fog of distilled water and rub with the cloth very gently. Then quit.
Special cleaners, such as “lens pens” that have a small impregnated pad for smudge removal, can also do an excellent job.
— Alan MacRobert