How can I clean my lenses without causing scratching or damages to the lens surface?
Clean your eyepieces in the same way you would clean a camera lens. An air blower, soft (i.e. camel hair) brush, lens tissue or microfibre cloth, and a solvent or alcohol will do the trick.

When I was an undergraduate I did some work with lasers and very expensive dielectric coated mirrors. The techniques here will show you how to carefully clean optics. Never completely wet your lens tissue or cloth. One drop on a lens tissue is usually enough, or a couple of drops on a microfibre cloth.

Essentially, you need to:


Use a blower to blow off as much dust as you can.
Then use a soft brush.
Dampen a small square of lens tissue with pure alcohol or acetone (see warning below). Gently draw or wipe the tissue across the surface once, and then discard. Repeat if necessary.
You may need to fog the lens with your breath to help remove water-soluble gunk.


Be insanely careful if you are using a solvent such as acetone. Although a good for cleaning optics, it's not very nice to paint and plastic so it should be a last resort for very stubborn marks well away from the edges of the lens (so you'll never touch the paint with it). Methyl or isopropyl alcohol are safer but less effective, or a generic lens cleaner from a camera store.