Cleaning Lenses

The quick and easy way to dust eyepieces is to lay a finger across the eye end (without touching the glass!) and suck air under your finger past the lens. (This moves the dust from the eyepiece to your lungs, but every astronomer knows which is more important.) The job takes about one second.

If that doesn’t do it, the traditional method for dusting optics is to brush lightly with a camel’s-hair brush. Sold in camera shops, these brushes have soft bristles with minimum tendency to scrape grit against a lens. Brush very lightly. Store the brush in its container or a plastic bag.

Camera shops also sell cans of compressed gas for blowing dust off lenses. Be careful with the kind that use liquid propellants; these have a reputation for spitting onto the glass and leaving a residue if the can is tipped or shaken in use. Blowing (instead of sucking) with your own breath is also likely to leave spit marks.

For tougher dirt or stains, various lens-cleaning solutions are available. Good ones are pure isopropyl alcohol or methyl alcohol (methanol), available in drug stores and hardware stores, respectively. Standard, diluted isopropyl rubbing alcohol works well too and is easier to find, but avoid alcohol preparations with other ingredients that may leave stains. Camera shops sell lens-cleaning fluids such as Crystal Clear, which is pure methanol, but you can get methanol much cheaper in a hardware store. Also available are “lens pens” with a soft, retractable, solvent-impregnated cleaning pad.

You’ll need a soft, grit-free wipe. A well-washed piece of pure cotton cloth works well. Moisten it with the fluid and swirl the fluid gently across the lens, applying no pressure. If necessary, rub dry with a fresh piece very gently. Don’t drop liquid directly onto the glass. It’s liable to seep around the edge of the lens into the cell and carry dissolved grime onto interior surfaces, staining them.

Eyelash and fingerprint oil may discolor coatings permanently if left on long enough. But such stainsare only cosmetic, eyepiece manufacturers insist, and should have no detectable effect on performance.

If problems develop inside the eyepiece, it’s best not to take it apart. You are almost certain to tilt and jam (“cock”) a lens element, and if you try too hard to uncock it, the edge will chip. Instead, call the manufacturer and ask about a professional cleaning.

The big front lenses of refractors and mirror-lens telescopes should not be taken out of their cells except by an expert. Again, the danger is cocking and chipping the glass — or not re-assembling everything exactly the way it was! Big lenses can be cleaned right in place the same way as small ones, by using more time and fluid. Resist any urge to hurry the job.