Cleaning Optics

by Bob Hess

Although new to astronomy and other telescopic activities, I have been cleaning coated lenses and laser optics professionally since 1980. The following procedure is generally used for all types of small coated optics in laser light applications and is a bit more specific than the recommendations from Televue.

• 1. Prepare the work area. This is very important because particulate contamination from the work surface or the worker is what usually scratches the coating. Clear the table of everything and wear a clean shirt straight from the laundry if possible. Work as still as possible so as not to shake particles out of your hair while cleaning. Work in a darkened room under a freshly cleaned high intensity desk lamp so that the reflection of a bright source of light on a dark background will allow you to see what you are doing. Be sure that the work surface will not be eaten by methanol (Formica or glass are good). Read all precautions regarding the safe use of methanol and follow all storage and handling instructions.

• 2. Assemble the tools. You will need the following:
  • a. Reagent grade methanol (available at the Science Shop in San Jose, 500ml will last a long time),
  • b. An eyedropper bottle for the methanol(also at the Science Shop),
  • c. Hemostat clamp, curved tip preferred (Science Shop or Enco Manufacturing Company also in San Jose),
  • d. Kodak lens paper (Ewert's Photo has it, get more than you think you will ever need),
  • e. Kimwipes, one large and two small boxes are nice to have (Science Shop),
  • f. Canned air duster (available at Ewert's Photo).

• 3. Clean the barrel of the lens. Fold a small kimwipe into a small square pad and apply a couple of drops of methanol to a corner. Wipe down the eyeguard rubber and the barrel. Do not use kimwipes on the coated surface because the fibers of these lint-free wipes may scratch it. I like to lay down a large kimwipe to set the tools and eyepieces on while working.

• 4. Prepare a cleaning pad. The technique is as follows:
  • a. Fold a single sheet of lens paper in half lengthwise, twice. Hold the sheet along its short ends only and do not touch the center.
  • b. Fold the resulting 3/4" wide strip in half by gently poking its center point.
  • c. Repeat step "b" and clamp the pad in the hemostat so that it will be easy to wipe the edge of the lens with an un-touched section of the lens paper. This step is hard to explain without illustrations though I will be glad to demonstrate at some gathering in the future or when I learn to communicate on the computer better. The cleanest part of the folded lens paper should end up hanging off the outside of the bent tip of the hemostat by about a quarter of an inch and off the tip by about an eighth of an inch.
  • d. Lay the hemostat on the table so that the cleaning pad hangs off the edge. This is done so that it can not pick up anything from the surface, just in case.

• 5. Blow off the lens. Use the canned air being carefull not to tip the can while doing so. Tipping canned "air" may spit nasty stuf onto the surface.

• 6. Wipe the lens. Apply a couple of drops of methanol to the cleaning pad and wipe all the way around the edge of the lens in one continuous motion. Be carefull not to let the metal hemostat contact the surface! Throw the cleaning pad out of the hemostat and into a garbage can after this single wipe. Touching the lens with a contaminated pad only re-deposits the dissolved eyelash oils back onto the lens.

• 7. Repeat steps 4 and 6 as many times as necessary to clean the lens. Each wipe should cover un-cleaned areas of the lens or remove streaks that may be left by the previous wipe. The idea is to dissolve the contaminant on the surface with the methanol and trap it in the paper fibers for disposal. The trick is to balance the wettness of the cleaning pad with the speed of the wipe so that the methanol dries immediately as the pad leaves an area of the lens. Methanol is the preferred solvent because it evaporates faster than isopropanol but slower than acetone, and it readily dissolves eyelash and fingerprint oils. Inspect the surface after each wipe to evaluate progress.

• 8. Attack difficult to remove spots with a cleaning pad folded into a sharp point and small circular scrubbing motions. If unsuccessful, try a couple of drops of acetone on the cleaning pad point.

• 9. Keep the lens clean. A clean lens is a happy lens. Cleaning a lens involves some risk to the coating, and while good technique will minimize the risk, the best situation is to not have to clean it often. Lens cleaning paranoia is dangerous as well, however, because little drops of spit will corrode the coating causing permanent scattering centers and accumulated grit makes the job of cleaning riskier.

A couple of final comments. NEVER blow on a lens! Spit and dielectric coatings do not get along. Control the methanol! It is a dangerous poison that can eat into nice wood floors or cause permanent blindness if a drop gets into your eye. It is also explosive. Keep it AWAY from kids. Same thing regarding acetone even though it is readily available in hardware stores. Isopropanol (99%) is probably the safest solvent but it is not as effective on bodily oils and dries much slower.