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
- 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
- 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
- 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
- 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.