The Pros' View...Cleaning Telescope Optics

John Hall - Pegasus Optics

The least destructive way is no cleaning at all until the coating is completely obscured by a coating of dust. Prevention of dust contaminant deposition is the first priority in keeping the coating operating as long as its expected life span. The second priority is avoid-ing cleaning it as long as possible. It's been esti-mated that a visible coating of dust, as seen from the front of the tube, causes about a 4% loss of contrast. This seems to match what I see at the eyepiece.

When you do have to clean it, what to AVOID is:

(1) Compressed air, even that advertised as "optical grade" or whatever. It will have contaminants in it and possibly propellant that will etch aluminum, also water vapor that will have concentrated contaminants in solution when sprayed.

(2) Direct pressure on the dry glass is the most destructive. It is not a good idea to use an acetone-soaked cotton ball or tissue on a dust-contaminated coating. Unfortunately, this is the substance of some recommended cleaning instructions.

(3) Contact or nearness of plastics or particle-board or formaldehyde-containing materials to the mirror. They will outgas, staining the aluminum and even the glass substrate. Remedy: regrind, repolish and refigure and recoat.

(4) Heavy concentration of soap in cleaning solution. Chlorides and other chemicals in dishwashing liquid or laundry soap will stain the aluminum right through the overcoat. Use a couple of drops of dishwashing liquid per quart, or not much more than that.

Soak the mirror first in room temp or tepid water. After about 5-10 minutes, direct a stream of water on all parts of the mirror. Submerge the mirror in a soapy water solution (2 drops/quart ratio). Get some cotton balls or surgical cotton. Make a pad of three and wipe under water with only the weight of the cotton balls. Go in one direction only, e.g., left to right, and don't reverse direction.

Make multiple passes from top to bottom until you've covered the mirror. Change cotton balls every couple of passes. Rotate the mirror 90 degrees and repeat.

Stand the mirror up, hose it down with tap water. Then use a jug of distilled water to "chase" the tap water droplets down to the bottom of the mirror. A few drops of distilled water will bead up on the mirror: touch the tip of an untouched part of a Kleenex tissue to these to wick them into the tissue. (It's a good idea to wear latex gloves while handling the mirror or cleaning it.) Take care not to touch the working surface of the Kleenex with your fingers. If you have a broad expanse of beadlets, use a paper towel folded a couple of times to form a soft pad. Press lightly against the surface without moving the towel. Don't re-use that pad surface again; get another paper towel and continue till done.

Air drying in the open is not a good idea because dust from the air quickly begins depositing on the surface. Also, residual contaminants in the droplets become concentrated as the droplets dry. It's better to provide an "umbrella" or cover such as a sheet of cardboard some distance over the mirror if you decide to air dry.

When the mirror is dry, place the mirror face up in its storage box. The box should have vents on the bottom to promote air circulation throughout its enclosure. This will prevent the buildup of whatever outgas products that could conceivably occur in the enclosure. The enclosure should not have any parts in direct contact with the mirror face. Store the mirror box someplace away from solvents, household or garden or garage chemicals or wherever motors are started up or welding rigs or other electrical equipment are in use. All these are "ideal" storage suggestions. I would avoid storing it where temp cycling occurs from one extreme to another, although putting the box outside a few hours before use will of course hasten equilibration. If you decide to use desiccant bags, dry them in your oven every couple of months or per manufacturer's instructions, else they do more harm than good, absorbing and releasing vaporized sulphates as they reach saturation.

Do not leave your dew-soaked telescope in the open covered with a near-airtight covering. If you have a scope cover with capability to "breathe", there should be no problem with enclosing the scope with it. If not, the high concentration of water vapor and high temperature in the tube will concentrate contaminants in this hostile environment. First let the telescope dry out, with the tube horizontal, and place a partial cover over the open end in front of the mirror, or in front of the mirror box, to prevent dust from settling on the mirror".

Thomas Back
TMB Optical

The best policy is not to let the lens get dirty and/or dusty in the first place. The use of the dust caps is highly recommended. However, no amount of preventive measures will stop the objective from eventually getting dust and airborne pollutants on the first optical surface. We recommend that you do not clean the objective often, as much as the urge may be. Depending on how often you use your scope, and the amount of pollutants in your air, you may have to clean as often as twice a year, or less than once a year. If you find it absolutely necessary to clean the lens, either find someone with the knowledge to properly clean optical surfaces, or use the following technique.

First, use a high quality compressed air duster (the R-134 type). Chemtronics sells a high quality unit. Do not tip and shake the can. Blow any particles off the lens surface, using short blasts. Next, using USP grade pure cotton, and a quality lens cleaner (such as Formula MC from Orion Telescopes, or your own mixture of distilled water and mild soap), blot the entire surface (use a small amount of liquid - not so much that the fluid could run between the lenses) to pick up any stubborn particles and to clean the surface. Do not, at any stage, apply hard pressure. Using a fresh piece of cotton or white facial tissue, carefully clean the surface of the lens, by wiping across in the radial direction. Repeat the process with denatured alcohol, blowing off any dust that may fall on the lens as you are cleaning it. If you want to take the ultimate step in cleaning, a final rinse with HPLC grade acetone, or any high grade acetone will clean the surface to new condition. You may notice a few faint streaks from the dried solvent. They will not affect performance, but they can be removed with light pressure and a Q-tip with a small amount of alcohol or acetone. Finally, a few short blasts of compressed air to remove any remaining dust.

If you feel uncomfortable with using pressurized air, you can substitute an air bulb or a clean, soft optical brush. I don't normally recommend the LensPen; however, if you make absolutely sure there are no hard dust particles on the LensPen, and the optical surface, you can use this method, if you find it to have worked well in the past. I might add that there are many good ways (and just as many bad ways!) to clean optics. If you have found a scratch free way that cleans well, continue to use it.

Collodian is an interesting way to clean lenses and mirrors. I have used it many times, but along with the dangers of ignition and ventilation concerns, there is always the possibility that it will pull off a part of the coating. This has happened to me with both mirrors and coated lenses. I cannot recommend this technique for the general cleaning of optics.

Published in the April 2004 issue of the NightTimes