It cannot be overemphasised that by far the best form of cleaning of optical surfaces is to prevent the dirt from accumulating there in the first place (i.e., use rain guards, lens caps, and cases whenever appropriate). The reason for this is that it is extremely easy to damage optical surfaces by cleaning them. In reality, an optical surface has to be quite filthy before the dirt has an optical effect that is significantly noticeable during visual observation and there is a real need to clean it. The objective lenses of my binoculars go for years without being cleaned, and the eyepieces may only be cleaned once a year, although those that I use for star parties tend to need a clean after each event.

My full binocular optical cleaning kit consists of the following items (Figure 5.2):

• Puffer Brush with Retractable Soft Bristles. This is my first line of attack. Most dust and so forth can be blown off with the puffer alone. If it is more stubborn, I deploy the bristles and use them in conjunction with the puffer. Flick the brush quickly over the lens surface while puffing the bulb. After each stroke across the

Figure 5.2. Cleaning kit. Top L to R: Lens tissue, microfiber cloth. Middle: camel-hair brush, blower brush, Opti-Clean. Bottom: Lens Pen.

lens, flick any accumulated dust off the brush, while giving a sharp puff to help dislodge it. Be very careful if you use canned air as a blower—the propellant can damage lens coatings.

• Microfiber Cloth. I keep one of these in each binocular case. To use it, hold the binocular so that the affected lens surface is facing down, then gently flick an edge of the cloth over the lens to remove any dust. If any deposit remains, breathe on the lens to moisten it slightly, then gently wipe the lens from center to periphery. Microfiber is quite good at removing grease and finger prints. Never rub the lens with a circular motion and never wipe from the periphery to the center. If there are trapped particles that could scratch the optical surface, they will be trapped in the lens surround; they are relatively safe there, so don't accidentally dislodge them.

• Lens Tissue. Lens tissue is particularly useful on small lenses. It can be made into a swab rather like a cotton bud by folding it into a strip and wrapping it around the end of a toothpick. Dampen it in cleaning fluid and use it in light strokes from the center to the periphery of the lens.

• Lens Pen®. A Lens Pen incorporates a retractable soft brush and a cleaning pad that is recharged with cleaning fluid from an impregnated piece of foam in its cap. It is particularly good at removing eyelash grease and fingerprints. First, use the brush to remove any dust. When you are sure that the lens surface is clear of particles, clean the deposits with the pad, taking care not to drag any particles from the lens surround.

• Opti-Clean. Opti-Clean is a polymer-based cleaning product that was developed to clean silicon wafers used in the microelectronics industry and is now marketed primarily for cleaning photographic lenses. It is suitable for any glass lens. It is a transparent liquid that is applied to the lens surface. As it dries, it forms a skin. When the skin is peeled off with an adhesive tab, it pulls any grease and grime with it, leaving the lens in pristine condition. It seems expensive, but it lasts for a very long time—I have had a 5 ml vial of it for about ten years. Note: There are different products with the same name used for contact lenses and in dentistry.

There are other proprietary cleaning fluids available from photographic outlets. Alternatively, you can make your own. My recipe is:

6 parts distilled water

1 part pure isopropyl alcohol (IPA).

2 drops liquid detergent (e.g., mild dishwashing liquid)

Apply it to the lens with a a lint-free cotton swab or a swab made of lens tissue wrapped around the end of a toothpick. Dampen the swab and swab the lens from center to periphery, rolling the swab so as to lift any grime away from the surface. Be careful not to overmoisten the swab so you don't get liquid into the lens surround. You can dry the lens with a dry swab or with a clean lens tissue.

However you clean the lens, be careful not to rub it any more than absolutely necessary. Not only does rubbing increase the likelihood of damage to the lens surface or coatings, but rubbing with a dry cloth or tissue can cause the build-up of a static electric charge on the lens surface. This charge will attract dust or lint, and it will be extremely difficult to dislodge. If this does happen, you will need to use a water-based cleaning solution (such as the one above) to get rid of the static electric charge.

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