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Cleaning eyepieces is an easy and essential step to getting the most out of your viewing. Eyepieces, if dirty, will provide degraded images. While care should be excercised to prevent scratching the anti-reflection coatings, if such care is used, the eyepieces can be cleaned regularly for decades without harm. Most eyepieces, it seems, do not come with cleaning instructions. But as an eyepiece is essentially the same as a quality camera lens, the same cleaning techniques can be used. I have camera lenses and eyepieces that have recevied regular cleanings for nearly twenty years will no ill effects.
WARNING: I do not know the effects of these cleaning techniques on telescope mirrors or flourite elements. Follow your manufacturer's directions for cleaning those products. You can also use these techniques to clean conventional refractor objectives and the corrector plates on compound telescopes. But if the manufcturer's cleaning directions vary from these, follow their directions.
Eyepieces need more frequent cleaning than objectives because they are handled more, and they collect fingerprints, eyelash grease, etc. Primary mirrors are tucked safely inside their tubes and refractor lenses are usually inside deep lens hoods. Compound telescopes, though, often have exposed corrector plates. Hoods and shades are also recommended for them, to keep both dew and fingers off the corrector plate, and to provide better contrast by blocking stray light. It is still best to not clean an optics that are not dirty, as then there is no chance of causing any scratches. But the oils in fingerprints and eyelash grease contain acids, and all sorts of other goop can collect on the eyepieces during normal use, and it is necessary to get these materials off your eyepiece to provide the best possible views and to prevent damage to the coatings.
I have an eyepiece cleaning kit that fits in an eyepiece poly-bottle. It is comprised of an anti-static brush, a microfiber cloth, a small bottle of cleaning solution, and Q-Tips (or ear buds). The anti-static brush is one designed for cleaning photographic negatives and is available at camera stores. I like an anti-static brush because it doesn't collect dust itself, and it doesn't create static charge on the eyepiece elements to collect dust during the cleaning process. I also have a microfiber cloth (lens tissue can be used as well), also available at camera stores, which is very absorbent, and will pick up any residual oils and residues left from the cleaning process that a tissue can miss. Plus, there is no worry of lint. The cleaning solution I use is 91% pure isopropyl alcohol. A pint bottle is less than two dollars, and is useful to have around the house anyway. Q-Tips are the 100% cotton type. They are used to apply the cleaning solution, and remove the excess.
A word on the alcohol. The 91% pure isopropyl alcohol contains two ingredients on the label, alcohol and purified water. Alcohol evaporate easily at room temperature, and leaves no residue. Purified water will not leave a residue, but will take longer to evaporate. Some rubbing alcohols may contain lanolin or other oils to prevent the alcohol from excessively drying the skin. These should be avoided. Read the label. We want alcohol and water as the only ingredients.
In my kit the alcohol is stored in a small bottle that once contained lens cleaning fluid. I partially filled and rinsed the bottle several times with alcohol to get any residual lens cleaning fluid out. Lens cleaning fluids should be okay, but you don't know what is in them. Plus they are expensive. Alcohol is a better deal.
To start cleaning, make sure the eyepiece is free of dust and grit. I examine the eyepiece under good lighting. I carefully blow on the surface, then lightly brush the surface with the anti-static brush. I don't use canned air as it is very easy to get the very cold liquid propellant on the eyepiece and damage the coatings or glass, or both. Once the surface of the eyepiece is free of dust and grit, examine it for smears or streaks of oil. If there are any, lightly moisten a Q-Tip with alcohol. Starting from the center, lightly rub the Q-Tip in a circular motion from the center out, until you are at the edge of the eyepiece element. Don't use a lot of alcohol, as you don't want the alcohol to pool and run under the edge of the eyepiece lens. While the alcohol should not harm the inside of the eyepiece, the alcohol will dissolve any grease and oil on the lens, an it will flow inside the eyepiece with the alcohol. The alcohol will evaporate, but the oils won't and they will remain.
Once you have cleaned the lens on the eyepiece, use the dry end of the Q-Tip and mop up the alcohol on the surface. Discard this used Q-Tip. It is important to mop up the excess alcohol as if it fully evaporates, it will redeposit the oils and grease back on the lens. You can safely perform the alcohol cleaning again with a fresh Q-Tip if necessary.
At this point I will examine the eyepiece again under a bright light. Often it is impossible to get all the grease and oil off the lens without irrigating the eyepiece with alcohol. This is where the microfiber cloth comes in. If necessary, I gently polish the surface of the eyepiece with the microfiber cloth, after again examining for dust, which traps any residual oil and grease within the cloth. When the cloth itself gets oil and grease marks on it, I wash it in warm water with a bit of Ivory soap. I rinse it thoroughly in warm water, and then rinse it again in distilled water or reverse-osmosis water. I hang it to dry in a lint-free place, a bathroom after taking a shower is ideal. This way the cloth stays dust- and residue-free, and won't deposit any oil or grease back on my eyepieces.
Once you are done cleaning, recap your eyepieces or put them back in their poly-bottles. I also put my little bottle of alcohol, the microfiber cloth, the anti-static brush and some Q-Tips in a poly-bottle, and put them in my eyepiece case for use in the field if necessary.
Remember, the only thing that is going to scratch your eyepieces is dust and grit. Remove that first. Keep your anti-static brush, Q-Tips, and cleaning cloth or tissues in a poly-bottle or similar container to keep them dust-free so that they can't cause any scratches. Use gentle pressure with the Q-Tips and cloth. Rubbing harder won't pick up more grease.
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