Telescope Maintenance

Cleaning precision optics is not the same as cleaning household windows or mirrors. Remember that the lenses and mirrors of your telescope are tasked with collecting miniscule amounts of light, then amplifying, magnifying and focusing that light into a viewable image. The slightest damage to the glass surface of your objective lens or mirror can ruin it as a viable scientific instrument. It is simply not acceptable to pick up a bottle of Windex and a paper towel and try to wipe the lens/mirror surface clean. An aviation mechanic who I was assisting in an inspection once chastised me about "using the right tool for the right job." There are tools you use on cars and tools you use on airplanes." The same is true for cleaning a telescope. There is a right way to clean a mirror in your bathroom. That is by no means the right way to clean the mirror of your telescope. Let's discuss how it should be done.

Any optical component that is exposed to the open air will eventually become contaminated by dust no matter how diligent you are about keeping the dust caps in place. After all, you must remove the cap if you wish to do any observing with the telescope. During the time that the scope is in use, surfaces are becoming contaminated. This can happen a lot faster than you might think. If you are an allergy sufferer, you no doubt have reflected many times on how your automobile seemed to change color overnight from black to green. The greenish tint is caused by pollen settling on the vehicle overnight. If your scope is outdoors on a high pollen night, the same thing could be happening to your lenses. If you work in a dusty area (something you should try to avoid just on general principles) you will have the same concern about contamination. Then there is the issue of liquid contamination. You've spent the whole night trying to find that faint dwarf galaxy that you read about in Sky & Telescope, then as fast as you found it, it is gone because the objective of your telescope has become fogged with dew.

In addition to the optical components discussed in the previous chapter, a good addition to any equipment setup is a good optics cleaning kit. Both Celestron and Meade offer cleaning kits, as should any camera store worth its salt. A typical kit will contain a box of special tissues for lens cleaning, a bottle of lens cleaning fluid and a can of compressed air. When cleaning your telescope, remember that you have two major objectives. Remove the dirt and avoid scratching any of the surfaces as you do so. Given a choice between a small amount of dust and a single scratch on my objective, I will take the dust any day. Contrary to popular opinion, objectives must become very dirty before optical performance begins to become impaired. It only takes one little scratch to render a precision scientific instrument all but useless. When cleaning your telescope, you should always start with the least intrusive method possible. That means that if you can remove the contamination without touching the telescope, so much the better. For dust removal therefore, your best friend is that can of compressed air. As long as the dust is not bound to the glass by some other contaminant such as tree sap, the dust should blow right off. If the dust resists, then it is time for step two. Prepare the lens cleaning fluid according to the directions that are provided with it; it should be diluted. Apply a tiny amount (using an eye dropper) to the area to be cleaned. Use the corner of a piece of lens cleaning tissue and by just barely allowing it to touch the top of that drop of fluid, clean the surface by dragging the fluid droplet across the lens. Do not if at all possible allow the tissue itself to touch your telescope. Only once you are absolutely certain that there is NO dust or pollen on your telescope, should you allow the tissue to touch the glass. Lens cleaning tissue is designed in such a way that the fibers of the paper are woven in a pattern that will not scratch the glass. If the tissue catches a dust grain however, that grain can be dragged along the glass surface leaving minute scratches behind. When the lens is spotlessly clean, you may then use the lens cleaning tissue to dry the optical surface. Dealing with sticky contaminants like the aforementioned tree sap requires a bit more care. You may apply a slightly more liberal amount of lens cleaning fluid to the contaminated area and allow it to break up the sap deposit. Then if there is no dust on the lens surface, use the lens cleaning tissue to dry the area. Take great care not to use too much pressure on the glass surface.

Certain types of telescopes require more care than others. Traditional Newtonian or Cassegrain reflectors have optical tubes that are exposed to the elements whenever they are in use and as such, the primary and secondary mirrors are continuously exposed to contamination when the covers are removed. These surfaces are exquisitely difficult to reach and clean and for that reason the popularity of these scopes fell off sharply when the closed-tube Schmidt-Cassegrain design appeared in the early 1970s. Refractors and Schmidt or Maksutov Cassegrain telescopes have tubes which are closed so interior surfaces are almost always contamination free. Just always remember to keep the dust cover on the visual back or focus tube. The objective lens of the refractor or the corrector of the catadiop-tric telescope is exposed to the elements and must be properly cared for.

Eyepieces are also finely machined optical components of your telescope and must be cared for in the same way as other optical surfaces. For removing dust, again use the compressed air wherever possible. The can will usually come with an extension to allow you to gain access to cramped areas near the edges of the lens. Don't forget to clean both sides of the eyepiece since there is an exposed glass surface inside the tube as well. Some pieces of equipment may be more difficult to clean than others. A Barlow lens for example may have the lens buried in the middle of a long tube where only a dust grain can get at it. Don't do what one friend did and place a piece of tissue on the end of a long toothpick. He got the lens clean, but only after the toothpick scratched the Barlow into retirement.

By the way, also take care to remember your finder scope as well. I've met more than one person around an astronomy club who does a great job caring for their main scope only to have a finder that is nearly useless because of the dust that has built up on it. Clean it with the same attention and care that you clean your main telescope with because it is an important part of the optical system. A telescope that cannot be aimed is completely useless. Use the compressed air can to clean the objective lens and the fixed eyepiece. If you need to resort to fluid and tissue cleaning, remember that if you rub too hard in addition to scratching the lenses you may also push the finder scope out of alignment. This will cause you to have to spend much time later realigning the finder scope, time you would rather spend doing other things.

As the evening wears on,the temperature invariably will begin to cool and things around you may begin to get a little soggy. Dew will form on any surface when its temperature cools below the dew point. Dew point is the temperature to which a parcel of air must be cooled in order to saturate it with only the existing moisture. The glass components of the telescope will cool faster than the air so even though the effects of the cooling temperature will not be immediately seen in the weather (fog), you will see it on the objective of your telescope. Different types of telescopes are affected in different ways. Open-tube reflectors are actually well protected because the glass elements are deep inside the tube of the telescope where they will cool much more slowly. Refractors are more susceptible since the lens is much closer to the outside air. Still a typical refractor is slow to dew because they usually have a tube extension that shields the lens. Catadioptric telescopes are the most vulnerable because the corrector plate is at the very end of the tube where it has no protection at all. The exposed corrector will often dew over very quickly at the beginning of a session. Most catadioptric owners (including yours truly) have added a dew cap to their collection. A dew cap is a plastic or metal tube that is designed to fit over the front end of the telescope to help retain some heat near the corrector plate and prevent the formation of dew. Once it has formed however, there is only one way to safely get rid of it. Falling temperatures put dew on your lens; only rising temperature can get rid of it. If dew is an issue for you, bring a battery powered hair dryer with you. Blowing some hot air over the corrector plate will quickly dissipate the dew. Always remember to do this after the end of an observing session. If the telescope lens is dew covered after bringing it inside, turn that hairdryer on the lens until it is dry. Do NOT ever cover a wet corrector or lens. The result of that will possibly be an accumulation of fungus growing in the greenhouse-like environment underneath the lens cover.

Caring for precision optical surfaces is not at all difficult, but it must be done correctly. Clean lenses and mirrors allow the sharpest possible views of the heavens. Just remember that if you're nervous about it, don't be afraid to get it cleaned professionally. If you can't do that, a slightly dirty lens or mirror is still far better than a scratched one.

Telescopes Mastery

Telescopes Mastery

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