Caring for Your Optics
Any telescope or binocular that you use for astronomy, no matter how humble it may be, deserves the best care you can give it. Much of the time you’ll be using it right at the limit of its capabilities, and when you’re trying to see very faint objects or fine detail, little things make a big difference.
Then again, life is full of imperfections, and there’s no point fretting about them. Every telescope gets dirty. Dirt on lenses or mirrors scatters light, making dark skies less dark and bright objects less crisp but not nearly as much as you probably think. The right attitude toward optics means knowing when to be vigilant and when to relax.
The first tactic against dirt is defensive, and this is when you should be vigilant. Keep the lens caps on when the instrument is not in use. If it’s missing a cap, make your own; a shower cap, or a plastic bag or dishcloth held over the front of the tube by a rubber band, works fine. As for the eyepiece holder, a plastic canister for 35-mm film fits the standard 1¼-inch focuser size. So does a wad of cloth.
I store my two reflectors with their main and secondary mirrors both facing somewhat down. That way dust won’t settle on them in storage. Eyepieces should be capped on both ends or kept in plastic bags or small plastic food containers. Telescope retailers sell cheap, durable cases to store and protect eyepieces.
Never touch the surface of a lens or mirror. The acids in skin oil can attack optical coatings over time. If you do leave a fingerprint on, say, a binocular lens, clean it off using the method described below.
So much for vigilance now to relax. Dirt happens, and in moderate amounts it has amazingly little effect on performance. In his book Star Testing Astronomical Telescopes, Harold Richard Suiter analyzes the effects of dirty optics in full mathematical detail. His conclusion? “The maximum amount of dirt [that a perfectionist] should tolerate on the optics is about 1/1000 of the surface area, the size of a single obstruction about 1/30 of the diameter.” In other words, on a 10-inch-diameter telescope mirror you can have as much dirt as in a completely opaque blot a third of an inch across. That’s quite a lot of crud to have no effect at all.
“Don’t decide to clean mirrors on the basis of shining a light down the tube at night,” advises Suiter. “All mirrors fail such a harsh inspection.” After you’ve done what you can to prevent dust, ignore it.
There’s a good reason to ignore dirt, aside from reducing the things to worry about in life. A dirty lens or mirror can always be made clean, but a scratched one is scratched forever. Cleaning causes tiny scratches, or sleeks, if you don’t do it right, and maybe even if you do. A few sleeks don’t matter, but a lot of them will. So clean your optics rarely.
But if things get really bad and you decide a cleaning has to be done, here’s how.