6. My mirror/lens/eyepiece is dirty. Should I clean it, and with what?

There are two schools of thought on this — think of their practitioners as the Felix Ungers and Oscar Madisons of the telescope world.

A few years ago, at a large star party, the owner of a very expensive refractor brought his prized telescope to the manufacturer’s tent, asking him to clean its lens. The optician disappeared into the back. “Oh, boy,” the excited owner thought to himself. “I’m about to get a cleaning lesson from the master himself!” The optician came back a few minutes later with a bottle of Windex and an old, yellowed piece of cheesecloth. “You guys worry about this too much,” he said, as he scrubbed the lens in front of the horrified owner.

I know lots of people who, like this guy, fret over every speck of dust on their optics and obsessively clean them. At the other end of the spectrum are people — and I used to be one of them — who never clean their optics. Their rationale is that you can possibly do more damage to your precision glass surfaces by cleaning them than by just letting them be dusty.

The right approach is probably somewhere in between these extremes. The key is prevention and moderation — use those dust caps, and clean only when absolutely necessary.

To clean your reflector’s mirror, take it out of its support cell and run distilled water over its surface. If the mirror looks OK after this, stop — you’re done. Carefully set the mirror on edge and let it air dry. If not, let the mirror soak in warm water with a tiny drop of dishwashing detergent added to it. (Make sure your sink is clean first!) Follow with a rinse of distilled water, and you should be good to go. If it’s still dirty, more drastic measures may be necessary. For the gory details, check out Alan MacRobert’s article on cleaning optics.

When it comes to grubby refractor lenses or dirty eyepieces, a different approach is needed. Whatever you do, don’t dismantle them. The best way to clean lenses is to gently wipe away the grime with a lint-free cloth or cotton swap moistened with pure isopropyl alcohol or a specialized lens-cleaning solution. But before you start, make sure there are no dust particles on the surface of the lens; a blast of air or a feather-light brushing will usually take care of this. A heavier-handed approach might scratch the coatings or perhaps even the glass itself. Refer to the above article for more suggestions.