Cleaning a Mirror

To clean the mirrors in a reflector telescope, you’ll need to be confident about taking the telescope apart and putting it back together again — and about collimating the telescope (aligning the mirrors) once it’s back together.

How do you know whether your mirrors need cleaning? It’s simple: if you’re asking the question, they don’t! Leave the mirrors alone unless they are so obviously crudded up (probably due to careless storage) that there’s no question.

If you do go ahead, here’s the cleaning procedure.

Undo the screws attaching the main mirror’s cell to the back end of the tube. Reach in the back and gently pull out the cell with the mirror inside it. Unscrew the clips holding the mirror in, and push the mirror out from the back without touching its shiny surface.

You’ll also need to remove the holder for the small secondary mirror inside the front end of the tube, and then get the secondary mirror out of the holder.

The first and most important cleaning step is to remove all grit correctly.Ordinary house dust contains bits of rock powder, and rubbing this stuff against glass (or, actually, against delicate optical coatings) causes sleeks. So you’ll need to get rid of grit without rubbing.

You’ll need the kitchen sink, two towels, liquid detergent, a bottle of distilled or demineralized (“de-ionized”) water (available in drugstores), and a package of sterile cotton (if it’s sterile it’s more likely to be grit free). Wash out the sink, rinse it well, and lay a folded towel on the bottom. Take off any jewelry from your hands and wrists. Put the mirror face-up onthe towel, and with the drain open, blast the mirror’s surface with room-temperature water for a few minutes. This will remove most dust and grit safely.

Turn off the tap and give the mirror a final rinse with a slosh of distilled or demineralized water. This will leave no mineral deposits when it dries. Stand the mirror on edge (on a folded towel to prevent slipping) and let it dry. You can draw off stubborn water droplets carefully with the corner of a paper towel. If the mirror looks reasonably clean, quit while you’re ahead. You can’t scratch a mirror you haven’t touched.

If it’s still cruddy, there’s more here than just surface dust, so you’ll need to go to Plan B. Plug the sink, put the mirror back in on the towel, and fill the sink halfway with lukewarm water. Add a squirt of liquid detergent and let the mirror soak for 5 or 10 minutes. Then, holding it underwater, swirl it around for a last chance at rinsing off loose grit.

Take a wad of cotton and, starting at one edge, swab the mirror in one direction, applying no pressure beyond the weight of the cotton itself. Grit is less abrasive wet than dry, so do this step under water if you can.

Turn the cotton over in a backward-rolling motion as you go, so that as soon as a part of it rubs the surface, that part is carried up and away from the glass. Throw out the wad when it has been turned completely. For a big mirror, the job may take a lot of cotton.

It’s good to work in complete silence. If you make sleeks, you may actually hear them! If so, stop and proceed to the rinse.

Drain the sink and run lukewarm water over the mirror for a minute. Finish with a rinse of distilled water, and tilt the mirror on edge to dry. Repeat the process with the small secondary mirror.

If you’re ever faced with a truly ghastly cleaning job — for instance, if you’ve just rescued a $2,000 telescope from 10 years of moldy exile in a relative’s basement (something we get calls about all too often) — call the manufacturer and ask about a professional makeover. It may not come cheap, but it’ll be cheaper than a new scope.

The right attitude is to be vigilant about preventing dirty lenses and mirrors — and then forget about them. Perfectionists are never happy, but astronomy should be fun. After all, what matters is not what you see on your telescope, but what you see through it.