Astronomy Hacks

Clean Your Eyepieces and Lenses Safely

Seven steps to better images.

All optical instruments use lenses in one form or another. Refractors have a single large objective lens, and binoculars have two. SCTs and other catadioptric scopes have a corrector plate, which is actually a large lens. Even Newtonian reflectors have lenses in their eyepieces.

Lenses inevitably become dirty with use. A dirty lens provides poor, lowcontrast images, so it’s necessary to clean your lenses from time to time. But lenses are fragile items—particularly their anti-reflection coatings—so it’s very easy to do more harm than good if you clean them improperly.

Tip

The first rule of cleaning eyepieces and lenses is to do everything you can to avoid the necessity. Store your optical gear in a dust-free location or in sealed containers. Keep the lens caps on when you are not using the instrument. Handle the equipment carefully to avoid getting fingerprints (or nose-prints) on the lenses. Avoid cleaning lenses unnecessarily. A few specks of dust or a tiny smudge has no effect on image quality. More lenses have probably been ruined by unnecessary, careless cleaning than by any other cause.

Fortunately, the proper lens-cleaning procedure is straightforward. Use the following steps to clean your lenses safely and effectively.

1. Establish a clean, uncluttered, well-lit working environment. The kitchen table is usually a good choice. You’ll also need a strong light, such as a flashlight, that you can direct at a grazing angle against the lens surface to reveal grit particles on the lens. If you are cleaning a large lens, such as a refractor objective, that you have removed from the scope, wear thin cotton gloves to avoid getting fingerprints on the glass.

   Warning

   Never clean a lens at the observing site if you can possibly avoid doing so. If you must clean a lens in the field, work inside your vehicle or another sheltered location where you can avoid the dust often present in outside air. Use a strong white light. It’s impossible to clean a lens safely under red light because you can’t see the grit particles that will scratch your lens.

2. Use an ear syringe or similar device to blow off any dust visible on the surface of the lens. Don’t use your mouth, or you will probably spray saliva on the lens. (In an emergency, we have blown through an ordinary drinking straw to remove dust from an eyepiece.) Don’t use canned air, either, because it may spray propellant onto the lens.

3. Examine the lens carefully under a strong light to make sure no dust or grit remains on the surface. You can remove a persistent particle by slightly moistening a tissue or cotton swab with Windex or a similar cleaning solution and blotting (not rubbing) the surface of the lens to pick up the particle. If necessary, use the blower again to remove any remaining dust.

   Tip

   Although you can buy lint-free lens cleaning tissue, we prefer to use ordinary facial tissue or toilet paper. If you do the same, make sure it’s unscented and not treated with lotions or other adulterants. Toilet paper is particularly safe because you can simply unroll the outer layer or two to get a piece certain to be free of grit.

   Some people prefer to use cotton swabs or cotton balls. If you do that, make sure they’re pure cotton (often described as surgical cotton) rather than an artificial fiber. Cotton balls and swabs sold for medical use are usually pure, grit-free, sterile cotton. Those sold for fingernail polish removal are often made from artificial fibers and may contain grit.

4. Moisten a tissue or cotton swab with cleaning fluid. You want it damp, but not dripping wet. Begin at the center of the lens and wipe outward very gently, using a circular motion, as shown in Figure 4-17. If you’ve used too much cleaning fluid, it will spread over the lens surface. You want the swab or tissue to be just wet enough that you can watch a bead of cleaning fluid follow the movement of the swab or tissue. If you stop moving the swab or tissue, it should immediately reabsorb the bead of cleaning fluid. If you’ve used too much cleaning fluid, or if you move the swab or tissue too quickly, the bead breaks away from the swab or tissue and will dry on the lens, leaving spots.

   

   Figure 4-17. Cleaning a binocular objective with a cotton swab

5. Replace the tissue or cotton swab frequently as you clean the lens. You may be able to clean an eyepiece or other small lens with one tissue or cotton swab. For larger lenses or smaller lenses that are badly smudged, using one tissue or swab too long merely spreads the gunk rather than picking it up.

6. The edge of the lens is the hardest part to get clean. As you approach the edge, replace your tissue or swab with a tissue folded into a sharp corner and moistened slightly with cleaning fluid. (Make sure not to touch the working area of the tissue with your finger to avoid contaminating it with skin oils.)

7. When you complete this procedure, the lens should appear clean when examined with a strong light at a grazing angle. If any lint fibers from the tissues or swabs remain on the lens, use your blower to remove them.

Some astronomers treat the choice of cleaning fluid as a religious issue, but the truth is that almost anything reasonable works fine. We have used commercial lens cleaning solutions, methanol (methyl alcohol), ethanol (ethyl alcohol), isopropanol (isopropyl alcohol), and acetone. (Be careful using acetone around painted or plastic surfaces; it may dissolve them.)

Tip

Some astronomers clean their lenses with a LensPen (http://www.lenspen.com), but the LensPen scares us despite the reassurances of its maker. The LensPen uses a two-step cleaning process. You first use the brush end to remove particles. (That in itself makes us uncomfortable; we fear using a brush on a lens may scratch the coatings by dragging an abrasive particle across the surface.) Once the lens is free of dust, you rub the other end of the LensPen against the lens using a circular motion to wipe away fingerprints, smudges, film, and other contaminants. The LensPen is said to be good for 500 or more cleanings, depending on the surface area of the lens and how dirty it is. The idea of reusing a cleaning surface once, let alone 500 times, scares us to death.

Perhaps we’re just timid. Thousands of photographers and astronomers use these devices to clean their lenses, and we’ve never heard a report of a lens damaged by a LensPen. Even so, the traditional method is cheap, safe, and effective—albeit less convenient than using a LensPen—so that’s what we continue to use.

Although some astronomers insist that only reagent-grade solvents are acceptable—and we suspect our organic chemist friend Paul Jones of using purer still spectroscopic grade solvents on his lenses—we’ve never had a problem using drugstore-grade USP solvents. Even the alcohols and acetone sold in metal gallon cans at the hardware store are fine. The important thing is to use a pure solvent rather than a mixture. For example, the acetone sold as fingernail polish remover often contains scents and oils that leave a film on the lens. Similarly, some drugstore alcohol contains scents, rubifacients, or other adulterants.

Our first choice of cleaning solution is isopropanol. We prefer 91% isopropanol, but that may be difficult to find. You can buy 70% isopropanol at any drugstore for about $1/pint, and it also works well. For water-soluble spotting, such as that caused by dried dew, commercial glass cleaners like Windex or Glass Plus work fine. (Just don’t spray them on the lens as you would on a window.)

Tip

For particularly filthy lenses, you may need to use heroic measures. Public star parties are notoriously hard on eyepieces, which is why most amateur astronomers use only “throw-away” eyepieces at public observations. After such events, we have seen eyepieces contaminated with an incredible array of substances. Fingerprints and noseprints are common, of course, as is mascara, but you can expect your eyepieces to be attacked by anything up to and including bubblegum at public events. For cleaning such eyepieces, we start with acetone, which is usually our last resort.