Cleaning Lenses and Mirrors

Jack Kramer & Grant Barlow

A few months ago, one of our members tried to be helpful by cleaning the mirrors of the club telescope. Unfortunately, he used a "lens cleaning" cloth that left a residue on the mirrors. There's only one right way to clean optics, and for the sake of your own telescope -- and the Club's -- we've resurrected this article that was first published a few years ago in NightTimes.

If you own any kind of optical instrument, it's important to master the fine art of cleaning the mirrors, lenses or corrector plates. Optical surfaces do get dirty from dust, dew, and airborne pollution. A dirty mirror or lens results in less light being transmitted or reflected, and the objects you see may have a halo of glare around them from light scattered by the dust.

First, the Materials - The optical surfaces in telescopes are very precise and are coated both to protect them and to increase light transmission or reflection. These coatings are more delicate than what you may be used to, so don't try to clean them like you'd clean some other optical surfaces. Above all, avoid "lens tissue" or a "lens cloth" like the's usually too abrasive and may do more harm than good! The best cleaning solution is just a bit of mild dishwashing detergent in warm water, and under certain circumstances, plain distilled water may do the trick. Above all, tap water should only be used as an initial rinse to get rid of the worst of the dust or in a soapy solution; whenever straight water is used, it should be distilled water. (Minerals in the tap water will leave spots.) We've also heard of using a cleaning solution of 50:50 Windex and water for lenses.

If the optics become dewed up while you're out observing, you can gently swab them off with a cotton ball. After all, dew is another form of distilled water; however, considering airborne acids and pollution, extensive dewing means that you should probably give the optic a proper cleaning at your first opportunity. Another form of distilled water is your breath which, together with gentle use of a Q-tip, is a respectable way to clean eyepiece lenses.

A cotton ball (100% real, sterile, surgical-quality cotton) is the best material with which to wipe the surfaces. But most cotton balls will leave little "hairs" on your optics. Some drug stores carry a type of cotton ball that is lint and fuzz free; though they cost more than the regular variety, they're well worth it. Change cotton balls several times so as not to spread around the same dirt you just cleaned off, scratching the surface in the process. Always hold the cotton ball by the same spot so that your skin oils aren't transferred to the optics. A good way to get a speck off your lens is to use a very fine camel's hair brush that's specially made for cleaning lenses, or use a Q-tip...or just blow it off. Speaking of blowing off the dust, it's not wise to use the aerosol "canned" air sold in camera stores. This air may contain traces of the propellant chemical.

Lenses and Corrector Plates - Both these items are cleaned in pretty much the same way. If all you want to do is remove dust, they may be gently swabbed with cotton balls and a small amount of distilled water. ("Gently" throughout this article means applying no more pressure than the weight of the cotton swap; merely drag it over the surface, applying no pressure.) Point the lens or corrector plate slightly down toward the ground so that the water runs off rather than seeping into the telescope body from around the lens. If the optics are really dusty, it's advisable to use a liberal rinse of distilled water before swabbing with a cotton. That way, most of the dust will be floated away rather than moved around with the cotton ball to possibly scratch the lens coating. If your lens harbors a coating of grime, liberally swab it off with a soap solution, then rinse with distilled water.

Unless you're very experienced in optical techniques, avoid removing the refractor lens cell from the telescope tube -- recollimation of a refractor is extremely difficult. Good refractor lenses are made as carefully matched sets of individual lenses; even rotating these individual lenses in the cell can destroy the ability of the system to bring objects to best focus. So never remove lenses from the cell. If some water happens to seep into the cell, leave the telescope with the lens uncovered in a warm room until it is dry.

What!! Remove the Corrector Plate? Maybe. Most Schmidt-Cassegrain telescopes will just need the outside cleaned. If there is dust or a film on the inside of your corrector plate, or you want to get rid of stains on the enclosed optical elements of your Schmidt-Cassegrain, then the corrector plate must be removed. It's not a good idea to remove the corrector plate unless you definitely need to clean the internal surfaces. For those brave souls who want to clean the inside, here's how to remove a corrector:

1. Place the telescope with the plate facing up so the name on the secondary holder is perpendicular to the top/bottom axis of the plate.

2. Carefully remove the screws and retaining ring.

3. Mark the positions of the shims around the plate with a permanent-type marker. (A small drop of colored fingernail polish at the position of each shim works fine.)

4. Very carefully and gently pry (yes, pry) the plate from its rubber/felt mounting with a very small screwdriver.

5. Keeping track of the shims, gently remove the plate to your work surface.

6. After cleaning, reassemble in reverse order. Make sure the shims are in their correct spots.

Some SCT owners report that after a good cleaning of both sides of their corrector plates, there is less of a tendency for the plates to fog up on humid nights. SCTs are closed systems, so their inside surfaces are well protected, but these scopes aren't air-tight. That means there will come a time when the inside surfaces have to be cleaned.

Mirrors - To do an effective job, you will have to remove the mirror from the telescope. If the mirror is very dusty, you can give it a preliminary rinsing with tap water. It's better to get rid of the major dust particles this way rather than by wiping, which may scratch the surface. To clean off the grime, bathe the mirror liberally with the soap solution, drawing the cotton ball over the entire surface to remove stubborn spots - use no pressure at all on the cotton ball. Then rinse the mirror by flooding it with distilled water. Now soak another cotton ball in distilled water and gently wipe the entire surface again. (You may note some tiny bubbles; that's an indication of residual soap film that this second swabbing will remove.) Then immediately flood the surface with distilled water again and hold the mirror on edge to let all the droplets run off. There should be virtually no droplets left on the surface, but if there are any, gently blow them off using a soda straw to concentrate and aim your breath. (The soda straw trick also helps when cleaning a lens or corrector.)

When Do My Optics Need a Cleaning? One indication that they need a cleaning is when normally bright objects look unusually dull in a good sky. Another indication is when bright stars have a persistent glow around them. This is caused by light from the object that's scattered by the dust and grime on the lens or mirror. Obviously, another indication is whether the optics simply look dirty. We've never heard a definitive rule as to how often optics (especially mirrors) should be cleaned. We have heard that it's better to tolerate some dust rather than always strive for a clean mirror. No matter how careful you may be, cleaning wears away minute traces of the coating and often creates tiny scratches. The loss of image quality due to dirty optics is minor...up to a certain point. Also, dust gets into even a scope in storage, unless it's very well sealed.

Finally, spots that will not come off a mirror when it's thoroughly cleaned may indicate a breakdown of the coating...if so, it's time for a re-aluminizing.

Published in the January 1997 issue of the NightTimes