Cruddy Things on Optics

Jack Kramer

Dust and grime on lenses is not something we take lightly. Like most amateur astronomers, I don't make a careful check of eyepieces, lenses, and mirrors each time I set up to observe. Then when I do take a close look at them, I wonder "How the heck did they get so filthy?!"

It seems as if there is no way to keep dust and grime off optical surfaces. You no sooner clean your primary mirror, then while admiring its glistening surface you begin to notice one, two, then an avalanche of dust motes beginning to settle on it. I don't have the answer to this, but here are some ideas that have worked for a number of observers.

If you can see dirt on your optics when you look at them face-on, then they are very dirty. Normally, they don't seem too bad until you hold them up to the light, looking at a shallow angle. That's when you see what's really on the surface. It also helps to use a magnifying glass to examine any small specks.

Don't get anal about dust. Dust is everywhere, so you know some of it will settle on your optics. Experts tell us that it's better to tolerate a certain amount of dust than to risk ruining the coatings on lenses and mirrors by excessive cleaning. For one thing, large observatory telescopes don't get cleaned very often, and if you've ever looked closely at one you've probably noticed that it was pretty dirty. That doesn't seem to bother the pros, so why should it bother us? It's surprising how much dust can accumulate on optics before the effects become noticeable. The main effect is that a large amount of dust scatters light so that a bright object tends to have a halo. I find that cleaning my primary mirror once a year is adequate unless it has been exposed to particularly dirty conditions. Others do it even less often.

Avoid dust and grime. Prevention is one answer, and there are steps we can take to prevent exposing our equipment to crud during an observing session. For example, if you must set up your scope on a dusty surface, lay down a tarp first. This minimizes the dust getting kicked up as you walk around. If you take a break from observing for awhile, cover the scope if it's convenient to do so. This will not only protect it from exposure to the elements but may also forestall your optics dewing up. A cover should just be draped over the scope, not wrapped tightly. This allows air to circulate while still slowing the radiational cooling.

A pocket is handy, but... Avoid the temptation to stick that extra eyepiece directly into your pocket, which is usually filled with lint and other stuff. To keep an eyepiece handy, first place it in a zip-lock sandwich bag before putting it in your pocket or else replace the lens caps on both ends. If, like me, you still tend to dump your eyepieces directly into a pocket, then for heavens sake, when you wash your jacket, turn the pockets inside out so they get a good cleaning and any lint or other crud gets washed away. And of course, if you do use a pocket to hold eyepieces, make sure there's nothing else in it like gloves or a flashlight. Some observers use the multi-pocketed photographers' vests for eyepieces.

How you store optics is important. If you keep a telescope in your garage, don't be surprised if the optics get dirty rather quickly. My garage is notoriously dusty, simply because those large doors admit a lot of wind-borne particles. On top of that, a car tracks in crud from the street, and along with the lawn mower and snow blower, it exudes oil and gas fumes. The dirt even seeps in around plastic covers. This produces a deposit of grime on the optics that is far worse than simple dust. Grime is much harder to clean off and has a more detrimental effect on the performance of the telescope. Damp basements have also been known to result in a deposit of mildew on optics. And bugs can get into almost anything. One report disclosed that tiny spiders had ruined an entire set of oculars by building their nests within the eyepiece barrels; the lens coatings had become etched by whatever chemicals the spiders exude. The best bet is to find a clean and dry area inside your house, then store the telescope on its "nose" - with the primary mirror or objective lens facing downward. (Most dust seems to settle from above.)

Keep covers clean. Okay, so you use plastic covers or end caps for the scope. If the covers get very dirty, every time you take them off you're stirring up whatever particles are clinging to the outside. And guess where some of those particles end up floating to! It's not necessary to keep covers scrupulously clean, because let's face it, they're there to take the dust instead of the optical surfaces. But once in awhile it's not a bad idea to clean off the covers. I've heard the same thing said about the telescope tube itself. If you take the mirror out to clean it, use that opportunity to clean up the body of the telescope, inside and out.

Cold optics need special care. Once you're through observing on a cold night, what's the best way to handle your scope? When you bring it in the house, the optics immediately fog up. As a result, I had always replaced the lens caps only after the condensed moisture had evaporated. However, an observer in Canada pointed out a different strategy; I tried it and it does seem to lessen the amount of condensation. Since the cold outside air can't hold nearly as much moisture as the warm indoor air, it's best to replace the lens caps while the telescope is still outdoors. Assuming your caps fit tightly, this traps the drier outdoor air next to your lens. Once you bring the scope back inside and the optics gradually re-acclimate to the warmth, less moisture will condense on the optics because of the drier nature of the trapped air. After awhile, remove the covers to let any remaining moisture evaporate. By doing it this way, you also minimize the amount of dust and other particles that might adhere to the wet optics. This results in fewer water spots and dirt on the optics.

Never rub hard. When giving your equipment a cleaning, a rule of thumb is to use no more pressure on the optics than the weight of the cotton ball or lens cleaning tissue. To do otherwise is to risk inducing a scratch on the optic.

Beware the speck that won't blow off. If you find a speck on a mirror or lens that won't blow off, it's better to leave it there for the time being. It might have landed when the optical surface was damp and become sort of "glued" in place. Or the speck may have some adhesive properties of its own. The pressure required to dislodge that speck might damage the coating of the mirror or lens. The particle might also be abrasive, in which case you could end up with a nasty scratch. If it's on a mirror, it'll probably come off when you flood the mirror with water during the cleaning process. Also, never use the "canned air" sold in many camera stores. Minute particles of the propellant are atomized and will likely be deposited on your optics while trying to blow off the dust. Most drug stores carry rubber bulb type syringes that work well for blowing off loose dust particles with minimum risk of depositing other substances on the lens or mirror. If a speck remains on an objective or eyepiece lens, then it will need a careful cleaning, which leads to the next point...

Never flood a lens. To clean a lens, use only a little distilled water or lens cleaning solution, then spread it with cotton or lens tissue. If possible, hold the lens so it's facing at a slight downward angle. This allows the excess liquid to run off and not seep in around the edge of the lens and into the lens cell. You can also use the same mild soap solution that is used to clean mirrors, then rinse the lens with distilled water. This does a good job of getting rid of grime and fingerprints. To apply any liquid, use a squirt bottle, or a dampened cotton swab or Q-Tip (again, use minimal pressure). Here is the URL for a web page devoted to cleaning eyepieces, courtesy of Tele Vue:

While you're cleaning the eyepieces, how about your filters? They're virtual magnets for fingermarks.

If a spot won't come off, leave it. If you've cleaned the mirror or lens and there are still some small spots, it's better to leave them as is. Applying pressure could do more damage. Sometimes permanent spots appear on optical surfaces due to pollutants in the atmosphere that are deposited when the optics become wet with dew. Faint spots could also be a sign that the optical coating is breaking down. It may be time to re-aluminize a mirror.

It's not uncommon for a lens to become covered with faint "speckles" in the top layer of the coating. Other than routine cleaning, there's not much you can do about this; lenses are seldom, if ever, recoated. Eyepieces are susceptible to spotting from bodily oils deposited by fingers and eyelashes. Women's mascara makeup is especially rough on the eye lens of an eyepiece. For this reason, it's a good idea not to use your best eyepieces for public observing sessions. A rubber eyecup on the eyepiece helps people position their eyes at the right place to view through the telescope and it also helps protect the lens from direct contact with eyelashes.

A few years ago, we had an article in the newsletter with specific steps on how to clean lenses and mirrors. If you have Internet access, you'll find it archived on our club web site. Otherwise, I can make a copy of the article for you. You'll also note some variation in the procedures that people prefer for maintaining optics, but the suggestions listed here find almost universal agreement. Most books that address telescope usage also cover cleaning and maintenance of the optics. One good source is Philip Harrington's book Star Ware.

Published in the January 2002 issue of the NightTimes