A new technique for entire mirrors or just small areas and minor spots
The bathing technique described elsewhere on this site works very well for a general cleaning but bathing has been suspected of allowing water to get under the coating if excessive cleaning is done. As an example, I had at least one customer who actually cleaned his mirror every time he went out and used it and eventually the coating began to deteriorate at the edges! - not recommended. For small areas, or even entire mirrors, a different approach has been found to work well and does not require submersion in water. Before attempting this I urge you to read the entire article very carefully. This technique has performed excellently for me but it requires careful attention and a little practice. I can assume no responsibility for damaged coatings or surfaces. A coated surface is a delicate thing and can be easily damaged. But I must say that I have cleaned incredibly dirty mirrors using this method and have made them look like new.
Excessive Cleaning and Impact of Stains
Some of my customers have had problems with coatings because they have cleaned too frequently. It's nice to have a pristine mirror that looks like it just came out of the chamber, but in an effort to keep it that way some people clean their mirrors too much. Constant cleaning can cause coatings to become thin and develop problems such as peeling away at the edge where the water gets under the coating. Old, incredibly dirty mirrors that look like they have hair growing on them actually work better than you might imagine but the trick is to keep the mirror from getting dirty in the first place. A cap for the tube or the mirror alone, if you have an open tube, is a good idea. Cover your mirror if you are not using your telescope even for a 1/2 hour warm up or equipment adjustment. Leaving a mirror uncovered all night will result in either a dust covered or stained mirror. In dry climates you are likely to get a dusty, dirty mirror; not so horribly bad but you may scratch the coating trying to clean it. In humid climates in warm weather you will likely have a spotty, dew-stained mirror in the morning; and the nature of the staining is particularly destructive, more so than just a finger print or loose dirt. Think about it. The water in the air is full of all kinds of chemicals in solution and this stuff comes out of solution as the water soaks into the porous silicon monoxide overcoating and eventually dries. Yes, porous. And it gets in there and under the overcoating and onto the aluminum where it reacts and does its irreversibly destructive work on the metal. Not good. Finger prints are less destructive because they just sit on the silicon surface where they do little damage. If you have a bare metal coating, finger prints can be more destructive because they can attack the metal directly.
Things You Will Need
Viva brand name paper towel. No other kind will do. Do not use Kim Wipes, lens paper, toilet paper or anything else. Viva is a special paper that works the best for this application. Viva is found in US supermarkets everywhere.
Acetone. About a pint at least. Acetone is a highly flammable liquid and should not be used when smoking, near open flames or stoves. It is also poisonous if taken internally, label all containers. It is sold to adults to be used by adults. Children are to be nowhere in the area and must not have access to it. Acetone also gives off fumes and should be used in a large room. Prolonged contact with the bare skin is thought to be dangerous, so you might want to use rubber gloves. I use acetone a lot without deleterious effects but don't use it recklessly. Do not use MEK (Methyl Ethel Ketone), it takes too long to dry and is not as aggressive a solvent.
Windex or Zep glass cleaner. These two types I have used with success.
You will need to be left alone. You can not be interrupted during this operation.
Protected aluminum is more durable than many people think. I have experimented and have tried to destroy protected aluminum coatings and other coatings by rubbing the surface with my hands and paper and have found it more difficult than I imagined to harm it. Eventually, the coating does pick up scratches and abrasions and scuffs, but it takes a bit of effort. Rubbing dirt into the surfaces causes instant scratches. Dirt is really the culprit. Grease and finger prints do not cause scratches, but smearing dirt-laden grease around the surface will. Bare aluminum mirrors and Beral coated mirrors are easier to scratch and damage. Beral is about five times harder than bare aluminum, about as hard as chrome.
Cleaning an entire Dirty Mirror
Preparation Take a single sheet of Viva, cut it into four square sheets. Fold a cut sheet twice over so you have a folded sheet about 3 inches square. Set the mirror on a level surface that can stand getting wet with acetone and water and glass cleaner. Acetone is a very aggressive solvent may wreck your counter top, so a sheet of plastic may be useful here.
Step 1 If the mirror has recently been purchased (or cleaned) and has not yet acquired stains and water spots, you can skip to Step 2, but if it's really dirty and stained, continue here. Pour some acetone on the mirror and soak the Viva sheet in acetone. Gently, nay, lightly, rub the mirror surface with the soaked paper in circular motions making sure to keep the surface wet. Acetone dries very quickly. This will get off a lot of the dirt and crud. The main job is to flush off dirt and get off stains that can be removed at this level. Rub only a minute or so and then change to a clean pad and rub again so you won't be smearing and grinding in the same dirt. The acetone will dry up quickly and you should stop rubbing just before it dries. The soaked paper will dry after the mirror, so when the mirror is dry the paper will still be wet with acetone. The mirror will not look particularly good at this point and may appear stained and smeary but don't worry just yet.
Step 2 Now comes the dicey part. Only experience will make you good at this and unfortunately you'll have to 'get good' on your own mirror. What needs to be done is to spray the mirror with Windex just covering the surface with a loose mist - not too much, just a thorough dusting. Another piece of folded paper is also sprayed with Windex until damp. Never wipe a coated surface (or any optical surface) with a dry piece of paper. This is critical. Dry paper will scratch - it's much harder than wet paper. Wet paper, especially Viva paper towel, is really quite soft. Pat the damp pad against the sprayed mirror a couple of times to make sure the pad is really damp and no dry fibers remain on the surface. The surface of the mirror is now swiped with gentle semi-circular motions working over the entire surface. The Windex will be seen as streaks and little beads and will gradually begin to dry, and the wiping should be stopped just as the surface dries. This last part is critical and the very essence of the cleaning operation. Once again, the mirror will dry while the pad remains wet. Even if the mirror has now dried, gently swiping the pad across the surface will cause a wet swipe across the mirror. This small amount of dampness will dry and leave nothing behind. If you see a spot you want to remove and the mirror is dry, breathe on the mirror, depositing a coating of condensed damp lung air, and then swipe with the damp pad. The spot should vanish. If it doesn't you can try a little added Windex and pressure but don't dig at it. Gradually, you will get to a point where the pad will wet and wipe the mirror perfectly. You can go after a spot and it will likely vanish. Work your breath and the pad together like team partners; a little breath, a little wipe. Practice will make perfect. It's a balancing act. Be careful not to expectorate one the mirror surface. If you do, just swipe up the little spots with a corner of the paper you do not use to clean the surface - spittle drops contain things you don't want on the surface. (Sorry for the disgusting part but we're engaging in professional work, so you get all the details.) This critically balanced damp condition is temporary and will not last forever, so work steadily and allow no interruptions - concentrate. Come back in five minutes and everything will be dried up and useless. You will need to start Step 2 again by re-dampening a new pad by batting it against a part of the mirror surface or another flat, clean surface. Using clean pads of the proper dampness is important.
Cleaning an Isolated Dirty Area
You can clean just a small area of the mirror, like an errant finger print or isolated drop of something gooey, without having to remove the mirror and make a big operation out of it. Just do Step 1 with an acetone soaked corner of a pad. Rub gently at the spot. Take another pad, soak a corner with Windex and do all you've learned about Step 2. That should do it. Good as new.