Let's say you have a mirror-- not some cheesey $2 makeup mirror, but a research-grade aluminum mirror-- and it has some crud on it, say a film of junk deposited during your Summer Institute for Hot MEtal Chemistry. Like, say, the mirror on the right in this picture:
How do you get that mirror clean?
Well, first, you assemble your tools:
You'll need the mirror, some lens tissue (special soft, lint-free paper), a hemostat (I prefer the curved-tip kind shown here, but you can use the straight ones, too), and some solvent (the bottle here is methanol, but you sometimes need to use acetone).
The goal here is to get the dirt off the mirror without scratching it or leaving new kinds of dirt on the mirror surface. Thus, it's crucial to avoid touching the mirror with your hands, or anything that's been in contact with your hands, lest you leave finger grease all over the optical surface. (You can, in principle, wear latex gloves to do this, assuming your hands are of a size that one size fits, but I don't generally bother.) Thus, take a sheet of lens tissue, and fold it in half along the long direction, holding it only by the ends. Then fold it in half again, making a long narrow strip.
Still holding the strip only by the ends, fold it in half along the short axis, then in half again. The easiest way to do this is to hold both ends and pull it taut, then sort of bump the middle part up against the edge of some (clean) object, like the table, to get the fold started. The key thing here is to make sure that the outside surface never comes in contact with anything dirty. Once you have it all folded up, clamp it in the hemostat, as shown in the (slightly blurry) picture at right.
Then, take your solvent, and put a drop or two onto the lens tissue. You don't want to overdo it-- at extreme levels, the paper will disintegrate, but long before that, you'll be leaving puddles of solvent on the mirror. I usually go with 2-3 drops from a dropper bottle like the one shown in the picture above, and then shake the hemostat for a few seconds to get rid of any excess.
Then take the hemostat, and bring the clean, wet surface of the lens tissue into contact with one edge of the mirror. Pressing the tissue firmly into the mirror, swipe the tissue across the mirror from one edge to another, once and only once. Do not swipe it back and forth-- then you're just moving the dirt around. Swipe it across, then discard the tissue.
(If the mirror is exceptionally dirty, you might need to make several passes, and in that case, you might as well make a couple of passes with the same tissue. But on the final cleaning pass, you use one piece of lens tissue for one swipe in one direction, to eliminate any traces of dirt or solvent left by previous passes.
(The choice and quality of solvents is also important here: you need to use the expensive spectroscopic grade methanol, not the reagent grade stuff, and if you need to use acetone, it should be the highest-purity acetone you can get from your chemistry department. If you end up using acetone (as I did here), you need to follow it up with a lighter cleaning with methanol-- acetone tends to leave more of a residue, that can cloud the mirror slightly.)
If all goes well, you will have cut a big swath through the dirt on the mirror, as seen in the picture at left. Good work.
Now, you just keep repeating those steps until you've cleaned the whole thing.
There aren't too many pitfalls to avoid, here. The biggest one is that you want to make sure you don't scratch the mirror surface. This means that you need to be extremely careful not to drag the metal tip of the hemostat across the surface of the mirror (duh), but also, you want to avoid trapping any dust or grit between the lens tissue and the mirror. If you do, and the trapped material is harder than the coating, you'll ruin the mirror (I did this to the output coupler on a Ti:Sapph laser once, which was not pretty). If you're not sure whether there's stuff on the mirror, blast it with canned air, which usually removes anything dangerous.
At the end of this process, you should have a nice, clean mirror-- maybe not exactly as good as new, especially if you baked soot onto it over the summer, but suitable for use in scientific experiments. And you'll also have a big pile of used lens tissue:
but that's just the price we pay for clean optics.
And that's how you clean a mirror. Any questions?
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Will I look more handsome if I clean my bathroom mirrors this way?
Speaking primarily from the point of view of one who has been cleaning his eyeglasses for close to 60 years, is there a specific reason for not beginning the process with distilled water containing a small amount of a mild detergent and then a pure water rinse? I'd think that a good amount of the atmospheric dirt deposited on a mirror is probably water soluble.
I'd certainly follow that up with acetone and/or methanol to remove any water or detergent residue.
Where do babies come from?
Will I look more handsome if I clean my bathroom mirrors this way?
You'll show up more clearly. I leave it to you and your ego whether that's likely to make you more handsome or not.
Speaking primarily from the point of view of one who has been cleaning his eyeglasses for close to 60 years, is there a specific reason for not beginning the process with distilled water containing a small amount of a mild detergent and then a pure water rinse? I'd think that a good amount of the atmospheric dirt deposited on a mirror is probably water soluble.
Generally, methanol will get the water-soluble stuff as well, so you can skip a step if you go straight to that. There was one occasion when we had to do a distalled-water rinse first, which was the time that a water line broke and rained water all over the optical table-- the really coarse crud came off with water, and then we did a methanol wash to get rid of residual dirt.
That sucked very much, for the record-- a couple hundred mirrors and lenses and waveplates, and every single one needed to be cleaned.
Where do babies come from?
Well, when a bottle of solvent and a piece of lens tissue really love one another...
Were you perhaps aiming for the "All or Nothing" questions thread?
Back when I had to clean some optics (really), I was taught to lay the lens paper on the lens, put a few drops of whatever on it and then pull the lens paper horizontally off the surface. Different technique? Different optics? It's been quite a while.
Back when I had to clean some optics (really), I was taught to lay the lens paper on the lens, put a few drops of whatever on it and then pull the lens paper horizontally off the surface. Different technique?
That works well for relatively light dirt (and, indeed, I took a picture of that technique, too, but it didn't make it into the post...). When you've got serious crud on the surface, though, you need to apply some pressure to get it off. That's what I'm describing here.
Back when I had to clean some optics (really), I was taught to lay the lens paper on the lens, put a few drops of whatever on it and then pull the lens paper horizontally off the surface. Different technique? Different optics? It's been quite a while.
That sucked very much, for the record-- a couple hundred mirrors and lenses and waveplates, and every single one needed to be cleaned.
Believe me, I know. Just two days ago I raked my table with a cooling water line full of water so nasty that you would have thought I was stealing it from the bathroom. So not fun.
Aaron's technique
That technique works better for optics that aren't mounted too. The lips on the mount tend to keep your tissue from flattening on the surface.
Chezjake: I would not recommend using distilled or even deionized water for cleaning lab mirrors. Since water will evaporate much more slowly than methanol or other solvents in a lab, any thin layer of water left on the surface while cleaning can pick up more particulates floating around after you think you've cleaned it. Also, you'll have some water bound to mirror surface, which can interact with whatever you're bouncing off the mirror (especially infrared light).
Chad: What shape of tool would you recommend for cleaning an elliptsoid mirror? By the way, great post. I'd like to see any of those "best" life science blogs give such practical lab advice!
We use a similar method to clean wafers in the semiconductor industry, except I like to use lint free swabs rather than tissue. They might be more expensive, but for ease of use they are so much better.
As an ethanol engineer, I should promote the stuff as preferable to methanol as it is less toxic. And if you buy the commercial preparations, you can have a little celebratory drink afterwards.
I used to have pretty good luck (for demounted optics with insoluble coatings, or no coatings at all) just using a stream of warm tap water to get rid of particulates, a quick swish in detergent-laden water (Micro worked pretty well), followed by a DI water rinse and a methanol rinse to get rid of the water.
Worked pretty well, except for the time some joker slipped in a NaCl window on me.
Next to getting finger prints or elbow grease applied to surfaces, I really hate it when I have to use acetone due to the amount of particle residue. It stinks.
From my semiconductor experience, heavier alcohols can certainly be used.
Hi Chad,
I am an amateur astronomer, and I own a 16" dobsonian scope. I had it fit with a very well-crafted, lambda/8ths mirror. Aluminum after depositing on the glass slowly builds an oxyde layer which is very resistant and protects it, while very thin.
After consulting with many people "in the know", I followed their advice to clean the mirror. I just put it under a shower and delicately but resolutely cleaned it with a soft sponge, and lots of water and soap.
Results ? I have done this four times in two years, and the mirror is as clean as new, and has no scratches whatsoever, not even microscopic ones.
It takes more effort to scratch a mirror than one would think!
Romano Zen, a true artist and crafter of optical instruments, is the builder of my mirror. When I argued I was not happy of having to mess with my own mirror so directly, he told me he once crawled on top of the mirror of the Asiago observatory (a 1.5 meters thing). No effects.
Cheers,
T.