Cleaning your mirror, and prolonging coating life

How to clean your mirror safely, and help the coating to last longer

All images and text Copyright Mike Lockwood, 2011


Mirror cleaning is something that I get asked about regularly, but it is just too large of a topic to put in my FAQ.  I also see very bad advice floating around on the internet on a regular basis.  So, I wrote my own article about it, and I asked the coaters that I worked with to "review" it, before I posted it.

Through communication with clients, coaters, and knowledgeable people in the industry, I have also come to understand some of the things you can do to prolong the life of the coating on your mirror.

Photos and images may be added in the future as I have time or as I acquire them.

This article assumes that your mirror is being used for normal astronomical observation, imaging, or even LIDAR data collection.

This article does NOT cover cleaning for applications that are more sensitive to stray light such as optics for high powered lasers, solar observation, etc.

DISCLAIMER:  All of the advice presented here is just that - advice - and I will not be held responsible if you damage your coating or mirror.  Use common sense, please.

When to clean your mirror

I clean my telescope mirrors about one or two times per year, and mostly this is just a quick rinse to knock off loose dust.  I rarely have to use more serious techniques.  This probably means I'm not observing enough, but that's another story....

You should clean your mirror more often if:
  • You get lots of dew on it frequently, or if there is a lot of humidity where you live or where you store your telescope and you notice the mirror dewing up regularly.  Dust, pollen, and other tiny particles attract moisture, which is not good for a coating, and removing this will help retard dew formation.
  • You find that mice or bugs are crawling on it, leaving their waste, trails, etc.  All of these substances will eat away at the coating and should be removed ASAP.
  • You are not stressed or scared by the process of cleaning an optic.
You should clean you mirror less often if:
  • The mirror is mostly stored indoors at relatively low humidity.
  • Your coating is brand new - brand new coatings can sometimes be more easily damaged than those that have been on for ~6 months or so, so resist the urge to clean for a while.
  • Cleaning your mirror stresses you out or scares you.
I can't say what the optimal cleaning schedule is - you'll just have to use your own judgement.  A little dust is no reason to clean, but a nasty looking mirror with crud all over it is going too long.  Big mirrors often require help, so this may factor into how often you clean them.

It is not difficult to determine when your mirror needs cleaning if you look at it properly.  Just look at it in daylight.  
(Looking at if properly does NOT include shining a flashlight down the tube and gasping in horror at the dusty optical surface that almost seems to scream back at you!)
  • If it looks noticeably dusty in normal daylight conditions (not under flashlight illumination at night), then it might be time for a rinse to knock off the dust particles.
  • If you see spots much larger than dust particles and mysterious trails or other features possibly caused by insects, it might be time for a more thorough cleaning with a nice organic solvent to cut through the crud.
  • If you see spots and areas that appear corroded, discolored, or not shiny/reflective, it might be time for a serious cleaning and evaluation of the coating condition.
  • If you see the mirror cell through the partially transparent coating, it's probably time for a cleaning and then a recoat!
Here is the most important thing about coatings - the enemy of a coating is moisture, in the form of liquid water and humidity.

Think of it this way - whenever moisture is in contact with the coating of your mirror, the clock is ticking.  There is only a finite time that the coating will stay undamaged when in contact with water, so it is best to minimize the time that the mirror is wet or is in very wet surroundings.

Humidity inevitably works its way into the coating and eventually damages it from the inside out.  Liquid water sitting on the surface for long periods of time also works its way in and does more localized damage because it enables or speeds up chemical reactions that take place in the dirt and grime that is sitting on the mirror, and these reactions can produce stuff that harms the coating.

Dust, pollen, and other particulates sitting on the coating will make it easier for condensation to form on the mirror.  This is akin to a tiny dust particle in the air acting as a "seed" for a raindrop to form around it - the moisture needs some nucleus on which to form/collect.  Once started, the droplet (perhaps of microscopic size) can form more easily, and then the chemical reactions start, possibly helped by the chemicals.

So, obviously we should keep the mirror covered loosely when not in use to keep dust and other things off of it.  Loosely means that air can circulate a little bit, which will help keep from trapping moist air around the mirror, but covered means that dust will not collect on it as quickly and insects will have a harder time getting to the mirror.

The photos below show what a nearly failed coating looks like.  The front surface shows greatly reduced reflectivity, an shining a light through from the back shows thousands of holes in the coating.  Personally I wouldn't let it go too far past this or else there may be a risk of actually having the contaminants start to slightly etch the polished surface of the glass.

Badly corroded coating
ABOVE:  This mirror's coating is failing badly.  Illumination is by a light from behind, and my reflection with the camera is barely visible in what is left of the coating.

Cleaning your mirror

I no longer use or recommend the technique of soaking the mirror in soapy water for a prolonged period.  Water is the enemy of a coating, so submerging the mirror is in some ways putting it in the worst possible situation.

Mirror coating damaged by soaking in water Not convinced?  Perhaps this story will help convince you.  I was asked to test a secondary mirror, and it turns out it was expoxied to some metal brackets and a piece of medium density fiberboard, the stuff that is basically sawdust combined with glue.  In order to remove the mirror, I decided to soak the "wood" in water for a few days to soften it up.  After this, I was able to break the "wood", and free the mirror eventually.  However, after a few days, the coating had developed some defects, and these are clearly shown in the image at right.

There could have been something in the wood that affected the coating, but I believe the water was the main culprit.  So, this combined with things I have been told by those in the know convinced me to change my cleaning ways.

There are two types of cleaning that I now use.

The first type of cleaning is a rinse with water, intended to remove loose dust and pollen only.

The second is several rinses, followed by blotting, followed by optional use of a solvent to remove any remaining crud.  The solvent will not affect the coating.

The steps are simple if you remember the following - rubbing on a mirror with dirt on it is like using sandpaper on the mirror.  Rubbing gently on a clean mirror is OK, but should be done as little as possible.

If you have a large mirror, or if it is difficult to handle or remove from its cell:
  • Consider getting the help of an assistant, or at least plan out how you will do it.  It won't hurt to rehearse your movements.  Certainly take a moment to discuss what you will do, and how.
  • Look for things you might trip over when carrying the mirror around, because you may not be able to see your feet.  Move items that you might trip over!  Consider putting the mirror on a cart rather than carrying it.
  • Look for things that you might potentially bang it into while carrying it.  Cover them with padding if you wish.
  • Watch out for the faucet if placing the mirror in a sink, and when taking it out.  Place a towel over it to be safe.
  • Be careful if you are wearing a ring, watch, metal belt buckle, or other metal jewelry - these items can potentially scratch a mirror.
In short, think ahead, and be careful.

TECHNIQUE 1 - "The rinse"

  • Remove the mirror from the telescope carefully, unless your telescope is one that is designed to have the mirror cleaned in the scope.  Ask the manufacturer if this is the case.  If the mirror cell contains electronics, fans, etc, I do not recommend cleaning it in the cell/telescope.
  • Provide for a soft spot to set the mirror on its edge.  Setting it on edge on hard materials like metal, ceramic tile, stone, concrete, etc. can easily chip the edge of the mirror.  Cardboard, carpet, hard foam, etc will support the edge of the mirror safely.
  • Place the mirror on its edge so that rinse water can drain off.  Lean it against something and make sure it cannot possibly fall over or roll off and fall.  If in doubt of its stability, have an assistant hold the mirror.
  • I like to place 20" or smaller mirrors in my plastic laundry sink, leaned up against the edge.  This way, they can't be broken if they fall over, and dropping them in the sink will not harm them.
  • I do NOT recommend doing this in metal or ceramic sinks unless you put some towels in first!  Make sure the sink can still drain, though.  You may put some wood or foam blocks under the towels and mirror to elevate the mirror from the bottom of the sink and allow water to drain off better.
  • A sprayer is best.  Faucets with a pull-out spray head are great - switch them to spray mode to maximize water velocity.  A garden hose works, too.  Do NOT use a power washer - the velocity of the water is too high and might remove the coating and damage the glass.
  • Make sure you keep the sprayer some distance from the mirror so you don't bang it into the mirror.
  • Rinse the mirror with the sprayer.  The kinetic energy of the water will knock lots of not-so-firmly attached crud off the mirror.
  • It is almost impossible to damage the coating unless you rinse for hours and hours (or unless the coating is already damaged).  A few minutes will usually do the trick.
  • Spray generally side to side, from top to bottom of the mirror so the dirt is washed downward and off the mirror.
  • Ideally the final rinse is done with distilled water, especially if your water has a lot of lime/calcium or iron it it.  The mirror can then be left to air dry, or it can be blotted dry.
  • If you don't have distilled water, then blot the excess water off the mirror's surface immediately with cotton, clean paper towel, etc.  Do not rub the cotton or paper towel around - just blot the surface until all moisture is gone.
  • Take a look at the mirror - it should look better.  Repeat again if you wish, but additional rinses will usually take off less crud.  If lots of stuff remains, consider use of a solvent in Technique 2 below.

TECHNIQUE 2 - Rinses, blotting, solvent

  • Repeat Technique 1 above a few times.  Do not let the mirror dry between rinses, but pause for a minute or two between them and let the water loosen the crud.
  • If you can see a lot of stuff remaining, take clean cotton and blot the mirror's surface between rinses.  Blotting means touching the mirror's surface with no sideways motion at all.  The goal here is to loosen dirt without rubbing it around on the mirror, and then rinse it away with water.
  • With a few rinse/blot cycles, you should remove most of the dirt on the mirror.  Chances are what remains is organic crud that water is not a good solvent for.
  • If you are happy that the rinsing is done, set the mirror down flat on its back.
  • Working in a WELL-VENTILATED AREA and observing all the other warnings and directions for the solvent, with the mirror dry, pour some acetone or alcohol on the mirror.  (Note that most drugstore alcohol is partly water, which you may have to blot off to get the mirror dry.)  You don't have to flood the mirror because the cotton will absorb it and move it around, but don't be too stingy.
  • Blot the mirror/solvent with clean cotton.
  • If you believe the mirror to be mostly free of dirt, you may drag the cotton across the surface, but do NOT apply any pressure to the cotton - the weight of the cotton and the solvent that it absorbs will be enough.
  • Switch the cotton frequently.  Don't wipe too long before rinsing.
  • If the solvent is helping to cut through the grime, then prop the mirror up again and rinse the mirror with water to help remove the crud and remaining solvent.
  • Set the mirror flat again and reapply solvent if desired, and repeat.  Otherwise, blot dry after rinsing and evaluate the surface.
You will never get a mirror perfectly clean, but you can remove most of the dirt and slime.  Do not obsess over perfect cleanliness - just get most of the crud off, and then then get the mirror safely back in the telescope and enjoy your cleaner mirror.

Secondary mirrors are smaller, and can be cleaned in a similar manner.  Take care to avoid fingerprints when handling them.

Recently there has been some discussion about the possibility of alcohol harming an aluminum coating by reacting with it.   Thus far, I have found no evidence or chemistry to support these claims, and in some of those that I have consulted have commented that water is far more likely to react with aluminum than alcohol.  Bottom line, unless I see real evidence, I will continue to use alcohol to clean coatings as countless people have done for decades.

Prolonging the life of a coating

Mirror coatings should last at least a few years, but possibly as long as 10-20 years in some ideal cases.  They will inevitably lose reflectance over time.  If removing the mirror is labor-intensive or dangerous, and/or removing it for a week or two will cause logistical problems or loss of important data, and/or the mirror is very delicate and shipping it is dangerous, then it makes sense to try to make the coating last as long as is reasonably possible.

If it is convenient, safe, and affordable to have the mirror recoated periodically (as most major observatories do, though many have on-site coating chambers tooled up precisely for this purpose), then it does not make as much sense to devote as much time and effort to making the coating last.

Here are some ways to make a coating last longer if that is desired and there are personnel available to do so.  Keep in mind that all non-dielectric coatings will fail eventually, but they are easily stripped and redone.

Cleaning the mirror at the appropriate time - when the mirror is not too clean, but also not filthy, is important to helping the coating last.

There are other things that can be done, but understand that the right thing depends on how the mirror is used and the type of environment in which it is stored.

Perhaps the best environment is a dry place with fairly steady temperatures (can be hot or cold).  Since dew formation is the main enemy for a mirror in storage, and dew formation is unlikely in a dry environment without large temperature swings, then just keeping the mirror loosely covered to keep dust and pests off should allow the coating to last a long time.  Dust is common in dry, desert environments, of course.

If a mirror is used frequently, then the user should try to get an idea of how often dew forms on the mirror.  If this happens nightly because the mirror is cooling below the dew point, then the coating will be under attack from moisture nightly.  In this case, fans circulating air around the mirror can help delay the formation of dew, but they also increase the volume of air moving through the mirror's area, and this can increase dust deposits.  Steps should be taken to keep the mirror clean, and the dew should be dried as soon as possible after use.  Fans can help with drying, and a low-wattage light bulb can gently heat the mirror and prevent or help remove dew.

The mirror should be checked while not in use to make sure dew is not dewing then.  If it is, then fans or a low-wattage light bulb may prevent it altogether.  Heat tape may even be an anti-dew option in applications where optical quality is not a high priority.  (The heat of the tape will create air currents, blurring telescope images, and the temperature gradient created within the glass will change the shape of the mirror slightly, though temporarily.  Well annealed glass goes back to its original shape when it is in thermal equilibrium.)

In very humid environments, dessicant packs can be attached to the underside of the mirror cover to help absorb humidity near the mirror when it is not in use.  If these packs are of the reusable type, the dessicant packs can be "recharged" by heating them in an oven periodically to drive out the moisture they have absorbed.  They are then placed back in service.

In general, high humidity and high temperatures make chemical reactions happen faster.  That's just how chemistry works.  So, avoiding one or the other or both can also help prolong coating life.  Ventilating the observatory or air-conditioning (and thus dehumidifying) it will help fight high temperatures and humidity, but exposing a cooler-than air mirror to evening air will often cause it to dew over immediately, so mind the thermostat setting!

A final note - if you use the mirror until the coating has obviously failed, it is possible that the chemical reactions have reached the glass (optical surface) and have reacted with it.  Also, the coating can react and be turned into something that resists normal, gentle chemical stripping.  This means stronger and more risky methods must be used to remove the old coating that has chemically reacted with something.  Periodic cleaning will of course reduce the chances of something reacting with the coating.

In extreme cases, such as mice or bugs dying on the mirror (I have seen it!), the glass can be etched, and this will destroy the figure of the mirror and require it to be reworked.

So, don't wait too long for a recoat, or the glass itself may be affected.

I wish you clear, dark skies, and reasonably clean optics.

  -Mike Lockwood, Lockwood Custom Optics

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