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 should clean you mirror less often if:
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!)
Here is the most important thing about coatings - the enemy of a coating is moisture, in the form of liquid water and humidity.

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.

So, obviously we should keep the mirror covered loosely when not in use.  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

There are two types of cleaning that I use.  Note that 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.

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:
In short, think ahead, and be careful.

TECHNIQUE 1 - The rinse

TECHNIQUE 2 - Rinses, blotting, solvent

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.  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

Lockwood Custom Optics Home page