your mirror, and prolonging coating
How to clean your mirror safely, and help the coating to last
All images and text Copyright Mike Lockwood, 2011
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
floating around on the internet on a regular basis. So, I
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
Photos and images may be added in the future as I have time or as I
assumes that your mirror is being used for normal astronomical
observation, imaging, or even LIDAR data collection.
article does NOT cover cleaning for applications that are more
sensitive to stray light such as optics for high powered lasers, solar
All of the advice presented here is just that - advice - and
will not be held responsible if you damage your
coating or mirror. Use common sense, please.
When to clean
clean my telescope mirrors about one or two times per year, and mostly
just a quick rinse to knock off loose dust. I rarely have to
more serious techniques. This probably means I'm not
enough, but that's another story....
You should clean your mirror more often if:
You should clean
you mirror less often if:
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.
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.
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,
a nasty looking mirror with crud all over it is going too long.
Big mirrors often require help, so this may factor into how
you clean them.
- The mirror is
mostly stored indoors at relatively low humidity.
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.
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.
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.
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.
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!
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
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
inevitably works its way into the coating and eventually damages it
from the inside out. Liquid water sitting on the surface for
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.
pollen, and other particulates sitting on the coating will make it
easier for condensation to form on the mirror. This is akin
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.
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.
photos below show what a nearly failed coating looks like.
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
may be a risk of actually having the contaminants start to slightly
etch the polished surface of the glass.
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.
mirrorI 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.
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
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.
second is several rinses, followed by blotting, followed by optional
use of a solvent to remove any remaining crud. The solvent
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
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.
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.
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
- 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.
the mirror from the telescope carefully, unless your telescope is one
that is designed to have the mirror cleaned in the scope. Ask
manufacturer if this is the case. If the mirror cell contains
electronics, fans, etc, I do not recommend cleaning it in the
for a soft spot to set the mirror on its edge. Setting it on
on hard materials like metal, ceramic tile, stone, concrete, etc. can
easily chip the edge of the mirror. Cardboard, carpet, hard
etc will support the edge of the mirror safely.
the mirror on its edge so that rinse water can drain off.
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.
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
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
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.
sprayer is best. Faucets with a pull-out spray head are great
switch them to spray mode to maximize water velocity. A
hose works, too. Do NOT use a power washer - the velocity of
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.
the mirror with the sprayer. The kinetic energy of the water
knock lots of not-so-firmly attached crud off the mirror.
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.
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
left to air dry, or it can be blotted dry.
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.
a look at the mirror - it should look better. Repeat again if
wish, but additional rinses will usually take off less crud.
lots of stuff remains, consider use of a solvent in Technique 2 below.
TECHNIQUE 2 -
Rinses, blotting, solvent
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.
Technique 1 above a few times. Do not let the mirror dry
rinses, but pause for a minute or two between them and let the water
loosen the crud.
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
to loosen dirt without rubbing it around on the mirror, and then rinse
it away with water.
a few rinse/blot cycles, you should remove most of the dirt on the
mirror. Chances are what remains is organic crud that water
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
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
the cotton will absorb it and move it around, but don't be too stingy.
- Blot the
mirror/solvent with clean cotton.
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
- Switch the
cotton frequently. Don't wipe too long before rinsing.
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
the mirror flat again and reapply solvent if desired, and repeat.
Otherwise, blot dry after rinsing and evaluate the surface.
Secondary mirrors are smaller, and can be cleaned in a similar manner.
Take care to avoid fingerprints when handling them.
there has been some discussion about the possibility of alcohol harming
an aluminum coating by reacting with it. Thus far, I have
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
evidence, I will continue to use alcohol to clean coatings as countless
people have done for decades.
the life of a coating
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
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.
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.
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.
the best environment is a dry place with fairly steady temperatures
(can be hot or cold). Since dew formation is the main enemy
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,
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
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.
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
prevent it altogether. Heat tape may even be an anti-dew
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
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.
are then placed back in service.
general, high humidity and
high temperatures make chemical reactions happen faster.
just how chemistry works. So, avoiding one or the other or
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!
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
into something that resists normal, gentle chemical stripping.
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
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
I wish you clear, dark skies, and reasonably clean optics.
Lockwood, Lockwood Custom Optics