THE CLEANING OF OPTICAL SURFACES: MIRRORS
Part Two: Front surface mirrors, secondary mirrors & mirrored diagonals
by P. Clay Sherrod, Arkansas Sky Observatories
Part One of the ASO Fine Optics Cleaning Procedures discusses the methods and protocol for
proper cleaning and maintenance of refractive optics: lenses, corrector plates, binocular lenses,
eyepieces, etc. The cleaning of front surface mirrored surfaces is much, much different than that
of refractive optics, however. Many times the reflective surfaces might consist of deposits of
enhanced silver or aluminum which may or may not be over-coated with a protective layer of
transparent material, usually a molecule-thick layer of Silicon Dioxide or something similar.
Your first step in attempting to clean ANY reflective optics is to first ascertain whether or not your
mirrored surface is indeed protected by such a coating, since the cleaning solution AND
procedure to clean without damage is quite specific for protected vs. unprotected mirror surfaces.
Note that the following recommendations include all reflecting optics: Primary mirrors, secondary
mirrors (both flats and curved), mirror diagonals, and any ancillary optical equipment that uses a
mirror in the optical interface.
DETERMINING MIRROR PROTECTION
Here’s a very simple rule for deciding whether or not your mirror surface is protected: if you do
NOT know, assume that it is NOT coated, and therefore, no protected.
Most manufacturers of Newtonian mirrors supply the finished product with a coating of silicon
dioxide over the final aluminized or silvered coatings. ASK whether your mirror is coated...if you
cannot get an answer, it is safest for you to assume that it is not coated.
On the other hand, most primary and secondary mirrors of popular catadioptric telescopes
(Schmidt-Cassegrain and Maksutov-Cassegrain) are NOT protective-coated unless otherwise
specified.
There is a good reason that many manufacturers do not put protective coatings on telescope
mirrors: they can reduce performance, both in terms of optical figure (irregularities in deposited
protective coatings can change the wave front of your mirror) and in terms of reflectivity (many
new mirror systems have "enhanced" coatings which contain highly reflective alloys in addition to
aluminum. However, since most enhanced coatings also contain the element SILVER, and since
silver tarnished instantly with exposure to oxygen, the chances of enhanced optics being overcoated are pretty good in your favor.
Attempts to clean uncoated optics can result very quickly in permanent damage: sleeking
(leaving streaks within the coatings themselves) or spotting is very common, even if the utmost
care has been used. Never, should any cleaning agent whatsoever be used on unprotected
mirror surfaces or damage will occur 100 percent of the time. Simply do not take the chance.
CLEANING IN RELATION TO MIRROR ACCESS
In some modern telescopes, it may be undesirable OR even impossible to totally remove the
primary mirror for the typical consumer and end-user; thus cleaning will likely take place less
often than it would if the mirror were smaller or easy to remove from the optical tube assembly.
Remember the Number One Rule on Optics Cleaning: "Don't...unless you absolutely have to."
The Number Two Rule is: "Brush first and then determine if cleaning is still necessary."
Brushing optics to remove particulates such as pollen and dust can usually get the mirror or
optics back into top shape, and cleaning should be done only if there are stains or excessive
spotting beginning to build up on the mirror's surface.
With commercial catadioptric telescopes, cleaning the primary should be avoided at all costs.
Prevention is the best care you can give the optics of these telescopes. Keep the back opening
plugged at all times. Even when briefly removing accessories, plug it up until you are ready to
insert a new gadget. This keeps both dust and insects from floating in AND it prevents humidity
and damaging environmental pollutants from entering the inside of the OTA. When cleaning of
the primary mirror IS necessary, leave the mirror in place. You should never remove the mirror of
a catadioptric telescope unless you have help from an experienced and competent assistant.
CLEANING SOLUTIONS FOR FRONT SURFACE MIRRORS
To preface any discussion about what is needed for cleaning mirrors with a protective
overcoating, it is important to note that complete immersion cleaning of most large (i.e,
Newtonian) mirrors is recommended, and thus the "solution" quantity is much greater. There is
no need to make up batches of cleaning solutions and store....just make it when you get ready to
clean your mirror. Essentially all that you are going to need are two pairs of surgical cotton
gloves, at least three clean terrycloth towels (have more on hand just in case), a small amount of
IVORY dishwashing liquid, a jug of distilled water and a few white Kleenex tissues.....oh, and
a large sink, bathtub, or basin.
Conversely, to clean unprotected mirrors requires ONLY a very small amount of high pure alcohol
content solution and nothing more, since ONLY spot cleaning should ever be attempted; if any
unprotected mirror surface becomes so pitted or stained that whole-mirror cleaning is needed...it
is time to send the mirror and/or OTA in for a complete re-coating job. No exceptions.
CLEANING OVERCOATED MIRRORS THAT CAN BE REMOVED
Again, if there is any doubt whatsoever that your mirror has protective over-coatings, assume that
it does NOT have protection or be prepared to face the consequences.
IMPORTANT NOTE: Rarely do secondary mirrors and diagonal mirrors have any protective
coating; always assume that they have front surface exposed enhanced coatings and never clean
except as described later.
1. You will need a basin large enough to hold your mirror and still have adequate room for your
hands to grasp the mirror around the edge.
2. Put a thick folded towel on the bottom of the sink or basin.
3. Add warm, not hot, tap water in which you have added ONE (1) TEASPOON (t.) of Ivory
Liquid dishwashing solution...do not be tempted to use more.
Please Note: The quantities for the solution described below are based on a one-half full kitchen
sink, but remember, this does not have to be exact! For larger basins, such as a bathtub or
washtub, use a proportionately similar detergent-to-water ratio).
4. Turn off all fans, vents and central heating/air during this process!
5. Remove all jewelry, including wrist watch and put one pair of the TWO pairs of new surgical
cotton gloves on your hands.
6. For Newtonian and similar mirrors, first remove the mirror and its cell from the telescope OTA;
then remove the mirror from the cell. Remember, NEVER touch the front surface of your
mirrors...your fingerprint contains acid and oils and can be the most damaging element to your
mirror!
NOTE: As with all glass, telescope mirrors become incredibly slippery and hard to handle when
wet. Make every precaution to protect the mirror! You will be safest if you "assume the worst"
and prepare for the mirror to slip. This means putting a large folded clean towel in the floor of the
basin in which the cleaning will be done, and having another clean towel folded and waiting
where the mirror will dry.
7. Place the mirror carefully in the basin FACE UP and rest it on the towel, making sure that you
have enough solution to completely cover the entire top surface completely.
8. Allow the mirror to soak for at least 5 minutes but NO LONGER than 15 minutes. Do not
touch the surface of the mirror at this time.
9. While the mirror is soaking, remove the cotton gloves and place them in the solution with the
mirror to prevent contamination.
10. After about 5 minutes put the gloves back on, but do not touch anything outside of the basin;
at this time you are going to very, very gently - with NO pressure - massage against the front
surface of the mirror with the tips of your fingers. DO NOT rub and do not use any type of cloth or
tissue* at this point, only your fingertips (*you MAY use Kim-Wipes or Intrinsic type pads for this
process). It is fine to "lay” your fingers down to cover more surface...you are essentially "buffing"
the entire surface with the dishwashing liquid using only fingertips.
11. Once done, rotate the mirror 90 degrees and massage the entire surface again.
12. Occasionally tilt the mirror out of the water for 30 seconds (maximum) and examine it. If
there are places that you missed, it will be obvious. If need be, run a very gentle stream of water
out of the tap (or pour from a pitcher) across the mirror and examine it while wet. Return the
mirror to the basin and massage needed areas until the entire mirror is uniformly clean and free
of streaks.
13. Leaving the mirror flat in the basin, remove all soapy water from the tub but LEAVE the towel
beneath the glass for safety. As the water recedes, begin flushing the surface of the mirror
immediately with cold tap water. NEVER ALLOW THE MIRROR SURFACE TO DRY!
14. Once the mirror has been flushed adequately with tap water, begin tilting the mirror upward
at about a 45-degree angle; placing an adequate mass of towels behind it is helpful, but be
careful to not let the mirror slip in the basin! If you can find someone to help you do this, it is a
LOT easier! Continue flushing the mirror with tap water while doing this...do not let the mirror go
dry.
15. Have a pitcher of distilled water within reach and shut off the tap water. Immediately flush
with distilled water; allowing the flush to drip off the mirror. Then do it again, using only distilled
water.
16. Remember, your gloves are soapy! Once you have reverted to the distilled water rinse,
remove the cotton gloves and work with your bare hands only, being careful to only touch the
edge of the glass and never touch the optical surface.
17. Lift the mirror out of the basin, keeping the surface vertical to the floor, and immediately place
it on the waiting towel on the floor and lean the mirror carefully at a sharp angle against a wall.
Use extra towels to assure that the mirror will not roll nor tumble. The angle allows the liquid to
roll off the surface, thereby reducing substantially the amount of dry water spotting that can
occur. NOW, put that second pair of cotton gloves on your hands for safe handling of the mirror
from this point forward.
18. After only TWO MINUTES maximum in the drying position, identify any beads of water that
are NOT rolling off the surface. These can be easily removed by "wicking"; a process in which
you roll up a white Kleenex tissue into a "pencil" and touch to the drop. Never rub; the tissue will
wick the water up off the glass and safely away.
19. Allow the mirror to air dry, (with ALL VENTS from air conditioning/heating closed!) for one
hour.
20. Some dust might accumulate during the drying process. If so, use a quality soft artist’s
square tip (see Cleaning Refractive Optics, Part One) brush to remove it, but wait until the hour of
drying time has passed before you do so!
Remember, the single most damaging thing that you can do to your precious telescope optics is
to CLEAN THEM. While it is perfectly safe to clean a protected primary mirror OR the front
corrector plate of a catadioptric, it is an entirely different undertaking to clean unprotected mirror
surfaces. If you have an unprotected mirror surface and you have no choice but to clean it,
search on Doc Clay’s website at www.arksky.org/ and you will find information on how to do so.
He will try to dissuade you, but if you insist, the instructions are there.