ASOtitle
THE CLEANING OF OPTICAL SURFACES:  MIRRORS

Part Two:
   Front surface mirrors, secondary mirrors and mirrored diagonals
 
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 solution and procedure is a tried and proven method for all coated (enhanced or non-reflective) surfaces as well as glass surfaces which have no coatings.
 
The cleaning of front surface mirrored surfaces is much, much different than that of refractive optics; many times the reflective surfaces might be of deposits of enhanced silver or aluminum which may or may not be overcoated with some protective layer (usually a molecule-thick layer of Silicon Dioxide or similar) of transparent material.
 
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 discussions include all reflecting optics:  primary mirrors, secondary mirrors (both flats and curved), mirror diagonals and any ancillary optical equipment which uses a mirror in the optical interface.
 
DETERMINING MIRROR PROTECTION

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.
 
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, then assume that it is not.
 
On the other hand, most primary and secondary mirrors of popular catadioptic (Schmidt-Cassegrain and Maksutov) 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 TERMS OF ACCESS TO MIRROR

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 my Number One Rule on Optics Cleaning:  "Don't....unless you absolutely have to."
Number Two Rule is:  "Brush first and then determine if cleaning is still necessary."
 
Brushing optics and carefully using compressed air to blow off 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 catadioptic commercial 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 prevent humidity and damaging environmental pollutants from entering the inside of the OTA.
 
In one of the following procedures, not that I discuss cleaning (essential cleaning only....) the mirror of a commercially-built catadioptic by leaving the mirror IN PLACE.  Never attempt to remove the mirror of these telescopes unless you have experienced and competent assistance.
 
CLEANING SOLUTIONS FOR FRONT SURFACE MIRRORS

To preface any discussion about what is needed for cleaning mirrors, 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, a clean terrycloth towel, a small amount of IVORY dishwashing liquid, a jug of distilled water and a few white Kleenex tissues.....oh, 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 MIRRORS WITH PROTECTIVE OVERCOATING

Again, if there is any doubt whatsoever that your mirror has protective overcoatings, 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)  In a basin large enough to hold your mirror and still have adequate room for your hands to grasp around the edges, prepare a solution of the following:
(based on one-half full kitchen sink quantity....this does not have to be exact!  For larger basins, such as a bath tub or wash tub, use proportionately similar detergent-to-water ratio)
    a) warm, not hot tap water in which you have added ONE TEASPOON of Ivory Liquid dishwashing solution....do not be tempted to use more.
    b) a thick folded towel placed on the floor of the basin;
    c) turn off all fans, vents and central heating/air during this process!
 
2)  Remove all jewelry, including wrist watch and put one pair of the TWO pairs of new surgical cotton gloves on your hands
 
3)  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 and you will be safest is 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; having another clean towel folded against a wall and resting on the floor where the mirror will dry.
 
4)  Place the mirror FACE UP carefully down in the basin, resting on the towel, making sure that you have enough solution to completely cover the entire top surface completely.
 
5)  Allow 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.
 
6)  While soaking mirror, remove the cotton gloves and place them in the solution with the mirror to prevent contamination.
 
7)  After about 5 minutes refit gloves 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 fingertips   [[note that you MAY use Kim-Wipes or Intrinsic type pads for this process]].  It is fine to "lay down" your fingers and cover more surface.....your are essentially "buffing" the entire surface with the dishwashing liquid using only fingertips.
 
8)  Once done, rotated the mirror 90 degrees and once again massage the entire surface.
 
9)  Occasionally tilt the mirror out of the water for only 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 while wet; return to basin and massage needed areas until entire mirror is uniformly clean and free of streaks.
 
10)  Leaving the mirror flat in the basis, 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!
 
11)  [note: an assistant is quite helpful at this point!] - 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 careful to not let the mirror slip in the basin!  CONTINUE flushing with tap water while doing this...do not let the mirror go dry......have a pitcher of distilled water within reach and shut off the tap water, and immediately flush with distilled water; allow the flush to drip off the mirror and do it again, using only distilled water.
 
12)  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.
 
13)  Lift the mirror out, keeping the surface vertical to the floor and immediately place 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 the second pair of cotton gloves on your hands for safe handling of the mirror from this point forward.
 
14)  After only TWO MINUTES maximum in the drying position (#12), 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.
 
15)  Allow to air dry, (with ALL VENTS from air conditioning/heating closed!) for one hour.
 
16)  Some dust might accumulate during the drying process....use a quality soft artists square tip (see Cleaning Refractive Optics, Part One) brush to remove such lint, but ONLY after one hour of drying time!
 
CLEANING PROTECTED MIRRORS WHICH CANNOT BE REMOVED:

The above procedure allows for cleaning a primary mirror which can be removed and immersed in a basin; some protected mirrors (i.e., larger Newtonians, some newer SCT and RC catadioptic telescopes) are made in such a way that mirror removal is very difficult or should NOT be attempted.  You clean this in three steps.
 
[DISCLAIMER:  From experience, I will state that such mirrors should NOT be cleaned, only brushed and blown off with compressed air; I do not, nor does ASO, recommend the following cleaning procedure; the following procedure can be used by skilled and experienced persons in telescope maintenance, but is not recommended for the normal owner/operator of telescope systems.  This is the procedure and technique used in the ASO Supercharge and only used when absolutely necessary, and we do not assume any liability from product damage from any attempts at such cleaning.]
 
CLEANING LIQUID:  For such surfaces, use essentially the same process, but instead of immersion, we are going to give your mirror a "sponge bath" applying the soapy liquid (about one gallon water to each one-half teaspoon of Ivory dishwashing liquid).  For this you will NEVER USE TAP WATER, only distilled water for both cleaning and rinsing.
 
RINSING LIQUID:  You will also need ONE OUNCE of pure (91% or higher) isopropyl alcohol and one capful of Kodak PhotoFlo per gallon of distilled water for RINSE (not wash).  The application of both solution AND rinse MUST be done using either Kim-Wipes (Kimberly-Clark) or Intrinsic Pads (Barnhardt Industries of South Carolina)...never, ever use any cloth, tissue or "lens cleaning cloth" for this cleaning or damage will occur.
 
FINAL RINSE:  A final rinse of pure distilled water is absolutely imperative...you must do this final step.
 
1)  Place first pair of cotton gloves on hands
 
2)  Have your one gallon of cleaning solution (distilled water with 1/2 teaspoon of Ivory Liquid) handy with a pad soaking in it; likewise you must have your gallon of RINSE solution (gallon of distilled water with one ounce of pure isopropyl alcohol and one small capful of Kodak PhotoFlo) ready with pad soaking it that as well!  Your final rinse with pure distilled water needs to be made immediately, so have that ready as well.
 
3)  Put the telescope so that the mirror is angled sharply, i.e., nearly vertical to the ground....your access to the mirror will limit what angles you might be able to achieve here.
 
4)  Making sure that the wipe or pad is ALWAYS completely soaked, but not dripping all over the inside of the OTA, gently begin wiping (Never rubbing!!) across the top 1/2 of the mirror surface; immediately ...during this process, it is absolutely imperative that you continue to resoak and freshen the cleaning pad...never let it dry out so much that surface tension increases against the glass!!
 
5)  Even though you have only done 1/2 of the mirror (always start at the top), you must now quickly RINSE what you have cleaned, using the fresh pad in the rinse solution; keep an abundant (but never dripping) amount of rinse liquid always against the glass!  Once the rinse is made, cover the mirror surface that you just cleaned with adequate distilled water final rinse and proceed to clean the lower one-half of the mirror.
 
6)  Once both halves have been cleaned and initial rinse completed, return to the entire mirror and wipe down with copious amounts of distilled water final rinse (no alcohol); repeat twice.  Never use so much that your pad is dripping into the telescope tube assembly.
 
7)  Check for water drops that are not quickly evaporating....use a "Kleenex pencil" as a wick to soak up those drops...never rub!
 
8)  Allow to dry in vertical position, with OTA end cap open but with soft cotton sheet over front, for about one hour.
 
9)  Remove any lint or dust with brush or blow off, but do not attempt until after one hour.
 
USE EXTREME CARE IF ATTEMPTING THE AFOREMENTIONED PROCESS and never attempt unless it is absolutely imperative.
 
CLEANING MIRRORS WITHOUT PROTECTIVE OVERCOATING
 
Very much unlike the previous discussion, cleaning unprotected mirror surfaces should be a "last resort" and is NOT recommended to the normal telescope user.  Only if fingerprints, bug droppings, pollen sap, etc. collects on the unprotected mirror surface should any attempt be made to clean.  Brushing is encouraged, but cleaning is discouraged.
 
For this cleaning, you need ONLY a quart bottle of pure isopropyl alcohol (91% is the minimum....94% is far better) and either Intrinsic wipes (Barnhardt Industries) or pure white Kleenex with no additives....NEVER use cotton or cotton balls to clean.  Never use Q-tips for cleaning small surfaces, only the pads or tissues as specified.
 
NEVER attempt to clean the entire surface of ANY unprotected mirror, whether it be a primary mirror or a small flat mirror in your diagonal assembly.  Clean ONLY spots and areas needing cleaning.
IMPORTANT:  never attempt to clean any spot larger than one inch using this procedure.  Use only the following procedure:
 
1)  Place a bright light so that it shines directly onto the surface to be cleaned; you need to be able to see the reflection of the light as well as move your line of sight to inspect so that the light does not shine directly back at you; viewing both ways allows you to examine for streaks and also can assist in preventing you from "over-rubbing" any cleaned area;
 
2)  Put on cotton surgical gloves and locate your area to be cleaned.
 
3)  Put ample alcohol onto your pad or tissue, making it soaked, but not dripping.
 
4)  Very gently wipe the solution across the stain...do NOT rub at all...not one bit.  Rubbing will remove your coatings!
 
5)  Follow that wipe with a second one using a totally different wipe or tissue, also soaked with alcohol.
 
6)  Finish by wiping off excess with a fresh dry wipe.....no rubbing, only a light swipe across the surface!
 
This method can be used on secondary mirrors, unprotected primary mirrors and enhanced coated diagonal mirrors.
HOWEVER, such cleaning is a last resort....never clean unnecessarily and never clean unless it must be done.
 
Remember what we have always preached at ASO:  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 the protected primary mirror OR the front corrector plate of a catadioptic (ASO Part One), it is an entirely different undertaking to clean unprotected mirror surfaces.
 
My utmost recommendations concerning cleaning of unprotected mirrors?
DO NOT ..... Let those with experience do it for you or live with the small imperfections....when they get too big, it is time for new coatings.
 
Best of luck and enjoy your telescopes...may the stars always shine their brightest through them.
 
 
Another quality service from your Arkansas Sky Observatory!
Go To Top