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Cleaning Optics

Before deciding if you actually need to clean your optics, read these instructions ALL of the way through first.  Here are two very informative articles on how to clean mirrors, eyepieces and filters. These are in the "Must Read" category if you are thinking about cleaning your optics!!


[This means that you can really screw up, BIG TIME, and it'll be your fault, if you're not VERY careful...]

bulletHow To Clean Mirrors and Lenses
bulletInformation on cleaning lenses and telescope optics from Televue


How To Clean Mirrors and Lenses

by Lenny Abbey

This file is an elaboration of a message sent in response to a request for help on ASTROFORUM in January, 1987. It is presented as an effort to assist those who have never had occasion to perform this delicate task.

The best advice on cleaning mirrors and lenses is ... you guessed it... DON'T.  But if things are so bad that you must, do it as follows:


  1. Blow all loose dirt off with "Dust Off" or another canned clean air product. Take care not to shake the can while you are using it, and be sure to release a little air before using it on the optical surface. This will assure that no liquid is dispensed to make things worse!
  2. Prepare a VERY dilute solution of mild liquid detergent (Dawn)
  3. Rinse the mirror off under a moderate stream of lukewarm water.
  4. Make a number of cotton balls from a newly opened package of Johnson & Johnson sterile surgical cotton, U.S.P. Soak 2 or 3 balls in the detergent solution. Wipe the surface of the wet mirror. The only pressure on the cotton should be its own weight.
  5. Throw cotton balls away.
  6. Repeat process with new cotton balls, using a LITTLE more pressure.
  7. Rinse mirror thoroughly under tap, which has been kept running for this step.
  8. Rinse mirror with copious amounts of distilled water (do this no matter how clean your tap water is).
  9. Set mirror on edge to dry, using paper towels to absorb the water which will all run to bottom of mirror. Keep replacing paper towels.
  10. If any beads of water do not run to bottom, blow them off with Dust Off.
  11. Replace mirror in cell, being careful to keep all clips and supports so loose that the mirror can rattle in the cell if it is shook. (Perhaps .5 to l mm clearance).
  12. Spend the next month realigning your scope.
  13. If you do anything more than this, you will damage the coating, and maybe the glass.




This restriction means that the above procedure must be modified. Only the front surface can be cleaned. If you remove the cell from the telescope, you will be in big trouble. There are probably not more than 25 people in this country who can effectively collimate a refractor! 

  1. Blow loose dirt off with Dust-Off, using the above precautions.
  2. Soak the cotton balls in a 50:50 solution of Windex and water. Squeeze slightly so that the balls are not dripping wet.
  3. Wipe front lens surfaces with the wet cotton. Follow immediately with dry cotton, using little or no pressure.
  4. Repeat procedure, using slightly more pressure.
  5. If some cotton lint remains on surface, blow off with Dust-Off.
  6. Repeat procedure if lens is not clean, but if one repeat does not do it give up and leave it as is.
  7. Inspect lens to make sure that no cleaning solution has found its way into the lens cell, or between the elements. If this has happened, leave the telescope with the lens uncovered in a warm room until it is dry.


Follow the procedure given for objective lenses, but use Q-Tips (with plastic sticks) instead of cotton balls. You may, of course, clean both surfaces. The eyebrow juice on the eye lens of eyepieces may require repeated applications. I think that this is OK in this case.


  1. Do not use any aerosol spray product, no matter who sells it, or what their claims are.
  2. Do not use lens tissue or paper. It DOES scratch.
  3. Do not use pre-packages cotton balls, they frequently are not cotton.
  4. Do not use any kind of alcohol.
  5. Do not use plain water.
  6. Do not use any lens cleaning solution marked by funny companies, like Focal, Jason, Swift, or even Edmund's. Dawn and Windex are cheap and commonly available.


ADDENDUM by astro-tom.com


There is a trick to cleaning SCT's.  You want to securely brace the scope so that you can hang the tube (corrector plate down) over the edge of a workbench or something.  You will be squirting distilled water UP at the corrector plate.  This operation is done with the scope facing downwards so that none of the water leaks in around the side and drips down on the mirror...which is bad in a relatively sealed system like a SCT.

If there are a lot of particles on the inside of the corrector plate, and I mean a lot, I would strongly recommend returning it to the manufacturer or a professional telescope cleaning service  for professional disassembly and cleaning.  It's worth it to not accidentally damaging the secondary spherical mirror or destroying the alignment of all of the optical surfaces.  Any scratch on the secondary mirror surface will severely damage your views.  Unless the dirt's on the outside surfaces of your SCT, I would NOT recommend tackling the job.  I also recommend that you DO NOT disassemble your SCT.




More On Cleaning Optics from Astralnomicon Observatory Systems: 

NOTE: These guidelines have been synthesized from several different sources from experienced astronomers to optics manufacturers. Although no one cleaning method or cleaning solution works for all surfaces, optical coatings or types of contamination, these guidelines apply to most commercially available optical surfaces and contaminates.

Since these guidelines are highly dependent your actual application to your optics, the purity and type of materials used or type of contamination present so...


Most of the components cited here are generally available from your local drugstore, pharmacy or optical supply company. Some of the more advanced solvents must be obtained from chemical supply houses.

WARNING: When in doubt as to whether to clean your optics or if you are unsure of the quality and purity of your cleaning components or your cleaning technique, DO NOT PROCEED! The cleaning of optical surfaces, especially those of first-surface mirrors, is the most delicate and exacting task which the astronomer is called upon to perform. At the time of cleaning, a lens is most vulnerable to damage that can not be repaired. Yet, if a telescope is to perform at its greatest potential, cleaning must be done time to time. First you must realize that usually the best advice on cleaning mirrors and lenses is this: DON'T DO IT unless absolutely necessary!

With this said, let's proceed.



Most dust that you see on your objective lenses, mirrors, correcting plates, eyepieces or filters is harmless and generally does not effect the quality of your optics or effect image quality, whatsoever. This dust should be left-alone or simply blown-off with high-quality compressed air product that uses dry nitrogen as its propellant. DO NOT use compressed air products that contain solvents as their propellants! Most compressed air products designed for blowing off electronics or computers contain such solvents. Do not use any other type of aerosol product that claims to clean glass of any type no matter what the claim of the manufacturer. If dry nitrogen propelled air is not available, the use of a compressed air product sold at photography stores such as Dust-Off may be used if your are careful not to tilt the can, thereby releasing the propellant onto your optical surface. Alternatively, you may obtain a rubber syringe bulb (found in the baby supply section of your local grocery store or pharmacy) or a blower bulb/camel hair brush combination from a local photography supply store. Blow the dust off starting at the center of lens or mirrors working your outward towards the outer edge. Stubborn dust particles can be ever so gently brushed outward using the same method only using a soft camel hair brush. If you observe anything other than dust (sand particles or eyelashes, for instance) DO NOT BRUSH THEM for they will scratch your optics!


Dirt, sand particles, grease, fingerprints, bugs or other contaminates that adhere to the surface of mirrors and lenses may degrade image quality, but they will not damage the delicate optical surface until they are moved against it. Certain organic residue or oils from fingerprints or bugs or cigarette smoke may eventually damage delicate coatings and must be cleaned. Removing dirt without allowing it to rub against the underlying optical surface is what makes cleaning such a critical task. If your mirrors and lenses are so dirty that they must be cleaned, then proceed carefully as instructed.


Many types of powerful commercial solvents are available including methyl ethyl ketone (MEK), methylene chloride (MEC), acetone or pure isopropyl alcohol, but these products should only be used by professionals in the correct dilution on the appropriate optical surface.

However, most professional observatory operators say the simplest solution is the best! Click here to see what they say at the Gemini Observatory in Hawaii.  

For personal use, you should prepare a solution that can be used safely on all optical surfaces based upon commonly available ingredients as follows:

1. Distilled Water. The basis of this solution is the most commonly available solvent on the planet (which also happens to also be responsible for life on earth): water. In our case, pure distilled, de-ionized water. Do not use tap water or bottled water for this purpose because they both contain minerals and other contaminates that defeat the purpose of what we are trying to accomplish: PURITY.

2. 99%-97% Isopropyl Alcohol. You may have to ask your pharmacist for this or look around your drugstore. Most off-the-shelf "isopropyl" alcohol is usually in the 70%-90% with the other 30%-10% containing regular water or other contaminates. Avoid this. Methyl hydrate (methanol) may be substituted for isopropyl alcohol.

3. Mild Detergent. Liquid dish detergent that is fragrance-free, color-free works best such as Dawn or other similar product (hint: the folks at Gemini say use horse shampoo!).

4. New plastic spray bottle. These can usually be found in the garden supply section of your local Wal-Mart or garden supply store. Do not be tempted to use a used bottle that previously contained some other solution such as glass cleaner, buy a new bottle; it only cost a couple of bucks. A one time solution can be mixed in a clean drinking glass, in a pinch, that has been adequately rinsed in distilled water.

Now the recipe. If you are cleaning a Newtonian type reflector you will be cleaning your primary and secondary mirrors. If they are aluminized mirrors (most are) do NOT add the isopropyl alcohol in step 3, below because alcohol reacts with aluminum. If you are cleaning a refractor or SCT you will be primarily cleaning your objective lens or front corrector plate, do add the alcohol in step 3. On occasion, SCT and other catadioptric telescope owners will want clean their primary or secondary mirrors, these are usually aluminized as well. You will need to mix two solutions, one with alcohol and one without, carefully marking each bottle as required with an "M" for mirror and a "G" for glass.

1. First start by rinsing your new, unused spray bottle with the distilled water. Fill and vigorously shake the water in the bottle at least three times to remove any inadvertent particles or residue that may be present in the bottle as a result of manufacturing.

2. Next, fill 3/4 of the bottle with the distilled, de-ionized water.

3. For glass cleaning only add 1/8 of the bottle with 99% isopropyl alcohol. Too much alcohol will leave streaks. This should leave the bottle 1/8th empty. For mirror cleaning omit the alcohol or any other solvent besides the mild liquid soap.

4. Finally, very carefully add 2-3 drops of the mild dish detergent. If your detergent bottle dispenses too much use an eye dropper. Too much soap will leave a cloudy residue.

Screw on the pump spray top immediately to avoid dust or other particulate material from entering the solution. Always keep it covered, in a cool dark place when not in use. Do not refrigerate.

WARNING: Care should be taken not to inadvertently get this solution on any other part of your telescope except the optical surfaces. Lens cells and corrector plates are often held in place by cork or rubber seals and gaskets to which this solution could damage. Always blot or wipe excess solution and prevent it from running down in these areas or entering the optical tube assembly.


You will need to obtain several different cleaning materials depending on whether you are cleaning mirrors, lenses or eyepieces. First and foremost, never use prepackaged bulk "cotton" balls, they always contain other miscellaneous fibers that can scratch delicate coatings and secondly they are not very absorbent. Additionally, also never use tissues, toilet paper, paper towels or lens cleaning paper that you may find at the photo store. All of these products may scratch coated optics or leave more dust than they remove. For lenses and mirrors, we recommend the use of precision quality optical wipes such as Opto-Alignment Technologies Inc.'s OPTO-WIPES(tm) available from AOS. Additionally, wearing OPTO-GLOVES(tm), also made by the same manufacturer, will prevent recontamination from fingerprints and body oils. If OPTO-WIPES are not available, obtain Johnson & Johnson's Sterile Surgical Cotton, (U.S.P.) which is sold in bulk pads at your local pharmacy. The pro's say to use a Natural Sponge! You will also need pure cotton swabs (Q-tips), that are mounted on the plastic sticks rather than the wood or paper sticks to clean your eyepieces and filters.


Many factors must be considered before attempting to disassemble your telescope. Always consult your manufacturer or your operating manual prior to disassembling any telescope. Disassembly may also void your warranty, check to be sure. Additionally, disassembling your scope can also cause it to be permanently out of alignment (collimation) requiring it to be returned to the factory for correction. This can be both time-consuming and expensive! Some types of scopes, such as Newtonian reflectors, require special tools or collimation devices to re-align the optics, make sure you have these on-hand prior to disassembly. In general, avoid disassembling your scope unless absolutely necessary! There are many user groups and experienced amateur astronomers that can give you good advice in this area for your particular scope, so we will not go into specific details regarding this process in these guidelines. In general, consider the following:

1. Refractor. Most refractor telescopes have multiple objective lens systems that are permanently mounted in cells or are permanently attached to the optical tube assembly (OTA). Because of their closed optical tube design, only the exposed surface of these optics will ever need to be cleaned. Again, most refractors do not have user "collimatable" optics and must be returned to the factory for optical alignment. Never disassemble a lens cell, under any circumstance, while many are air-spaced, others are vacuum sealed or filled with an inert gas such as nitrogen or even oil!

2. Catadioptric. Schmidt-Cassegrain, Maksutov-Cassegrain, Schmidt-Newtonian, Ritchey-Cretien and other similar telescopes employ multiple optical surfaces that must be perfectly aligned for correct collimation. For simplicity sake we will refer to these as CATs. Most telescopes of this type generally have closed optical tubes assemblies minimizing any debris that can enter into the OTA except through the visual port. Although these telescopes are usually user collimatable, the front correcting plates are rotationally figured and aligned to perfectly match their primary mirrors. Care must taken to precisely mark the position of each component prior to disassembly as well as taking care not to drop screws or the secondary mirror down onto the primary mirror causing permanent damage. Again, disassembly of these scopes is not recommended, except by experienced personnel. Material (such as bugs or dust) can sometimes enter inside the optical tube assembly or debris can be seen on the secondary mirror. These items can usually be carefully vacuumed out by attaching a long drinking straw to a vacuum cleaner hose and gently inserting it into the back of the telescope through the visual port to suck the debris or critter out without disassembling the scope. Like refractors, usually only the exposed surface of the front correcting plate need ever be cleaned.

3. Newtonian/Dobsonian/Truss-type reflector. The mirrors of these scopes are the most susceptible to accumulating dirt and dust because of their open, exposed design. But, these scopes are also the easiest to disassemble and clean as well as put back together and collimate. Again carefully marking the positions of the secondary and primary mirror will promote successful reassembly. Also, as previously advised, care must be taken not to drop components and fasteners down the optical tube onto your primary mirror.


1. Mirrors usually have to be removed from their optical tube assembly (OTA) prior to cleaning. As stated above, this is usually not recommended except for Newtonian telescopes. Aluminized mirrors should only be cleaned once a year. Further cleaning will degrade the optical surface. Care should always be taken not to drop your mirror while cleaning or otherwise bump it with other items in the area. If cleaning your mirror in the kitchen sink place some towels or other material in the sink (not obstructing the drain, of course) to cushion the mirror in case it slips.

2. Before any cleaning operation, always blow all loose dirt off with a compressed air product prior to proceeding. Take care not to shake the can or tilt it while you are using it and be sure to release a little air before applying it to the optical surface. This will assure that no propellant is dispensed onto the mirror. You can also use the rubber syringe bulb for this purpose, though it is not as effective. Blow the particulate matter from the center of the mirror outwards towards the edge of the mirror and eventually off it.

3. Rinse the mirror off under a moderate stream of lukewarm water for two or three minutes. Test the temperature of the water with your wrist, just as you would when warming a baby's bottle. Allow the water to run at this temperature during the entire cleaning process.

4. Spray the entire mirror surface with your cleaning solution. If you made your solution in a cup instead, tear a swatch of cotton from your newly opened package of Johnson & Johnson sterile surgical cotton, soak the swatch in the cleaning solution and squeeze the solution onto the mirror surface.

5. First, we want to remove any visible solid particulate matter from the mirror surface by using the Opto-Wipes or small cotton balls torn from the surgical cotton. Starting at the perimeter of the mirror and spiraling around toward the center of the mirror, gently blot the Opto-Wipe (or cotton ball) onto the particles lifting them off the mirror surface. The Opto-Wipes are specifically designed to accomplish this feat. Replace the wipe or cotton ball with each and every blot and discard until all visible particles have been lifted off the mirror surface.

6. Next, re-wet the mirror surface with the cleaning solution or soak more cotton balls. Begin gently wiping the wet mirror surface, again starting at the circumference and spiraling your way towards the center using absolutely no pressure on the wipe. The only pressure applied should be that of the weight of the wipe or soaked cotton balls. Change wipes or cotton balls often, after each short stroke and discard.

7. Rinse the mirror with copious amounts of distilled water. Visually inspect the mirror surface for stubborn spots or splotches that were not removed by steps 5 or 6.

8. If particulate matter is still present repeat step 5. If organic splotches are still present repeat step 6 using ever so slightly increased pressure. Repeat step 7.   DO NOT REPEAT AGAIN.  If particles still remain, they are probably embedded in the mirror surface and have to be removed professionally.

9. Once the mirror has been thoroughly rinsed with the distilled water, turn it upon it's side for the excess water to drain. Then use the clean Opto-Wipes to blot the excess water and the compressed air to chase away the remaining droplets. Do not use dry cotton, as it may leave behind dust or threads.

10. Replace the mirror into the telescope, loosely applying the retaining clips enough to gently hold the mirror and follow your manufacturer's instructions for optical realignment.


WARNING: Never remove a refractor's objective lens or remove the objective lens from their cell under any circumstances! Only remove an SCT's corrector plate under extreme circumstances and only after carefully marking its rotational position in relation to the optical tube assembly. This procedure will assume that only the front surface of either a lens or corrector plate is being cleaned and that it is still attached to the telescope.


1. Blow off all dust from the lens or plate as described above, working from center outward until all particles that can be removed, are removed.

2. Apply cleaning solution to the Opto-wipes or surgical cotton balls squeezing and discarding any excess solution. Never apply or directly spray the lens or corrector plate which could cause the solution to drip down into the cork or rubber gaskets and enter the optical tube assembly.

3. Gently blot any visible solid particulate matter, lifting it off the glass surface, being careful not to apply any lateral motion in the process which could resulting in scratching the glass surface or its coatings. Discard each wipe or cotton ball with each and every blot and discard.

4. Next reapply cleaning solution to wipes or cotton balls and begin working your way from the center outward like spokes on a bicycle. The only pressure being that of the weight of the wipe or cotton ball itself. Replace and discard each wipe or cotton ball with each spoke as your proceed around the entire glass surface.

5. Examine the glass surface for stubborn particles or organic splotches that were not removed by step 4. If particles or spots remain, repeat step 4 gently, using slightly more pressure.

6. Once complete, use a dry Opto-Wipes or cotton balls and repeat the spoke pattern, drying up any excess moisture, again replacing each wipe or cotton ball as your proceed.

7. Blow off any excess dust that may have been left by the cotton balls or that attached from the air onto the moist surface using the compressed air.


Follow the procedure given for objective lenses, but use Q-Tips (cotton on plastic sticks) instead of cotton balls. You may, of course, clean both surfaces. The eyebrow sweat on the eye-ward surface of eyepieces may require repeated applications.


Some filters that use diachronic, gel, or metal coatings (such as Solar filters) or those made of plastic may not react well to the presence of alcohol or other solvents in the cleaning solution, so mix a small batch of the solution as directed for cleaning mirrors (distilled water and soap). Then follow the cleaning procedure as described for objective lenses.



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