Mirror Cleaning

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Revision as of 23:03, 23 February 2019 by WikiSysop (talk | contribs) (Methods of cleaning large telescope mirrors and other optics)
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Steward Observatory on Mount Lemmon

This procedure for cleaning large mirrors was demonstrated to us by Joseph Hochscheidt when he cleaned the Manner Telescope primary and secondary in June 2018. Use it with caution since this is based on notes taken at the time. Obviously the first rule is do no harm, and that means be reasonable, gentle, and follow the advice of experts when it is available.

For this cleaning process the mirror was left in the telescope. It was a warm sunny day, and the telescope enclosure was opened slightly to allow for ventilation and to provide illumination needed to judge the effectiveness of the process.

1. Blow off all dust and loose particles with dry nitrogen gas from a compressed gas cylinder.

This requires bringing a cylinder to the telescope, attaching a regulator, and using a flexible hose that provides enough gas flow for the purpose. Do not use compressed air! It will contain moisture that will condense on the optics, and it may also contain fine particulates that could damage the coatings. Also do not use dust-off gas often sold for blowing dust out of electronics and even for photographic lenses. Typically these contain fluorocarbons and they will leave a residue that is hard to remove. An alternative that is often used with large optics is to use gas withdrawal carbon dioxide. This is available in small cylinders, and is released at full pressure through a hose to direct the flow onto the mirror and its shrouds. Carbon dioxide cleaning will produce CO2 snow which can effectively scrub a mirror of dust. However it will be insufficient to remove deposits left by evaporating dew or by insects. If a supply of dry nitrogen or carbon dioxide is not available the surfaces may be dusted with a new very clean and soft brush. A photographic lens camel's hair brush, or a soft paint brush with fine bristles will be safe for this task. Compressed gas is preferred, however.

Assure that all loose material is blown out or wiped from the housing that baffles the mirror.

2. With the telescope pointed approximately horizontally, use large quantities of Kimwipes or paper towels rolled to absorb any fluids that drain from the mirror. These may be replaced several times during the cleaning process. The purpose is to prevent fluids from draining into critical components behind the primary, or auxiliary instruments or components that may be on the lower side of the truss. Each case will be unique and should be evaluated before beginning. It may be beneficial to point the telescope slightly below the horizontal if the light baffle around the primary is solid and would hold liquid that could drain to an area which is not critical.

It is not advisable to remove the primary. This adds considerable risk to the cleaning process, and would surely require telescope realignment. Also, primary removal would normally be preceded by mechanical work to assure the telescope remains stable, and would add days to a job that can be done in hours.

3. Wash the interior and all exposed surfaces with water.

4. Prepare a solution of distilled water and detergent. The distilled water can be purchased at a grocery or drug store in gallon jugs. It should be mixed in a clean 5 gallon bucket reserved only for mirror washing. For mirrors of the order of 0.5 meter in diameter, 2 gallons of water for the washing solution should be adequate. Add a small quantity of Liquinox detergent to the water. Test the solution to see that it wets a surface easily. Excess detergent leaves a residue that requires more rinsing and the compromise needed is to have enough in solution to clean, and not so much it is difficult to rinse. Only a few milliliters of liquid detergent should be needed in 2 gallons (about 8 liters) of distilled water. If Liquinox is not available, then Dove dishwashing liquid also works well.

5. Prepare suitable washing wipes. At Steward the preferred ones are treated sea sponges that have been soaked in vinegar to remove minerals and then very thoroughly washed and rinsed. The final wash and rinse is done with TX609 Technicloth wipes bundled to make a poof ball perhaps 10 cm in diameter and held by the back where the edges of the cloth are gathered. You will need many of these and it helps to have an assistant making them as you proceed. At this stage both the person doing the mirror cleaning and the assistant should have on new clean nitrile gloves so that skin oils are not collected and left on the mirror.

6. Apply the water and detergent mix to the mirror working in thirds. It may take two or more treatments to achieve a clean surface, and a preliminary rinse should be done on the work area before moving to the next third.

7. Rinse twice with distilled water and pat dry with a TX609 wipe poof. The rinse should be using a separate rinse bucket of water into which to dip the wipe. The final rinse is with a wet but not dripping wipe. The goal is not to leave water droplets on the surface, and not to pick up debris into the wipe that will scratch the coating. By using a large clean bucket and fresh wipes on each pass, you are assured that contamination of the water and the wipe is minimized. When you dip a previously used wipe into distilled water you will contaminate the water with residual detergent and you risk contaminating with particulates that were picked up off the mirror. A final rinse with a second filling of distilled water will give a better result than making do with one rinse and dipping back into he water several times.

Ulmt primary cleaning.jpg

The primary mirror of the 0.6-meter Manner Telescope on Mt. Lemmon during cleaning. Packing around the lower edge absorbs cleaning fluids. The upper right third of this mirror has been cleaned and washed. The lower portion has not been cleaned for two years.

Materials required:

  • Nitrile gloves
  • Dust off or cleaning gas such as
    • Prepure N2 tank with regulator and hose
    • CO2 tank with a siphon tube (liquid withdrawal) low velocity snow generator (see below)
  • Bounty paper towels for handling excess liquid and runoff
  • White Kleenex tissue free of fragrances and oils for spot cleaning
  • Texwipes as a technical alternative to off-the-shelf paper towels (caution, these will scratch some coatings)
  • TX609 Technicloth for direct use on mirror coatings during the wash and rinse stages
  • Soft brush such as a very high quality new fine bristle paint brush or a camera lens brush (these collect oils and abrasive dust; best not to reuse)
  • Isopropyl alcohol 90% for removing stubborn organic contaminants
  • Distilled water (several gallons)
  • Clean 5 gallon buckets
  • Detergent
    • Liquinox
    • Dove dishwashing liquid (alternative to Liquinox)


  • Disposable nitrile gloves are available from all drugstores and most grocery stores for medical purposes. Use the dust-free variety. Replace them frequently if you are handling the material that will be in contact with the mirror surface.
  • Distilled water is available off the shelf in drug and grocery stores. You will need several gallons. Pour a supply into a clean bucket and let it settle. Use the liquid at the top of the bucket
  • Buckets. Start with new clean plastic 5-gallon buckets and label them for wash and rinse liquids. After a first use rinse with distilled water and store covered for reuse on the next cleaning. Do not use buckets that have had other uses. New buckets are available from home suppliers (Home Depot), hardware, and paint stores.
  • CO2 snow cleaning equipment
  • N2 or CO2 gas cylinders
    • Local welding gas suppliers will refill tanks
  • Texwipes
    • Amazon search for texwipes and then for TX609 to have the lint free version
  • Liquinox
    • Amazon links to suppliers for this too. A small amount of Alconox will do.
  • Isoproponol
    • Isopropyl alcohol is a readily available substitute for pure ethanol to be used as a cleaning agent to remove oils and to hasten drying. It is available off the shelf in drug and grocery stores. Use the 90% pure antiseptic medical alcohol in a new previously unopened bottle. Alcohol will adsorb water from the air once opened.

Planewave Instruments

Joe Haberman at Planewave Instruments recommended the cleaning system described by the Arkansas Sky Observatory website. While aimed at smaller telescopes, the advice on this site is similar to the Steward Observatory process in that they dust off first, clean with liquids only when necessary, work in small areas, use a cleaning solution, and rinse thoroughly. The difference is that they use Kleenex for the wipes, and the cleaning solution is a mixture of distilled water, alcohol, and Photo-Flo (a wetting agent). They also use Photo-Flo in the final rinse.

Photo-Flo was commonly available in the days of chemical photography for rinsing film and prints after processing to minimize water spottting. It is harder to find now. If you follow the recipe at the ASO site, substituting a small amount of Dove liquid detergent for the Photo-Flo is a workable option. A final rinse with pure distilled water and judicious use of poofs of several while Kleenex tissues to pick up any residual drops may give a better result than leaving some residue from the Photo-Flo. The TX609 wipers will have more surface area and be easier to manage for a large mirror than Kleenex, and some generic white tissue may not be as lint and abrasive free as the tissue that was manufactured when the ASO established this protocol.

In the event the link to the ASO site is not available, here is a summary of the process abstracted from their page in February 2019.

Materials needed:

  • Three clean jars or quart containers
  • Distilled water from a supermarket
  • Isopropyl alcohol (high percentage)
  • Coffee filters (to filter water and alcohol)
  • Blue Windex
  • Kodak Photo-Flo wetting agent (camera store or Amazon)
  • Spray bottle to dispense solutions onto cleaning pads
  • Pure white Kleenex with no additives
  • Synthetic cotton pads (not obviously identifiable now. See TEX wipes above for a better alternative.

Cleaning solution:

  • Filter an entire bottle of Windex through the coffee filter into a thoroughly washed and dried container.
  • Filter the distilled water using a second clean coffee filter into another jar.
  • Mix solutions in these amounts in another quart jar.
    • 1 ounce of filtered Windex
    • 1.5 ounce of isoproponal
    • Two (!!) small drops of Photo-Flo
  • Stir gently and thororoughly and to not shake.
  • Add 12 ounces of distilled water.
  • Pour liquid into a clean marked spray bottle for use.


  • Clean optics only in the daytime with the optical surface "looking" out of a window or toward a bright open sky
  • Never....never...attempt to surface clean large optics when the humidity is above 65% !! streaking will result. if you attempt to clean your optics when the humidity is high, you will be very disappointed in the results.
  • Plan to use at least one tissue per inch aperture being cleaned....always keep a dry tissue to the surface for best results!
  • Make sure you have dusted off the particles from the glass prior to further cleaning!


  1. Turn your telescope so that you are the optics head-on; you are not going to use so much liquid that you need to be worried about cleaning solution getting away from you and down inside the retaining rings of the optics. Make yourself comfortable....you may be here a while! I prefer placing the telescope if possible in a position where I can sit down to clean. You must have a small table or area within reach where you will have your cleaning pads, solutions and Kleenex waiting.
  1. Imagine your optics in quadrants or quarters, like large sections of pie. You are going to begin at the top left and work your way down to the bottom left piece of pie.
  1. Wet the cleaning pad with cleaning solution, not the glass surface. You want the pad wet, but not dripping; make sure you hold the pad only on one side and do not do not use the side where your fingers have been.
  1. Begin in your upper left quadrant and gently daub (not rub) this section until you have generously smeared the cleaning solution across that area. The idea here is to move only the liquid across the surface to break the adhesion of film and dirt deposits against the glass. Proceed quickly to the next step.
  1. Before the liquid begins to collect into large areas and before any drying takes place, immediately begin wiping the quadrant just soaked with Kleenex tissue to dry it

Plull the Kleenex across the surface in in one direction only letting surface tension of the cleaning agent remove the liquid from the surface. You will see any residual thin film of liquid rapidly drying behind the stroke of the Kleenex. Follow each swipe with another dry Kleenex tissue. Keep changing tissues promptly.

  1. When entire quadrant is reasonably dry, buff gently with a totally dry Kleenex; repeat a second time with another Kleenex while gently "puffing" a bit of your breath against the corrector plate or lens to expose possible areas of streaking. Again, change the tissue constantly.
  1. Repeat same procedure on remaining three quadrants with a bit of overlap on each.
  1. Check each areas overlapped during cleaning and as needed use a fresh pad sprayed with a very small amount of cleaner. Swab touch-up areas nearly dry with just enough moisture remove defects in cleaning.
  1. Using your breath as a guide, gently use a pado buff the final cleaned surface and remove residual spots and streaks.
  1. Optionally rinse as needed only if there are problem areas remaining after the previous work. For this wet a pad with rinse solution and cover a quadrant while following immediately to promptly dry as described above. After rinsing, buff entire surface as needed with a fresh and dray pad to finish.